Loading ...
Sorry, an error occurred while loading the content.
 

C/2010 X1 remnant, and other things

Expand Messages
  • Alan Hale
    Hi everyone, I apologize in advance for what is almost certainly going to be a lengthy post. The recent events, specifically involving C/2010 X1 and
    Message 1 of 24 , Oct 26, 2011
      Hi everyone,

      I apologize in advance for what is almost certainly going to be a lengthy
      post. The recent events, specifically involving C/2010 X1 and observations
      of it, have prompted me to write this, but this also strikes me as a good
      opportunity to comment on issues and trends that have led up to the recent
      events. As a faithful observer of comets for over four decades I'd like to
      think I've earned a right to address these issues.

      First off, of course, there is now no doubt that a faint dust stream remnant
      of C/2010 X1 does indeed continue to exist. I was certainly skeptical of
      this, up until the time I saw Rolando Ligustri's image that he took on the
      22nd; but that image, and the other images that he and other observers have
      taken since then, clearly demonstrate that this dust stream is there. I
      rather agree with John Bortle's comparison to the "head-less" Kreutz
      sungrazer of 1887; one other example that occurred to me (although the
      physical circumstances are obviously different) is the recent P/2010 A2.

      It so happened that Rolando took his October 22 image at the precise time
      that I was making an unsuccessful visual attempt for this comet.
      Accordingly, as soon as I saw the image I noticed that there was nothing at
      the ephemeris position of the comet itself; the closest relatively bright
      portion of the "tail" (which is how I will refer to it from here on) was
      displaced several arcminutes to the west-northwest of the ephemeris
      position. I had concentrated my search efforts on the ephemeris position,
      and thus I should *not* have seen anything when I was making my search. I
      verified this displacement with the image that Rolando took the following
      morning.

      Armed with this knowledge, and also the knowledge of what precisely to look
      for, I made another attempt on the morning of October 24. I did in fact seem
      to see a shapeless but extended "something" in about the expected position,
      and moreover this "something" seemed to move westward at the expected rate.
      This was an extreme borderline observation, however, and was akin to some of
      the very extended diffuse nebulae I've occasionally looked for (the
      supernova remnant S147 in Auriga/Taurus being an example); I could "see"
      something by detecting the very slight differences in the background sky
      brightness between the locations where it "was" and where it "wasn't."

      For what it's worth, I did attempt a brightness measurement by an extreme
      defocusing of stars, and would say somewhere around m1 ~12: I agree with
      John, though, that this is essentially meaningless -- for one thing, there
      is no coma, and that is usually what is meant by a comet's brightness -- and
      at best it refers to a shapeless, ill-defined portion of the tail remnant.

      I had to be out of town for a family emergency on the morning of the 25th
      (and I think it was cloudy here anyway), but I tried again this morning (the
      26th). Again, I seemed to see this extremely faint "something" that moved in
      the right direction at the right rate, and, again, it was an extreme
      borderline observation. At this point I am reasonably sure that I have
      indeed detected this tail remnant of C/2010 X1 on the two mornings in
      question, although I would greatly appreciate it if anyone who was imaging
      the comet on those dates, and especially around the times of my observations
      (roughly 8:30 to 9:30 UT on October 24, 7:50 to 8:50 UT on October 26) could
      either send those images to me, or post them somewhere.


      Now, having reported all this, I would like to comment on some visual
      cometary observations, not only of C/2010 X1 but some other comets, and in
      general. While Juan Gonzalez has been a bit on the "hot seat" with his
      reports of C/2010 X1 -- and I freely admit that I have been among his
      doubters, and I will get to that in a moment -- my comments are directed
      more towards the current community of comet observers in general. I will
      occasionaly use the word "you," but, except in the specific case of the
      recent observations of C/2010 X1, I am not referring to any observer in
      particular.

      Before proceeding, I wish to reiterate something I said in an earlier post,
      i.e., at no time do I mean anything personal about any individual. I
      consider everyone here friends and colleagues, and I have high respect for
      all of you. I am speaking strictly of the scientific observations involved,
      as I would like to believe that the visual brightness (and morphology)
      reports of comets continue to have scientific value. That requires, though,
      that they be done correctly at all times.

      Let's look at Juan's two recent reports of C/2010 X1. The first one, on
      October 9, specifically refers to a "coma;" we now know that there is no
      "coma" for this comet, and anything that might have been visible was not at
      the ephemeris position, and Juan makes no mention of this. Furthermore, he
      remarks that there was zodiacal light present (with which I will concur,
      with my own unsuccessful observation attempt around that same time), and,
      considering the extremely low surface brightness of the "tail remnant," any
      kind of background lighting should have wiped it out completely. (Recall
      that all the CCD images at that time showed nothing, or at best, an
      extremely faint and uncertain fragment that may or may not have been real.)
      And while I accept at face value Juan's comments about the clarity of the
      atmosphere down to the horizon, at an altitude of 23 degrees you are still
      looking through over 2.5 air masses, and you are going to be encountering a
      non-trivial amount of extinction.

      The second observation, on October 21, does seem at face value to be
      reasonably consistent with what the later images showed, although I note
      that he reported one of his "bright areas" to be at the ephemeris position.
      There would have been no zodiacal light at the time of the observation,
      although the reported altitude of 16 degrees works out to over 3.5 air
      masses and thus a significant amount of extinction; if this was at the
      beginning of the observation (Juan doesn't say, but does report that he
      followed his object for 40 minutes) then the extinction would obviously have
      decreased throughout the observation.

      For the record, the altitude at the beginning times of my two recent
      observatons was 44 degrees (1.4 air masses) on both mornings. I was using a
      41 cm Newtonian, with a fairly fast optical system (f/4.5); I again note
      that while I am reasonably certain of my sightings (although perhaps not
      100% certain until/unless I can see some images taken around then), these
      were extreme borderline detections.

      The upshot here is that I must remain very skeptical of the October 9 report
      as, all things considered, it is inconsistent with what the CCD images (both
      then and now) have revealed. The October 21 report is much less clear to me;
      I do have some problems with it, which I've discussed, but on the other hand
      it is consistent enough with the CCD images that I cannot simply dismiss it.
      "Benefit of the doubt" would suggest that I accept it as a positive
      observation, but to be honest, I don't believe I can go either way at this
      time. The best I can do right now is consider it an open issue, which I may
      never be able to resolve to my complete satisfaction either way. (This
      definitely is not the first such "open issue" I have encountered over the
      years . . . )


      But to turn now to the issues which have contributed to the difficulties I
      have had with some of the reported observations (and, reminder, from here on
      "you" is generic):

      In addition to my one comet discovery, during the days before the web and
      the NEOCP I was constantly being called or e-mailed by Dan Green or Brian
      Marsden to confirm various comet discoveries, and over a period of about two
      decades I successfully confirmed numerous such discoveries (as well as
      dis-confirmed numerous other reports). One of the comets I confirmed was as
      faint as m1=13.5. Some of the comets were several degrees away from their
      "expected" locations, and yet I was still able to track them down (and
      perhaps it's worth keeping in mind that we were not even sure these comets
      were real, although we considered some more likely than others).

      While I am not infallible, with this kind of track record it is
      inconceivable to me that I could be looking *directly* at a 9th- or
      10th-magnitude comet, and *not* see it. Yet, if I am to believe some of the
      reports that have been submitted in recent years, that is precisely the
      case. There are several examples I can cite, but I will focus on two recent
      ones.

      We've already discussed C/2011 M1 to an extent. Some of you were
      consistently reporting this comet to be as bright as m1 ~9-10, with a large,
      diffuse coma, and yet, despite numerous search attempts, I never saw it. And
      in this case, the CCD images backed me up:

      With a telescope of any size, a CCD image with an exposure time of only 15
      or 20 seconds will reveal stars fainter than what can be visually detected
      with that telescope. In the case of the C/2011 M1 images, the total exposure
      times were in the range of 16 *minutes* -- and yet, they only revealed an
      extremely faint smudge of light on the verge of complete disintegration that
      was barely detectable.

      Another example that I have not seen discussed here yet is C/2011 A3. Again,
      several of you have reported this comet as being bright at m1 ~9-10 and with
      a large coma, and, again, despite numerous attempts I have failed to see it.
      (Other observers have privately informed me that they, too, have failed to
      see it.) Once again, the CCD images back me up: even when processed with an
      unreasonably high contrast, they show only a small and relatively condensed
      coma that is no brighter than the 15th-magnitude stars in the field (an
      appearance, incidentally, that is entirely consistent with a comet that is
      2.5 to 3 AU from both the earth and the sun).

      These are *not* cases of "visual photometry vs. CCD photometry;" these are
      cases of CCD images showing what is and what isn't there. It is very
      disingenuous to cite the CCD images as positive evidence of the C/2010 X1
      observations, and then turn around and dismiss the CCD images of C/2011 M1
      and C/2011 A3 which reveal that those two comets were much, much fainter
      than the visual reports at the times in question.


      There are two other issues I would like to touch upon. Back when they were
      active, I was constantly cross-checking my observations against those of
      John Bortle and Charles Morris, and I was almost always within a
      half-magnitude or so of their reports. This doesn't necessarily mean that
      any particular individual is "right," and, yes, we all should "call them as
      we see them," but if everyone involved is doing it correctly, we should all
      be seeing something fairly similar to each other. After all, if the depth of
      a lake is 10 meters at a certain point, then all measurements of the depth
      at that point should be somewhere close to 10 meters. Similarly, if a
      comet's "true" brightness is magnitude 10.0, then all measurements should be
      somewhat close to that (say, between magnitude 9.5 and 10.5).

      But I am not seeing that now; I am seeing reported brightnesses that are
      two, three, sometimes more magnitudes different from what I am seeing.
      Again, I am not saying that my measurements are always "right," but I find
      it hard to believe that I've lost my ability to make reasonably accurate
      measurements within the past decade. It's like we're back to those depth
      measurements; if one person is reporting a depth of 2 meters, while someone
      else is reporting a depth at the same point of 28 meters, something is
      wrong. Likewise for our 10th-magnitude comet; if one observer reports it at
      7th magnitude, and another observer at 13th magnitude, something is wrong.

      And then, there is the topic of limiting cometary magnitudes for telescopes
      of various apertures. I have been utilizing the 41 cm telescope for over two
      decades now, and can state with reasonable confidence that its limiting
      magnitude for comets is around 14.5. (I'm referring to "cometary" comets,
      not completely stellar ones, which I can obviously detect fainter.) Over the
      years, observers I know with similar-sized telescopes have reported
      something similar, and those with smaller or larger telescopes have reported
      limiting magnitudes that are consistent with this.

      And yet nowadays, I am seeing some of you, including some of you with
      telescopes significantly smaller than mine, routinely reporting comet
      magnitudes up to a full magnitude -- sometimes even more -- fainter than the
      limit of my telescope. How is this possible? I think I've already shared the
      story of the occasion where, by examining CCD images that I took almost
      simultaneously with a faint comet I was attempting visually, I was able to
      detect a comet at m1=14.8 -- but this took four nights (and two hours each
      night) of this simultaneous visual/CCD work before I could finally convince
      myself that that tiny speck of light that flickered at the extreme limit of
      vision was indeed the comet I was after. Yet someone with a telescope
      smaller than mine easily detects comets a full magnitude fainter than that??

      A question I would ask is, do you ever look for a comet and *not* see it? If
      you're constantly looking for comets near the limit of your telescope, there
      should be quite a few of them that you never see. If you somehow are able to
      "see" every comet you attempt, then I submit that something is wrong. You
      all know how many comets I have visually observed over the years, but you
      may not know that there are over 200 other comets that I have looked for,
      and *not* seen.

      I guess I am reminded of the old story of the "boy who cried 'wolf.'" If you
      consistently report 9th-magnitude comets that aren't there, report comet
      brightnesses that are three magnitudes different from what I'm seeing, and
      report seeing comets with a small telescope that are far too faint for me to
      see with my larger telescope, then you will have to pardon my skepticism
      when it comes to reports of extremely unusual cometary phenomena like what
      we've recently seen with C/2010 X1. Even if you got it right this time . . .

      The enthusiasm for visual comet observations that I am seeing now is
      refreshing to see, and again, I really would like to think that the data we
      obtain is scientifically useful. (It might even help in resolving the
      "visual photometry vs. CCD photometry" issue, although I sometimes think
      that's an unsolvable problem.) But it can only be useful if it's done right.
      I implore everyone here -- veterans included, and that includes me -- to be
      careful and cautious in obtaining these observations, and to see to it that
      we are making the most reliable observations that we can possibly make.


      Sincerely,

      Alan
    • Rodney Austin
      Hear! Hear! I am in total agreement with your comments Alan. Over the years I have tended not to report such magnitudes as I have estimated simply because I
      Message 2 of 24 , Oct 26, 2011
        Hear! Hear! I am in total agreement with your comments Alan. Over the years
        I have tended not to report such magnitudes as I have estimated simply
        because I have found that mostly my estimates are a long way from the 'mean'
        of more experienced magnitude estimators. I have just concentrated on
        looking for fuzzy things that move; preferably new ones.
        Rod Austin


        [Non-text portions of this message have been removed]
      • Alan Hale
        Hi Rodney, ... And I expect you to find some! Especially now that it seems that the Siding Spring Survey is shutting down (unless there s been some very recent
        Message 3 of 24 , Oct 26, 2011
          Hi Rodney,

          > I have just concentrated on looking for fuzzy things that move; preferably
          > new ones.

          And I expect you to find some! Especially now that it seems that the Siding
          Spring Survey is shutting down (unless there's been some very recent news on
          that front that I'm unaware of . . . )


          Sincerely,

          Alan
        • Uwe Pilz
          Alan wrote ... Dear Alan and all other observers, I would like to inform you that we in our German group developed a method for getting visual-like magnitudes
          Message 4 of 24 , Oct 26, 2011
            Alan wrote
            > weobtain is scientifically useful. (It might even help in resolving the
            > "visual photometry vs. CCD photometry" issue, although I
            > sometimes thinkthat's an unsolvable problem.)

            Dear Alan and all other observers,

            I would like to inform you that we in our German group developed a method for getting visual-like magnitudes form CCD observations. What we need to get ar mult aperture measurements. I publish the results together with visual values at our observation site http://kometen.fg-vds.de/obsaktinh.htm .
            A description on how it works can be found starting here http://kometen.fg-vds.de/FGProjekte.html . I prepared a rough English translation of my German lession ( http://kometen.fg-vds.de/Photometrie/kphot_e.pdf ).

            The results fit very fine to the visual values. I expect that not all questions are answered her, but I see it as a good starting point. What I wish to have would be more of the multi aperture measurements. Most of the values came from one observer, Berhnhard Häusler, which may be a constraint of the result we see so far.

            --

            Uwe Pilz, German group.


            .
            .
            .
            .
            .
            .
            .
            .

            Hol Dir das Notizbuch von uni.de: Schreibe dazu einen Artikel zum aktuellen Monatsthema und mit etwas Glück wird dein Artikel veröffentlicht. Alle Infos hier: http://uni.de/redaktion/nachwuchsjournalisten-gesucht
          • biver_nicolas
            Dear Alan and other, Yes, I mostly do follow your comments, Alan. In my case (I also failed to see C/2011 M1 this summer in 8-16 telescopes and C/2010 X1 last
            Message 5 of 24 , Oct 27, 2011
              Dear Alan and other,

              Yes, I mostly do follow your comments, Alan. In my case (I also failed to see C/2011 M1 this summer in 8-16" telescopes and C/2010 X1 last 2 weeks) I also had hard time to see those diffuse objects or get magnitude estimates as bright. But in several cases I know that I did not have ideal skies (i.e. not the darkest or most transparent ones, and light pollution makes things getting worse and worse every time - e.g. from Pico Veleta (2850m elevation) last weeks, light pollution from Granada has significantly degraded the skies over the past 10 years, even from this high point in the mountain, 40km away. I am also getting older, and may not be as senstive as before...

              I have also another concern about a kind of "race for the brightest estimate" of any comets: it may not be true (hopefully) but I feel like there is a trend to look for being the one that will "see" the comet beeing the brightest (I recently observed C/2009 P1 barely brighter than 7.5, but several are reported it brighter than 7). It may be a bias due to method or star catalog selection, and also a trend not so confirmed in full ICQ dataset...

              And last, I wanted to comment on Tail versus Coma for comets: we do what we can visually, but for low phase angles (and generally distant comets) I am sure that sometimes our coma encompasses a large fraction of the tail. In the case of comet 17P/Holmes (which was very special in the sense that it was not generating coma and tail in a classical stationnary regime) I had made simple simulation of particule trajectories (including the radiation pressure) and found out that the slightly parabolic shape shown by the bright (up to 1 degree at some point) envelope we reported as a coma, was in fact a 100-millions km long tail seen face on, and instead of reporting 2.5-3 magnitude m1 we should have reported values closer to m1=10-13 at some point, as seen for the inner small active coma that was visible in telescopes close to the center!

              Nicolas
            • Dave Herald
              Alan – well said! There is another issue to keep in mind with visual observations. It is one thing to compare estimates of total magnitude – as there the
              Message 6 of 24 , Oct 27, 2011
                Alan – well said!

                There is another issue to keep in mind with visual observations. It is one thing to compare estimates of total magnitude – as there the object is clearly visible ‘to all’. But when you get into the area of whether or not a visual observer could actually see an object, you run into issues of credibility that can too easily be treated as personal. The fundamental problem is that an observer’s assertion that they can see an object fails the test of repeatability until others can also see it. That is, until others can see it, the situation reduces to one of whether or not you believe the observer – or (perhaps more correctly) whether you accept the attribution given by the observer to what they saw. In this regard, getting an image of an object is far more satisfactory, as ‘everyone’ can dissect the image in detail and for independent conclusions.

                Net result is (in my opinion) that while visual observations of objects that are ‘generally’ visible are valuable and reliable, visual observations involving the detection of objects that others can’t see are properly subject treated with scepticism – purely on the basis of the lack of independent ‘repeatability’ or other forms of independent verification.


                Dave Herald
                Murrumbateman, Australia


                From: Alan Hale
                Sent: Thursday, October 27, 2011 12:18 PM
                To: comets-ml@yahoogroups.com
                Subject: [comets-ml] C/2010 X1 remnant, and other things
                Hi everyone,

                I apologize in advance for what is almost certainly going to be a lengthy
                post. The recent events, specifically involving C/2010 X1 and observations
                of it, have prompted me to write this, but this also strikes me as a good
                opportunity to comment on issues and trends that have led up to the recent
                events. As a faithful observer of comets for over four decades I'd like to
                think I've earned a right to address these issues.

                First off, of course, there is now no doubt that a faint dust stream remnant
                of C/2010 X1 does indeed continue to exist. I was certainly skeptical of
                this, up until the time I saw Rolando Ligustri's image that he took on the
                22nd; but that image, and the other images that he and other observers have
                taken since then, clearly demonstrate that this dust stream is there. I
                rather agree with John Bortle's comparison to the "head-less" Kreutz
                sungrazer of 1887; one other example that occurred to me (although the
                physical circumstances are obviously different) is the recent P/2010 A2.

                It so happened that Rolando took his October 22 image at the precise time
                that I was making an unsuccessful visual attempt for this comet.
                Accordingly, as soon as I saw the image I noticed that there was nothing at
                the ephemeris position of the comet itself; the closest relatively bright
                portion of the "tail" (which is how I will refer to it from here on) was
                displaced several arcminutes to the west-northwest of the ephemeris
                position. I had concentrated my search efforts on the ephemeris position,
                and thus I should *not* have seen anything when I was making my search. I
                verified this displacement with the image that Rolando took the following
                morning.

                Armed with this knowledge, and also the knowledge of what precisely to look
                for, I made another attempt on the morning of October 24. I did in fact seem
                to see a shapeless but extended "something" in about the expected position,
                and moreover this "something" seemed to move westward at the expected rate.
                This was an extreme borderline observation, however, and was akin to some of
                the very extended diffuse nebulae I've occasionally looked for (the
                supernova remnant S147 in Auriga/Taurus being an example); I could "see"
                something by detecting the very slight differences in the background sky
                brightness between the locations where it "was" and where it "wasn't."

                For what it's worth, I did attempt a brightness measurement by an extreme
                defocusing of stars, and would say somewhere around m1 ~12: I agree with
                John, though, that this is essentially meaningless -- for one thing, there
                is no coma, and that is usually what is meant by a comet's brightness -- and
                at best it refers to a shapeless, ill-defined portion of the tail remnant.

                I had to be out of town for a family emergency on the morning of the 25th
                (and I think it was cloudy here anyway), but I tried again this morning (the
                26th). Again, I seemed to see this extremely faint "something" that moved in
                the right direction at the right rate, and, again, it was an extreme
                borderline observation. At this point I am reasonably sure that I have
                indeed detected this tail remnant of C/2010 X1 on the two mornings in
                question, although I would greatly appreciate it if anyone who was imaging
                the comet on those dates, and especially around the times of my observations
                (roughly 8:30 to 9:30 UT on October 24, 7:50 to 8:50 UT on October 26) could
                either send those images to me, or post them somewhere.


                Now, having reported all this, I would like to comment on some visual
                cometary observations, not only of C/2010 X1 but some other comets, and in
                general. While Juan Gonzalez has been a bit on the "hot seat" with his
                reports of C/2010 X1 -- and I freely admit that I have been among his
                doubters, and I will get to that in a moment -- my comments are directed
                more towards the current community of comet observers in general. I will
                occasionaly use the word "you," but, except in the specific case of the
                recent observations of C/2010 X1, I am not referring to any observer in
                particular.

                Before proceeding, I wish to reiterate something I said in an earlier post,
                i.e., at no time do I mean anything personal about any individual. I
                consider everyone here friends and colleagues, and I have high respect for
                all of you. I am speaking strictly of the scientific observations involved,
                as I would like to believe that the visual brightness (and morphology)
                reports of comets continue to have scientific value. That requires, though,
                that they be done correctly at all times.

                Let's look at Juan's two recent reports of C/2010 X1. The first one, on
                October 9, specifically refers to a "coma;" we now know that there is no
                "coma" for this comet, and anything that might have been visible was not at
                the ephemeris position, and Juan makes no mention of this. Furthermore, he
                remarks that there was zodiacal light present (with which I will concur,
                with my own unsuccessful observation attempt around that same time), and,
                considering the extremely low surface brightness of the "tail remnant," any
                kind of background lighting should have wiped it out completely. (Recall
                that all the CCD images at that time showed nothing, or at best, an
                extremely faint and uncertain fragment that may or may not have been real.)
                And while I accept at face value Juan's comments about the clarity of the
                atmosphere down to the horizon, at an altitude of 23 degrees you are still
                looking through over 2.5 air masses, and you are going to be encountering a
                non-trivial amount of extinction.

                The second observation, on October 21, does seem at face value to be
                reasonably consistent with what the later images showed, although I note
                that he reported one of his "bright areas" to be at the ephemeris position.
                There would have been no zodiacal light at the time of the observation,
                although the reported altitude of 16 degrees works out to over 3.5 air
                masses and thus a significant amount of extinction; if this was at the
                beginning of the observation (Juan doesn't say, but does report that he
                followed his object for 40 minutes) then the extinction would obviously have
                decreased throughout the observation.

                For the record, the altitude at the beginning times of my two recent
                observatons was 44 degrees (1.4 air masses) on both mornings. I was using a
                41 cm Newtonian, with a fairly fast optical system (f/4.5); I again note
                that while I am reasonably certain of my sightings (although perhaps not
                100% certain until/unless I can see some images taken around then), these
                were extreme borderline detections.

                The upshot here is that I must remain very skeptical of the October 9 report
                as, all things considered, it is inconsistent with what the CCD images (both
                then and now) have revealed. The October 21 report is much less clear to me;
                I do have some problems with it, which I've discussed, but on the other hand
                it is consistent enough with the CCD images that I cannot simply dismiss it.
                "Benefit of the doubt" would suggest that I accept it as a positive
                observation, but to be honest, I don't believe I can go either way at this
                time. The best I can do right now is consider it an open issue, which I may
                never be able to resolve to my complete satisfaction either way. (This
                definitely is not the first such "open issue" I have encountered over the
                years . . . )


                But to turn now to the issues which have contributed to the difficulties I
                have had with some of the reported observations (and, reminder, from here on
                "you" is generic):

                In addition to my one comet discovery, during the days before the web and
                the NEOCP I was constantly being called or e-mailed by Dan Green or Brian
                Marsden to confirm various comet discoveries, and over a period of about two
                decades I successfully confirmed numerous such discoveries (as well as
                dis-confirmed numerous other reports). One of the comets I confirmed was as
                faint as m1=13.5. Some of the comets were several degrees away from their
                "expected" locations, and yet I was still able to track them down (and
                perhaps it's worth keeping in mind that we were not even sure these comets
                were real, although we considered some more likely than others).

                While I am not infallible, with this kind of track record it is
                inconceivable to me that I could be looking *directly* at a 9th- or
                10th-magnitude comet, and *not* see it. Yet, if I am to believe some of the
                reports that have been submitted in recent years, that is precisely the
                case. There are several examples I can cite, but I will focus on two recent
                ones.

                We've already discussed C/2011 M1 to an extent. Some of you were
                consistently reporting this comet to be as bright as m1 ~9-10, with a large,
                diffuse coma, and yet, despite numerous search attempts, I never saw it. And
                in this case, the CCD images backed me up:

                With a telescope of any size, a CCD image with an exposure time of only 15
                or 20 seconds will reveal stars fainter than what can be visually detected
                with that telescope. In the case of the C/2011 M1 images, the total exposure
                times were in the range of 16 *minutes* -- and yet, they only revealed an
                extremely faint smudge of light on the verge of complete disintegration that
                was barely detectable.

                Another example that I have not seen discussed here yet is C/2011 A3. Again,
                several of you have reported this comet as being bright at m1 ~9-10 and with
                a large coma, and, again, despite numerous attempts I have failed to see it.
                (Other observers have privately informed me that they, too, have failed to
                see it.) Once again, the CCD images back me up: even when processed with an
                unreasonably high contrast, they show only a small and relatively condensed
                coma that is no brighter than the 15th-magnitude stars in the field (an
                appearance, incidentally, that is entirely consistent with a comet that is
                2.5 to 3 AU from both the earth and the sun).

                These are *not* cases of "visual photometry vs. CCD photometry;" these are
                cases of CCD images showing what is and what isn't there. It is very
                disingenuous to cite the CCD images as positive evidence of the C/2010 X1
                observations, and then turn around and dismiss the CCD images of C/2011 M1
                and C/2011 A3 which reveal that those two comets were much, much fainter
                than the visual reports at the times in question.


                There are two other issues I would like to touch upon. Back when they were
                active, I was constantly cross-checking my observations against those of
                John Bortle and Charles Morris, and I was almost always within a
                half-magnitude or so of their reports. This doesn't necessarily mean that
                any particular individual is "right," and, yes, we all should "call them as
                we see them," but if everyone involved is doing it correctly, we should all
                be seeing something fairly similar to each other. After all, if the depth of
                a lake is 10 meters at a certain point, then all measurements of the depth
                at that point should be somewhere close to 10 meters. Similarly, if a
                comet's "true" brightness is magnitude 10.0, then all measurements should be
                somewhat close to that (say, between magnitude 9.5 and 10.5).

                But I am not seeing that now; I am seeing reported brightnesses that are
                two, three, sometimes more magnitudes different from what I am seeing.
                Again, I am not saying that my measurements are always "right," but I find
                it hard to believe that I've lost my ability to make reasonably accurate
                measurements within the past decade. It's like we're back to those depth
                measurements; if one person is reporting a depth of 2 meters, while someone
                else is reporting a depth at the same point of 28 meters, something is
                wrong. Likewise for our 10th-magnitude comet; if one observer reports it at
                7th magnitude, and another observer at 13th magnitude, something is wrong.

                And then, there is the topic of limiting cometary magnitudes for telescopes
                of various apertures. I have been utilizing the 41 cm telescope for over two
                decades now, and can state with reasonable confidence that its limiting
                magnitude for comets is around 14.5. (I'm referring to "cometary" comets,
                not completely stellar ones, which I can obviously detect fainter.) Over the
                years, observers I know with similar-sized telescopes have reported
                something similar, and those with smaller or larger telescopes have reported
                limiting magnitudes that are consistent with this.

                And yet nowadays, I am seeing some of you, including some of you with
                telescopes significantly smaller than mine, routinely reporting comet
                magnitudes up to a full magnitude -- sometimes even more -- fainter than the
                limit of my telescope. How is this possible? I think I've already shared the
                story of the occasion where, by examining CCD images that I took almost
                simultaneously with a faint comet I was attempting visually, I was able to
                detect a comet at m1=14.8 -- but this took four nights (and two hours each
                night) of this simultaneous visual/CCD work before I could finally convince
                myself that that tiny speck of light that flickered at the extreme limit of
                vision was indeed the comet I was after. Yet someone with a telescope
                smaller than mine easily detects comets a full magnitude fainter than that??

                A question I would ask is, do you ever look for a comet and *not* see it? If
                you're constantly looking for comets near the limit of your telescope, there
                should be quite a few of them that you never see. If you somehow are able to
                "see" every comet you attempt, then I submit that something is wrong. You
                all know how many comets I have visually observed over the years, but you
                may not know that there are over 200 other comets that I have looked for,
                and *not* seen.

                I guess I am reminded of the old story of the "boy who cried 'wolf.'" If you
                consistently report 9th-magnitude comets that aren't there, report comet
                brightnesses that are three magnitudes different from what I'm seeing, and
                report seeing comets with a small telescope that are far too faint for me to
                see with my larger telescope, then you will have to pardon my skepticism
                when it comes to reports of extremely unusual cometary phenomena like what
                we've recently seen with C/2010 X1. Even if you got it right this time . . .

                The enthusiasm for visual comet observations that I am seeing now is
                refreshing to see, and again, I really would like to think that the data we
                obtain is scientifically useful. (It might even help in resolving the
                "visual photometry vs. CCD photometry" issue, although I sometimes think
                that's an unsolvable problem.) But it can only be useful if it's done right.
                I implore everyone here -- veterans included, and that includes me -- to be
                careful and cautious in obtaining these observations, and to see to it that
                we are making the most reliable observations that we can possibly make.


                Sincerely,

                Alan



                ------------------------------------

                Comet Observations List: http://groups.yahoo.com/group/CometObs/
                Comet Images List: http://groups.yahoo.com/group/Comet-Images/

                NOTICE: Material quoted or re-posted from the Comets Mailing List should be indicated by:

                Comets Mailing List [date]
                http://www.yahoogroups.com/group/comets-ml
                Yahoo! Groups Links




                [Non-text portions of this message have been removed]
              • Mike Begbie
                Wow! What a brilliant post, Alan! Although I do not discount Gonzales’ observations...I still think that he did amazing work, and he really did see what was
                Message 7 of 24 , Oct 27, 2011
                  Wow! What a brilliant post, Alan!

                  Although I do not discount Gonzales’
                  observations...I still think that he did amazing
                  work, and he really did see what was there...I’m
                  sort of worried about the definition of a m1
                  estimate, and a “coma”. What are we recording
                  here? A real comet as defined many years ago...or
                  a dust cloud where the comet is dead? Where do we
                  draw the line? I’m absolutely behind John Bortle’s
                  last post...the definition of a comet.

                  I am sort of thinking we need to re-evaluate this
                  group, sorry Maik, no insult intended...But maybe
                  we need to set up a separate group dealing just
                  with CCD obs. and just visual stuff. Keep comets
                  ml as a discussion group only, and divide cometObs
                  into visual and electronic....separate groups.
                  Just mulling over this.... don’t really know where
                  I am going with this...do know that I would would
                  wish to see Hale’s post today and Bortle, as the
                  sort of post I like to absorb.

                  Best

                  Mike


                  [Non-text portions of this message have been removed]
                • RICHARD MILES
                  Dave, Alan, John, Juan Jose, and others with an interest, Comparing and contrasting human vision and area detectors such as CCD cameras is a fascinating
                  Message 8 of 24 , Oct 27, 2011
                    Dave, Alan, John, Juan Jose, and others with an interest,

                    Comparing and contrasting human vision and area detectors such as CCD
                    cameras is a fascinating topic. Between the 1930s and the 1970s tremendous
                    advances were made into the study of vision, and the history of the
                    development of our understanding of the mechanism of human vision is a
                    fascinating story in its own right. Many of the findings have direct
                    relevance to the subject of visual observation of comets and back in the
                    early 1990s I gave a talk at the BAA in London on "The Eye and The
                    Astronomer" when I alluded to the topic. It would make a good subject for a
                    written article if not an entire book!

                    Anyway, back to the pertinent point, it's this. People these days are so
                    used to using cameras and the like that they fall into the trap of thinking
                    the eye operates on the same principle as a camera or CCD. The truth is
                    that in many of its functions IT DOES NOT. Indeed it is the very mechanism
                    by which the eye works that makes it so valuable when estimating the
                    appearance and brightness of diffuse extended objects such as comets. It's
                    an extension of Ricco's Law and the fact that the brain is able to sum the
                    incident stimuli (both in time and apparent area), especially with averted
                    vision, that is relevant here. Amazingly, if you use averted vision in the
                    range 15-40 degrees from the fovea (point of highest acuity) then the eye
                    has the ability to integrate ALL of the light falling over a region some
                    100' in diameter (Ref. 1). [Importantly, spatial summation continues for
                    even larger areas but at the expense of gradually falling efficiency.]
                    That's pretty amazing when you think about it:- so long as the light from an
                    extended object falls within about a 2-degree apparent field then the
                    eye/brain is able to distinguish it as being real and its sensitivity is the
                    same as if the same amount of light fell within a much smaller circle, say
                    10x less in size (but not if it were contained within a point source, I
                    should add).

                    A CCD camera is a difficult tool to use for measuring the integrated
                    intensity of an extended coma because as you use increasingly larger
                    measuring apertures, three effects work against you; (i) you add more noise,
                    (ii) larger areas contain increasing numbers of background stars, and (iii)
                    the same error in the measured background intensity leads to an increase in
                    the systematic errors in the final result. A well-trained visual observer
                    can avoid these pitfalls of the CCD! (btw: The recent work of Sostero, Guido
                    and Howes on C/2010 X1 has been useful re. image background subtraction
                    techniques for amateurs.)

                    Advanced amateurs working visually have honed their observing techniques to
                    be near optimal. However, someone new to the subject could elucidate the
                    optimal technique by merely going to the physiology literature to work out
                    how to do this. Here's how I'd sum up working at the optimal limit for
                    seeing a very extended coma from a physiological basis:

                    (a) The telescope focal length and magnification should be such that the
                    apparent size of the coma in the eyepiece is kept as small as possible,
                    preferably no larger than about 2 degrees in size.
                    (b) Vision should be averted by about 30 deg from the fovea.
                    (c) The exit pupil at the eyepiece should be 3-5 mm in diameter.
                    (d) Scattered light should be reduced to a minimum to minimise the intensity
                    of the background sky.

                    The subject of the apparent intensity of the background sky is also crucial
                    in this debate and much work has been done on this subject in physiology
                    labs. So the choice of eyepice design is relevant, so is the screening of
                    stray light entering the telescope, as well as any light pollution present.

                    So if we take a real example, what would be the ideal scope to view the
                    remnant cloud of C/2010 X1? If we consider that the cloud has a minor axial
                    dimension of 4' (from the Oct 23 image of Sostero et al.) then the ideal
                    magnification to use would be 30x (i.e. 2deg/4'). So to fit that through a
                    5mm pupil, the entrance aperture would need to be 150mm across. So a
                    30x150mm instrument would be close to the ideal setup. In practice, sky
                    brightness is always a limiting factor (compared with the pitch dark
                    situation in a laboratory) so increasing the magnification to 35-40x should
                    be additionally beneficial since this darkens the background a little.

                    We can now see that JJGS's setup of 77x200mm was not so far from the ideal,
                    which would have been say 40x150mm. However, using a 400mm scope would push
                    the apparent size subtended by the coma well past the 2-degree criterion.

                    Many comet observers have a range of scopes / binoculars of various size and
                    so the key is to select the instrument which is best suited to each target.

                    Richard


                    Ref.1
                    Psychophysical experiments on spatial summation at threshold level of the
                    human peripheral retina.
                    Vision Research, Vol. 17, Issue 7, Pages 867-873 (1977)
                    A.M.W. Scholtes, M.A. Bouman


                    For anyone interested, here's another earlier paper which looked at spatial
                    summation by the eye:

                    Temporal and spatial summation in human vision at different background
                    intensities
                    Journal of Physiology, Vol. 141, Issue 2, Pages 337–350 (1958)
                    H. B. Barlow
                    Available at: http://jp.physoc.org/content/141/2/337.full.pdf

                    (Comment - In this earlier work, the stimulus was situated a bit too close
                    to the fovea (6.5 deg away) for best performance of the eye, i.e. not
                    sufficiently averted.)

                    Also, anyone reading the literature should avoid studies of scotopic
                    (colour) vision: as astronomers we are usually only concerned with photopic
                    vision of the dark-adapted eye.
                  • Jakub Černý
                    Hello Alan, Nice post that brigs a lot of issues to discus. 1) Statement that the JJ s observations from 21.10. is real but the 9.10. one is not. For me it is
                    Message 9 of 24 , Oct 27, 2011
                      Hello Alan,



                      Nice post that brigs a lot of issues to discus.



                      1) Statement that the JJ's observations from 21.10. is real but the 9.10.
                      one is not. For me it is pretty hard to believe that someone would construct
                      a dust cloud with "tail" extending in p.a. which was later been confirmed on
                      CCDs without knowin that. If that observation was not real then JJ must have
                      really luck in imagination ;-). I also think that while the comet was on
                      9.10. more distant, the difference between could and comet nucleus expected
                      position should be smaller!



                      We should not forget that the recovery of dust cloud using CCD was made
                      after big effort inspired by JJ's visual observations. TO me it looks
                      simply, that it was not recovered on 9.10. due people did not even tried to
                      find it with small instruments.



                      2) The discussion about if we are observing coma or tail seems to be
                      pointless to me. Definitively we are observing cloud of particles what are
                      most probable dust one. But what is exact definition of particle what is
                      part of "coma" and the "tail"? It seems to me similar to past discussion if
                      Pluto is the planet or no, so it is just nomenclature. The object itself
                      does not change because we will call that with other name.



                      If the visual photometry of this, lets call it "object", is usefull or not
                      is another part of question. For creating light curve analysis comparing to
                      other comet, there is no way to mix it with theese observations. However it
                      may still helps to some theory. For comet disruptions final phase we can
                      expect that nucleus will break up into small pieces and then the gas
                      production will stop. We can expect that some small pieces (20-100 m) can
                      continue supply coma with some small amount of gas but not for long. The
                      result is cloud of dust and its photometry will depends on two functions,
                      first is same as surface of asteroids (reflected sunlight) and the second
                      will be the speed of cloud dispersion. The total brightness would be mix of
                      thoose two functions so we should recieve photoemtric parameters for it with
                      n = >2 . While one part of it is constant, the second one - dispersion -
                      depends on dust ejection speed.



                      So the photometry of dust debris is still usefull, just it will be extremely
                      inaccurate.



                      I can also show you another example of practical use of photoemtry dust
                      debris. I used photometry of C/1999 S4 dust debris cloud to calculate
                      expected total brightness of C/2010 X1 debris cloud (see image of C/1999 S4
                      debris cloud on http://www.ing.iac.es/PR/AR2000/linear.jpg ). Result was
                      that the C/2010 X1 debris cloud should have brightness between 11.6 - 12.1
                      mag (16. to 21. October). That is in very good agreement with actual visual
                      observations thanks to visual observations from past!



                      3)rd issue does not have anything with C/2010 X1 but for me it is most
                      important. The case of "bright" visual comets C/2011 A3 and C/2011 M1.
                      Honestly, always when I recieve a visual observations from J.J. then I have
                      on my mind something like "oh my god, he went crazy" with thoose magnitudes.
                      Somehow, JJ's magnitudes are always something like 2 mag brighter then other
                      observers. The reason is unknown, it can be anything started with bad
                      methodology to some specifications of JJ's sight or conditions.



                      In case of C/2011 A3 i started to observe this comet while it was 15.8 and
                      almost stelar. That day I was bored with time at telescope, so I have tried
                      something "funny", and it surprised me that the comet was visible, thanks to
                      its stellar apperance. Due that I had no usefull starts magnitude down to 16
                      mag I asked friend on Ondrejov 0.65-m to calibrate the fields for stars V
                      mags and I have confirmed that I cas clearly see stars, close to comet, down
                      to 16.2(+/-0.02) mag. From that time I continued to follow this comet, but
                      long time had no sign of anything different from starlike condensation
                      untill I see first JJ's observation of outter coma. That time I first tried
                      comet with smaller magnification in order to check if there is not anything
                      else then the starlike center. I was very surprised that I can see some very
                      faint outside coma in it, which I never noticed before. Once I even tried it
                      with 25x100 because I was expecting in so big field such a small coma
                      visible. But nothing was seen down to 11.3 mag and I never had positive
                      observations with any other instrument then 0.35-m dobson.



                      The other comet - C/2011 M1 seems to me very similar to comet from past -
                      177P/Barnard ! Extremely large, diffuse coma with low surface brightness and
                      almost no sign of central condensation.



                      Alan you speaking about that you never had more then 0.5 mag difference to
                      other observers, but I know al ot of historical issues when there was
                      terrible differences in comet magnitudes! Always it was caused by large,
                      faint diffuse coma.



                      Very interesting is the 177P case.

                      The multiaperture CCD total magnitudes gives for different apertures
                      magnitudes from 15.3 to 11.4 mag while my visual estimate give brightness
                      9.7 mag.

                      177 2006 09 25.79 dk 11.4 LB 14.5L 8a800 3 ICQ XX
                      SRB a 10C 5.25mST7 K40 GAI 5 9* 0.3 7.3 3.0s 3.0 Jiri Srba
                      177 2006 09 25.79 dk 12.5 LB 14.5L 8a800 3 ICQ XX
                      SRB a 10C 1.6 mST7 K40 GAI 5 9* 0.1 7.3 3.0s 3.0 Jiri Srba
                      177 2006 09 25.79 dk 15.3 LB 14.5L 8a800 3 ICQ XX
                      SRB a 10C 0.2 mST7 K40 GAI 5 9* 0.1 7.3 3.0s 3.0 Jiri Srba

                      177 2006 09 25.07 M 9.7 TI 20.0L 6 80 7 1 ICQ XX
                      CER01 Jakub Cerny



                      The total magnitude does not says anything about comet visibility !!!



                      If one measure 11 mag comet with 6' coma diameter, and you are looking for
                      1' object in field of view, then you can easily miss it.



                      We also need to inspect the comet DC, which is in math language function
                      which tells you how the surfacebrightness decreasing with distance from
                      center. In case of DC 1, the function is not important and surface
                      brightness is almost constant.



                      m1 = surface brightness * surface



                      Lets do the math, Juan Jose Gonzales reported C/2011 A3 reported 24.82
                      september magnitude 9.7 for 6' coma. We know that JJ's magnitudes are 2 mag
                      higher then from other observers, so we use this correction and gets the
                      11.7 mag comet. The formula for surface brightness S is m +
                      2.5*log(Diameter/2*60), that gives 23.7 mag per square arcsecond. So now we
                      can calculate magnitudes for expected object of diameter x:



                      1': 14.7 mag

                      2': 13.7 mag

                      3': 12.7 mag



                      Well 14.7 mag object would be hard even with your instrument in good
                      conditions, even the outer coma lower the contrast and if you are looking
                      for a object to focus then you can be easily lost with this kind of comet.



                      But there is DARK SIDE of this issue. And maybe it is most important of what
                      i want to say. (I hope someone readed my message until this)



                      As you correctly wrote, the question, if the observers of this even did some
                      negative observation is on the place. Issues when many visual observers
                      (even skilled) reported a comet that did not exist happened already. Many of
                      those reports of weak comets can always be mixed with false detections. You
                      can only be sure when you are handling with observations of (well condensed)
                      comets 2-3 mag up to star limit magnitude! Rare estimating of weaker comets
                      can be done, but if observer that do that too offten you are flooding the
                      database with datas which can contain high value of false detects.



                      I thing it is pitty that ICQ database accepting very poor data nowdays and
                      only observers on discussions like Alan Hale are saying something L.



                      Best regards,

                      Jakub Cerny



                      From: comets-ml@yahoogroups.com [mailto:comets-ml@yahoogroups.com] On Behalf
                      Of Alan Hale
                      Sent: Thursday, October 27, 2011 3:19 AM
                      To: comets-ml@yahoogroups.com
                      Subject: [comets-ml] C/2010 X1 remnant, and other things





                      Hi everyone,

                      I apologize in advance for what is almost certainly going to be a lengthy
                      post. The recent events, specifically involving C/2010 X1 and observations
                      of it, have prompted me to write this, but this also strikes me as a good
                      opportunity to comment on issues and trends that have led up to the recent
                      events. As a faithful observer of comets for over four decades I'd like to
                      think I've earned a right to address these issues.

                      First off, of course, there is now no doubt that a faint dust stream remnant
                      of C/2010 X1 does indeed continue to exist. I was certainly skeptical of
                      this, up until the time I saw Rolando Ligustri's image that he took on the
                      22nd; but that image, and the other images that he and other observers have
                      taken since then, clearly demonstrate that this dust stream is there. I
                      rather agree with John Bortle's comparison to the "head-less" Kreutz
                      sungrazer of 1887; one other example that occurred to me (although the
                      physical circumstances are obviously different) is the recent P/2010 A2.

                      It so happened that Rolando took his October 22 image at the precise time
                      that I was making an unsuccessful visual attempt for this comet.
                      Accordingly, as soon as I saw the image I noticed that there was nothing at
                      the ephemeris position of the comet itself; the closest relatively bright
                      portion of the "tail" (which is how I will refer to it from here on) was
                      displaced several arcminutes to the west-northwest of the ephemeris
                      position. I had concentrated my search efforts on the ephemeris position,
                      and thus I should *not* have seen anything when I was making my search. I
                      verified this displacement with the image that Rolando took the following
                      morning.

                      Armed with this knowledge, and also the knowledge of what precisely to look
                      for, I made another attempt on the morning of October 24. I did in fact seem
                      to see a shapeless but extended "something" in about the expected position,
                      and moreover this "something" seemed to move westward at the expected rate.
                      This was an extreme borderline observation, however, and was akin to some of
                      the very extended diffuse nebulae I've occasionally looked for (the
                      supernova remnant S147 in Auriga/Taurus being an example); I could "see"
                      something by detecting the very slight differences in the background sky
                      brightness between the locations where it "was" and where it "wasn't."

                      For what it's worth, I did attempt a brightness measurement by an extreme
                      defocusing of stars, and would say somewhere around m1 ~12: I agree with
                      John, though, that this is essentially meaningless -- for one thing, there
                      is no coma, and that is usually what is meant by a comet's brightness -- and
                      at best it refers to a shapeless, ill-defined portion of the tail remnant.

                      I had to be out of town for a family emergency on the morning of the 25th
                      (and I think it was cloudy here anyway), but I tried again this morning (the
                      26th). Again, I seemed to see this extremely faint "something" that moved in
                      the right direction at the right rate, and, again, it was an extreme
                      borderline observation. At this point I am reasonably sure that I have
                      indeed detected this tail remnant of C/2010 X1 on the two mornings in
                      question, although I would greatly appreciate it if anyone who was imaging
                      the comet on those dates, and especially around the times of my observations
                      (roughly 8:30 to 9:30 UT on October 24, 7:50 to 8:50 UT on October 26) could
                      either send those images to me, or post them somewhere.

                      Now, having reported all this, I would like to comment on some visual
                      cometary observations, not only of C/2010 X1 but some other comets, and in
                      general. While Juan Gonzalez has been a bit on the "hot seat" with his
                      reports of C/2010 X1 -- and I freely admit that I have been among his
                      doubters, and I will get to that in a moment -- my comments are directed
                      more towards the current community of comet observers in general. I will
                      occasionaly use the word "you," but, except in the specific case of the
                      recent observations of C/2010 X1, I am not referring to any observer in
                      particular.

                      Before proceeding, I wish to reiterate something I said in an earlier post,
                      i.e., at no time do I mean anything personal about any individual. I
                      consider everyone here friends and colleagues, and I have high respect for
                      all of you. I am speaking strictly of the scientific observations involved,
                      as I would like to believe that the visual brightness (and morphology)
                      reports of comets continue to have scientific value. That requires, though,
                      that they be done correctly at all times.

                      Let's look at Juan's two recent reports of C/2010 X1. The first one, on
                      October 9, specifically refers to a "coma;" we now know that there is no
                      "coma" for this comet, and anything that might have been visible was not at
                      the ephemeris position, and Juan makes no mention of this. Furthermore, he
                      remarks that there was zodiacal light present (with which I will concur,
                      with my own unsuccessful observation attempt around that same time), and,
                      considering the extremely low surface brightness of the "tail remnant," any
                      kind of background lighting should have wiped it out completely. (Recall
                      that all the CCD images at that time showed nothing, or at best, an
                      extremely faint and uncertain fragment that may or may not have been real.)
                      And while I accept at face value Juan's comments about the clarity of the
                      atmosphere down to the horizon, at an altitude of 23 degrees you are still
                      looking through over 2.5 air masses, and you are going to be encountering a
                      non-trivial amount of extinction.

                      The second observation, on October 21, does seem at face value to be
                      reasonably consistent with what the later images showed, although I note
                      that he reported one of his "bright areas" to be at the ephemeris position.
                      There would have been no zodiacal light at the time of the observation,
                      although the reported altitude of 16 degrees works out to over 3.5 air
                      masses and thus a significant amount of extinction; if this was at the
                      beginning of the observation (Juan doesn't say, but does report that he
                      followed his object for 40 minutes) then the extinction would obviously have
                      decreased throughout the observation.

                      For the record, the altitude at the beginning times of my two recent
                      observatons was 44 degrees (1.4 air masses) on both mornings. I was using a
                      41 cm Newtonian, with a fairly fast optical system (f/4.5); I again note
                      that while I am reasonably certain of my sightings (although perhaps not
                      100% certain until/unless I can see some images taken around then), these
                      were extreme borderline detections.

                      The upshot here is that I must remain very skeptical of the October 9 report
                      as, all things considered, it is inconsistent with what the CCD images (both
                      then and now) have revealed. The October 21 report is much less clear to me;
                      I do have some problems with it, which I've discussed, but on the other hand
                      it is consistent enough with the CCD images that I cannot simply dismiss it.
                      "Benefit of the doubt" would suggest that I accept it as a positive
                      observation, but to be honest, I don't believe I can go either way at this
                      time. The best I can do right now is consider it an open issue, which I may
                      never be able to resolve to my complete satisfaction either way. (This
                      definitely is not the first such "open issue" I have encountered over the
                      years . . . )

                      But to turn now to the issues which have contributed to the difficulties I
                      have had with some of the reported observations (and, reminder, from here on
                      "you" is generic):

                      In addition to my one comet discovery, during the days before the web and
                      the NEOCP I was constantly being called or e-mailed by Dan Green or Brian
                      Marsden to confirm various comet discoveries, and over a period of about two
                      decades I successfully confirmed numerous such discoveries (as well as
                      dis-confirmed numerous other reports). One of the comets I confirmed was as
                      faint as m1=13.5. Some of the comets were several degrees away from their
                      "expected" locations, and yet I was still able to track them down (and
                      perhaps it's worth keeping in mind that we were not even sure these comets
                      were real, although we considered some more likely than others).

                      While I am not infallible, with this kind of track record it is
                      inconceivable to me that I could be looking *directly* at a 9th- or
                      10th-magnitude comet, and *not* see it. Yet, if I am to believe some of the
                      reports that have been submitted in recent years, that is precisely the
                      case. There are several examples I can cite, but I will focus on two recent
                      ones.

                      We've already discussed C/2011 M1 to an extent. Some of you were
                      consistently reporting this comet to be as bright as m1 ~9-10, with a large,
                      diffuse coma, and yet, despite numerous search attempts, I never saw it. And
                      in this case, the CCD images backed me up:

                      With a telescope of any size, a CCD image with an exposure time of only 15
                      or 20 seconds will reveal stars fainter than what can be visually detected
                      with that telescope. In the case of the C/2011 M1 images, the total exposure
                      times were in the range of 16 *minutes* -- and yet, they only revealed an
                      extremely faint smudge of light on the verge of complete disintegration that
                      was barely detectable.

                      Another example that I have not seen discussed here yet is C/2011 A3. Again,
                      several of you have reported this comet as being bright at m1 ~9-10 and with
                      a large coma, and, again, despite numerous attempts I have failed to see it.
                      (Other observers have privately informed me that they, too, have failed to
                      see it.) Once again, the CCD images back me up: even when processed with an
                      unreasonably high contrast, they show only a small and relatively condensed
                      coma that is no brighter than the 15th-magnitude stars in the field (an
                      appearance, incidentally, that is entirely consistent with a comet that is
                      2.5 to 3 AU from both the earth and the sun).

                      These are *not* cases of "visual photometry vs. CCD photometry;" these are
                      cases of CCD images showing what is and what isn't there. It is very
                      disingenuous to cite the CCD images as positive evidence of the C/2010 X1
                      observations, and then turn around and dismiss the CCD images of C/2011 M1
                      and C/2011 A3 which reveal that those two comets were much, much fainter
                      than the visual reports at the times in question.

                      There are two other issues I would like to touch upon. Back when they were
                      active, I was constantly cross-checking my observations against those of
                      John Bortle and Charles Morris, and I was almost always within a
                      half-magnitude or so of their reports. This doesn't necessarily mean that
                      any particular individual is "right," and, yes, we all should "call them as
                      we see them," but if everyone involved is doing it correctly, we should all
                      be seeing something fairly similar to each other. After all, if the depth of
                      a lake is 10 meters at a certain point, then all measurements of the depth
                      at that point should be somewhere close to 10 meters. Similarly, if a
                      comet's "true" brightness is magnitude 10.0, then all measurements should be
                      somewhat close to that (say, between magnitude 9.5 and 10.5).

                      But I am not seeing that now; I am seeing reported brightnesses that are
                      two, three, sometimes more magnitudes different from what I am seeing.
                      Again, I am not saying that my measurements are always "right," but I find
                      it hard to believe that I've lost my ability to make reasonably accurate
                      measurements within the past decade. It's like we're back to those depth
                      measurements; if one person is reporting a depth of 2 meters, while someone
                      else is reporting a depth at the same point of 28 meters, something is
                      wrong. Likewise for our 10th-magnitude comet; if one observer reports it at
                      7th magnitude, and another observer at 13th magnitude, something is wrong.

                      And then, there is the topic of limiting cometary magnitudes for telescopes
                      of various apertures. I have been utilizing the 41 cm telescope for over two
                      decades now, and can state with reasonable confidence that its limiting
                      magnitude for comets is around 14.5. (I'm referring to "cometary" comets,
                      not completely stellar ones, which I can obviously detect fainter.) Over the
                      years, observers I know with similar-sized telescopes have reported
                      something similar, and those with smaller or larger telescopes have reported
                      limiting magnitudes that are consistent with this.

                      And yet nowadays, I am seeing some of you, including some of you with
                      telescopes significantly smaller than mine, routinely reporting comet
                      magnitudes up to a full magnitude -- sometimes even more -- fainter than the
                      limit of my telescope. How is this possible? I think I've already shared the
                      story of the occasion where, by examining CCD images that I took almost
                      simultaneously with a faint comet I was attempting visually, I was able to
                      detect a comet at m1=14.8 -- but this took four nights (and two hours each
                      night) of this simultaneous visual/CCD work before I could finally convince
                      myself that that tiny speck of light that flickered at the extreme limit of
                      vision was indeed the comet I was after. Yet someone with a telescope
                      smaller than mine easily detects comets a full magnitude fainter than that??

                      A question I would ask is, do you ever look for a comet and *not* see it? If
                      you're constantly looking for comets near the limit of your telescope, there
                      should be quite a few of them that you never see. If you somehow are able to
                      "see" every comet you attempt, then I submit that something is wrong. You
                      all know how many comets I have visually observed over the years, but you
                      may not know that there are over 200 other comets that I have looked for,
                      and *not* seen.

                      I guess I am reminded of the old story of the "boy who cried 'wolf.'" If you
                      consistently report 9th-magnitude comets that aren't there, report comet
                      brightnesses that are three magnitudes different from what I'm seeing, and
                      report seeing comets with a small telescope that are far too faint for me to
                      see with my larger telescope, then you will have to pardon my skepticism
                      when it comes to reports of extremely unusual cometary phenomena like what
                      we've recently seen with C/2010 X1. Even if you got it right this time . . .

                      The enthusiasm for visual comet observations that I am seeing now is
                      refreshing to see, and again, I really would like to think that the data we
                      obtain is scientifically useful. (It might even help in resolving the
                      "visual photometry vs. CCD photometry" issue, although I sometimes think
                      that's an unsolvable problem.) But it can only be useful if it's done right.
                      I implore everyone here -- veterans included, and that includes me -- to be
                      careful and cautious in obtaining these observations, and to see to it that
                      we are making the most reliable observations that we can possibly make.

                      Sincerely,

                      Alan





                      [Non-text portions of this message have been removed]
                    • Jakub Černý
                      Hello Richard, This was an excelent reading and i very enjoyed that! Thank you, Jakub Cerny From: comets-ml@yahoogroups.com [mailto:comets-ml@yahoogroups.com]
                      Message 10 of 24 , Oct 27, 2011
                        Hello Richard,



                        This was an excelent reading and i very enjoyed that!



                        Thank you,

                        Jakub Cerny



                        From: comets-ml@yahoogroups.com [mailto:comets-ml@yahoogroups.com] On Behalf Of RICHARD MILES
                        Sent: Thursday, October 27, 2011 9:13 PM
                        To: comets-ml@yahoogroups.com
                        Subject: [comets-ml] Vision and the remnant of C/2010 X1





                        Dave, Alan, John, Juan Jose, and others with an interest,

                        Comparing and contrasting human vision and area detectors such as CCD
                        cameras is a fascinating topic. Between the 1930s and the 1970s tremendous
                        advances were made into the study of vision, and the history of the
                        development of our understanding of the mechanism of human vision is a
                        fascinating story in its own right. Many of the findings have direct
                        relevance to the subject of visual observation of comets and back in the
                        early 1990s I gave a talk at the BAA in London on "The Eye and The
                        Astronomer" when I alluded to the topic. It would make a good subject for a
                        written article if not an entire book!

                        Anyway, back to the pertinent point, it's this. People these days are so
                        used to using cameras and the like that they fall into the trap of thinking
                        the eye operates on the same principle as a camera or CCD. The truth is
                        that in many of its functions IT DOES NOT. Indeed it is the very mechanism
                        by which the eye works that makes it so valuable when estimating the
                        appearance and brightness of diffuse extended objects such as comets. It's
                        an extension of Ricco's Law and the fact that the brain is able to sum the
                        incident stimuli (both in time and apparent area), especially with averted
                        vision, that is relevant here. Amazingly, if you use averted vision in the
                        range 15-40 degrees from the fovea (point of highest acuity) then the eye
                        has the ability to integrate ALL of the light falling over a region some
                        100' in diameter (Ref. 1). [Importantly, spatial summation continues for
                        even larger areas but at the expense of gradually falling efficiency.]
                        That's pretty amazing when you think about it:- so long as the light from an
                        extended object falls within about a 2-degree apparent field then the
                        eye/brain is able to distinguish it as being real and its sensitivity is the
                        same as if the same amount of light fell within a much smaller circle, say
                        10x less in size (but not if it were contained within a point source, I
                        should add).

                        A CCD camera is a difficult tool to use for measuring the integrated
                        intensity of an extended coma because as you use increasingly larger
                        measuring apertures, three effects work against you; (i) you add more noise,
                        (ii) larger areas contain increasing numbers of background stars, and (iii)
                        the same error in the measured background intensity leads to an increase in
                        the systematic errors in the final result. A well-trained visual observer
                        can avoid these pitfalls of the CCD! (btw: The recent work of Sostero, Guido
                        and Howes on C/2010 X1 has been useful re. image background subtraction
                        techniques for amateurs.)

                        Advanced amateurs working visually have honed their observing techniques to
                        be near optimal. However, someone new to the subject could elucidate the
                        optimal technique by merely going to the physiology literature to work out
                        how to do this. Here's how I'd sum up working at the optimal limit for
                        seeing a very extended coma from a physiological basis:

                        (a) The telescope focal length and magnification should be such that the
                        apparent size of the coma in the eyepiece is kept as small as possible,
                        preferably no larger than about 2 degrees in size.
                        (b) Vision should be averted by about 30 deg from the fovea.
                        (c) The exit pupil at the eyepiece should be 3-5 mm in diameter.
                        (d) Scattered light should be reduced to a minimum to minimise the intensity
                        of the background sky.

                        The subject of the apparent intensity of the background sky is also crucial
                        in this debate and much work has been done on this subject in physiology
                        labs. So the choice of eyepice design is relevant, so is the screening of
                        stray light entering the telescope, as well as any light pollution present.

                        So if we take a real example, what would be the ideal scope to view the
                        remnant cloud of C/2010 X1? If we consider that the cloud has a minor axial
                        dimension of 4' (from the Oct 23 image of Sostero et al.) then the ideal
                        magnification to use would be 30x (i.e. 2deg/4'). So to fit that through a
                        5mm pupil, the entrance aperture would need to be 150mm across. So a
                        30x150mm instrument would be close to the ideal setup. In practice, sky
                        brightness is always a limiting factor (compared with the pitch dark
                        situation in a laboratory) so increasing the magnification to 35-40x should
                        be additionally beneficial since this darkens the background a little.

                        We can now see that JJGS's setup of 77x200mm was not so far from the ideal,
                        which would have been say 40x150mm. However, using a 400mm scope would push
                        the apparent size subtended by the coma well past the 2-degree criterion.

                        Many comet observers have a range of scopes / binoculars of various size and
                        so the key is to select the instrument which is best suited to each target.

                        Richard

                        Ref.1
                        Psychophysical experiments on spatial summation at threshold level of the
                        human peripheral retina.
                        Vision Research, Vol. 17, Issue 7, Pages 867-873 (1977)
                        A.M.W. Scholtes, M.A. Bouman

                        For anyone interested, here's another earlier paper which looked at spatial
                        summation by the eye:

                        Temporal and spatial summation in human vision at different background
                        intensities
                        Journal of Physiology, Vol. 141, Issue 2, Pages 337–350 (1958)
                        H. B. Barlow
                        Available at: http://jp.physoc.org/content/141/2/337.full.pdf

                        (Comment - In this earlier work, the stimulus was situated a bit too close
                        to the fovea (6.5 deg away) for best performance of the eye, i.e. not
                        sufficiently averted.)

                        Also, anyone reading the literature should avoid studies of scotopic
                        (colour) vision: as astronomers we are usually only concerned with photopic
                        vision of the dark-adapted eye.





                        [Non-text portions of this message have been removed]
                      • Alan Hale
                        Dear Jakub, While there are several items in your message that are perhaps worth responding to, and I may at some point, there is one I want to address right
                        Message 11 of 24 , Oct 27, 2011
                          Dear Jakub,

                          While there are several items in your message that are perhaps worth
                          responding to, and I may at some point, there is one I want to address right
                          now:

                          >
                          > The total magnitude does not says anything about comet visibility !!!

                          Yes, it does. A 9th magnitude comet is easily detectable with my 41 cm
                          telescope. A 15th magnitude comet is not.

                          >
                          > If one measure 11 mag comet with 6' coma diameter, and you are looking for
                          > 1' object in field of view, then you can easily miss it.
                          >

                          This is absurd.

                          When I look for comets, I do *not* look for comets of a specific size; I
                          look for the comet -- period. If the comet is bright enough to see in my
                          telescope, I will see it. An 11th-magnitude comet with a 6' coma is *easy*
                          is my telescope, and there is no way I can miss it. (I swept up comets of
                          that description during my comet-hunting days.)

                          With C/2011 M1, it was only after numerous unsuccessful attempts that I saw
                          the CCD images in question, which revealed the comet to be an extremely
                          faint and barely detectable smudge of light. Likewise, with C/2011 A3 it was
                          only after numerous unsuccessful attempts that I saw the CCD images, which
                          revealed it to be a small and relatively condensed object (even when
                          processed past the point of anything reasonable). To assert in either case
                          that there was a large diffuse coma is to assert that something existed
                          which the CCD images showed did not exist.

                          You cited the case of 177P. I measured a peak brightness of m1=9.2 for that
                          comet, in 10x50 binoculars. The comet was large, and diffuse -- and
                          extremely easy to see in both the 41 cm scope and the 10x50s.


                          Sincerely,

                          Alan
                        • Jakub Černý
                          Dear Alan, You claimed that there are CCD images proving that theese comets had no faint outter coma. I can t continue in this discussion because this simply
                          Message 12 of 24 , Oct 28, 2011
                            Dear Alan,



                            You claimed that there are CCD images proving that theese comets had no
                            faint outter coma. I can't continue in this discussion because this simply
                            evidence is missing to me.



                            1. I can not see any proper CCD photometry from the period when JJ reported
                            theese comets 9 - 10 mag

                            2. Therefore I can not find any multiaperture photometry, which can tell us
                            about its surface brightness.

                            3. Magnitude and diameters reported in case of theese comets are pretty
                            similar to last case of C/2010 X1, which you also claimed to detect at
                            magnitude ~12. But for detecting it on CCD images there must be used special
                            effrot and techniques. I have no informations that same techniques (for
                            example, long exposures and removing stars to improve s/n ratio) has been
                            used in case of theese comets.

                            4. Case of C/2011 X1 was favourable to ccd observers, because it is dust
                            remnant that produce continuum with peak in red. The C/2011 A3 and C/2011 M1
                            are active comets, which implies gas outter coma, with peak in green (where
                            most ccd's are not sensitive). I dont know the spectral sensitivity of
                            images you talk about.



                            Best regards,

                            Jakub Cerny



                            From: comets-ml@yahoogroups.com [mailto:comets-ml@yahoogroups.com] On Behalf
                            Of Alan Hale
                            Sent: Friday, October 28, 2011 2:10 AM
                            To: comets-ml@yahoogroups.com
                            Subject: Re: [comets-ml] C/2010 X1 remnant, and other things





                            Dear Jakub,

                            While there are several items in your message that are perhaps worth
                            responding to, and I may at some point, there is one I want to address right
                            now:

                            >
                            > The total magnitude does not says anything about comet visibility !!!

                            Yes, it does. A 9th magnitude comet is easily detectable with my 41 cm
                            telescope. A 15th magnitude comet is not.

                            >
                            > If one measure 11 mag comet with 6' coma diameter, and you are looking for
                            > 1' object in field of view, then you can easily miss it.
                            >

                            This is absurd.

                            When I look for comets, I do *not* look for comets of a specific size; I
                            look for the comet -- period. If the comet is bright enough to see in my
                            telescope, I will see it. An 11th-magnitude comet with a 6' coma is *easy*
                            is my telescope, and there is no way I can miss it. (I swept up comets of
                            that description during my comet-hunting days.)

                            With C/2011 M1, it was only after numerous unsuccessful attempts that I saw
                            the CCD images in question, which revealed the comet to be an extremely
                            faint and barely detectable smudge of light. Likewise, with C/2011 A3 it was
                            only after numerous unsuccessful attempts that I saw the CCD images, which
                            revealed it to be a small and relatively condensed object (even when
                            processed past the point of anything reasonable). To assert in either case
                            that there was a large diffuse coma is to assert that something existed
                            which the CCD images showed did not exist.

                            You cited the case of 177P. I measured a peak brightness of m1=9.2 for that
                            comet, in 10x50 binoculars. The comet was large, and diffuse -- and
                            extremely easy to see in both the 41 cm scope and the 10x50s.

                            Sincerely,

                            Alan





                            [Non-text portions of this message have been removed]
                          • Alan Hale
                            Dear Jakub, One of the C/2011 M1 images is here: http://www.fototime.com/ftweb/bin/ft.dll/picture?PictId={FB5D920C-F948-4AA7- 8F78-77EEEE1A1AD6}&size=ORIG
                            Message 13 of 24 , Oct 28, 2011
                              Dear Jakub,

                              One of the C/2011 M1 images is here:

                              http://www.fototime.com/ftweb/bin/ft.dll/picture?PictId={FB5D920C-F948-4AA7-
                              8F78-77EEEE1A1AD6}&size=ORIG

                              Unfiltered CCD, total exposure time 15 minutes.

                              Two others were taken by Michael Jaeger and posted to the comet-images
                              group, but they seem no longer to be there. I will e-mail those to you
                              privately.

                              The C/2011 A3 images were e-mailed to me privately, and I will have to ask
                              permission to forward them along.


                              Sincerely,

                              Alan
                            • Artyom Novichonok
                              Dear Alan, Jacub and All! You know, I try to do the best quality CCD photometry of comets and walk over to it carefully. I and Vladimir Gerke imaged C/2011 A3
                              Message 14 of 24 , Oct 28, 2011
                                Dear Alan, Jacub and All!

                                You know, I try to do the best quality CCD photometry of comets and
                                walk over to it carefully. I and Vladimir Gerke imaged C/2011 A3 in
                                September from TAU-station( very dark skies of Northern Caucasus).
                                This comet was compact object (near 1' coma) with integral magnitude
                                13-14m. No large external coma was not found. Of course I knew about
                                the visual estimates at the time and would especially like to
                                clarify this issue.

                                C/2011 M1.. hm... It was ~14 mag near discovery. But then... I agree
                                with the statement S. Yoshida regarding this matter (although it has
                                been said about the X1).

                                The comet, observed visually, especially when the brightness of 10-12
                                mag, should be not just smeared blot on CCD images with deep
                                permeation...

                                --
                                Best regards,
                                Artyom.

                                My comets light curves page:
                                http://severastro.narod.ru/sla/com/lc.htm
                              • Jakub Černý
                                Dear Alan, Artyom and all, I take a look from pictures provided by Alan and I agree that I have same feeling from that images as you guys. But in science there
                                Message 15 of 24 , Oct 28, 2011
                                  Dear Alan, Artyom and all,

                                  I take a look from pictures provided by Alan and I agree that I have same
                                  feeling from that images as you guys.

                                  But in science there is no place for feelings when we hace scientific
                                  methods.

                                  In the case of C/2011 A3, Artyom is for me very trustable person, because
                                  his V-mags photometry results are mostly in very good agrement with visual
                                  observations. I can see that SBIG STL-11000M camera has been used, with max.
                                  QE 50% near 500nm (see
                                  http://www.scribd.com/doc/54988295/100/Model-STL-11000M-Typical-Specificaito
                                  ns) so it is favourable camera for scanning cometary gas and it should be
                                  consistant with human eye spectral sensitivity.

                                  However when I am working with photometry data In many cases I found that
                                  there is some systemic difference between Artyoms m1s and visual m1s. There
                                  already happened case that Artyom CCDs was even almost 1 mag higher then
                                  visuals (29P this spring) or 1 mag weaker then visuals in case small and
                                  condensed comet (C/2010 S1 at end of the summer). This giving me some
                                  uncertainity because this differences I can not understand.

                                  In case of this comet, from some latitudes it could be very low over
                                  horizon. The question to Artyom, what was comet altitude, air condition and
                                  light pollution over the horizon?

                                  In the case of C/2011 M1, there are images Alan sent me, but there are no
                                  known photometric data to me. There will be another chance to také some
                                  pictures of this comet in November, unfortunately still low over horizon :(.

                                  There could be done theoretical test of this observations. Through coma
                                  diameter. Real coma diameters can be cross checked with other comets with
                                  same activity level (heliocentric magnitude in same distance).

                                  Best regards,
                                  Jakub Cerny

                                  -----Original Message-----
                                  From: comets-ml@yahoogroups.com [mailto:comets-ml@yahoogroups.com] On Behalf
                                  Of Artyom Novichonok
                                  Sent: Friday, October 28, 2011 11:14 PM
                                  To: comets-ml@yahoogroups.com
                                  Subject: Re[2]: [comets-ml] C/2010 X1 remnant, and other things

                                  Dear Alan, Jacub and All!

                                  You know, I try to do the best quality CCD photometry of comets and
                                  walk over to it carefully. I and Vladimir Gerke imaged C/2011 A3 in
                                  September from TAU-station( very dark skies of Northern Caucasus).
                                  This comet was compact object (near 1' coma) with integral magnitude
                                  13-14m. No large external coma was not found. Of course I knew about
                                  the visual estimates at the time and would especially like to
                                  clarify this issue.

                                  C/2011 M1.. hm... It was ~14 mag near discovery. But then... I agree
                                  with the statement S. Yoshida regarding this matter (although it has
                                  been said about the X1).

                                  The comet, observed visually, especially when the brightness of 10-12
                                  mag, should be not just smeared blot on CCD images with deep
                                  permeation...

                                  --
                                  Best regards,
                                  Artyom.

                                  My comets light curves page:
                                  http://severastro.narod.ru/sla/com/lc.htm






                                  ------------------------------------

                                  Comet Observations List: http://groups.yahoo.com/group/CometObs/
                                  Comet Images List: http://groups.yahoo.com/group/Comet-Images/

                                  NOTICE: Material quoted or re-posted from the Comets Mailing List should be
                                  indicated by:

                                  Comets Mailing List [date]
                                  http://www.yahoogroups.com/group/comets-ml
                                  Yahoo! Groups Links
                                • Juan José González Suárez
                                  Alan, Richard, Jakub, Nicolas, Dave, Mike, and friends, Some days ago, I started a mail to this list with some lines of California Dreamin (1965). Let me
                                  Message 16 of 24 , Oct 28, 2011
                                    Alan, Richard, Jakub, Nicolas, Dave, Mike, and friends,

                                    Some days ago, I started a mail to this list with some lines of
                                    "California Dreamin" (1965).
                                    Let me begin today with other song of the same year, another one of my
                                    most favorite songs, ( for obvious reasons ... ), "Sound of Silence":
                                    "Hello darkness, my old friend ... I've come to talk with you again ...
                                    ... and the vision that was planted in my brain ... still remains ..."
                                    Yes, the Oct. 9 and Oct. 21 visions ( of the C/2010 X1 remnant ) that
                                    then were planted in my brain still remain, crystal clear, similar to
                                    many previous estimates of very low DC "normal" comets with tail and no
                                    observable central condensation.

                                    Let's go to this ( complex ) subject.

                                    0 - Sorry for the extreme length of the text.

                                    1 - This will be my final post on these C/2010 X1 - and related -
                                    threads, like another kind of "silence": this subject is closed for me.
                                    I've been physically tired lately, under antibiotic treatment. I'm
                                    simply a visual observer, doing my work under mountain skies ... not
                                    wishing to go on forever into dialectical discussions.

                                    2 - Recent CBET 2876 ( including Sostero, Guido and Howes imaging
                                    efforts, visual photometry, and mainly, Sekanina's analysis on the
                                    comet's disintegration process ) gives a good resume of the actual
                                    knowledge about this final evolution of the comet.

                                    3 - I suppose that most members of this list are some kind of tired
                                    about this X1-related threads. I also suppose that many members are
                                    tired of my long posts ( so am I ...). We must remember that I didn't
                                    begin the long posts. I ONLY reported to the list what I thought were
                                    interesting observations. Some people can justifiably think that I'm
                                    seeking some notoriety. I can assure them that I'm not. At my near 60
                                    years of varied life I have near fulfilled almost all my life goals. I
                                    have had enough experiences ( including near death situations :
                                    exploring underground rivers in deep caves, mountain-climbing on ice,
                                    ..., potential severe accidents after hundreds of thousands of
                                    kilometers on the road ), and my actual perspective of life is familiar
                                    and quiet ... But including, of course, the serious observation of comets.

                                    ( sorry for this personal digression ).


                                    4 - Sincere thanks to Richard Miles for his very interesting
                                    contribution ( directly related to this thread ) on contrasting human
                                    vision and CCD cameras.
                                    These paragraphs are of the most pertinence and interest :

                                    >
                                    > Anyway, back to the pertinent point, it's this. People these days are so
                                    > used to using cameras and the like that they fall into the trap of
                                    thinking
                                    > the eye operates on the same principle as a camera or CCD. The truth is
                                    > that in many of its functions IT DOES NOT. Indeed it is the very
                                    mechanism
                                    > by which the eye works that makes it so valuable when estimating the
                                    > appearance and brightness of diffuse extended objects such as comets.
                                    > even larger areas but at the expense of gradually falling efficiency.]
                                    > ...
                                    >
                                    > A CCD camera is a difficult tool to use for measuring the integrated
                                    > intensity of an extended coma because as you use increasingly larger
                                    > measuring apertures, three effects work against you; (i) you add more
                                    noise,
                                    > (ii) larger areas contain increasing numbers of background stars, and
                                    (iii)
                                    > the same error in the measured background intensity leads to an
                                    increase in
                                    > the systematic errors in the final result.
                                    > ...
                                    > A well-trained visual observer can avoid these pitfalls of the CCD!
                                    > (btw: The recent work of Sostero, Guido and Howes on C/2010 X1
                                    > has been useful re. image background subtraction techniques for
                                    amateurs.)
                                    >
                                    > Advanced amateurs working visually have honed their observing
                                    techniques to
                                    > be near optimal.
                                    > ...
                                    > Here's how I'd sum up working at the optimal limit for
                                    > seeing a very extended coma from a physiological basis:
                                    >
                                    > (a) The telescope focal length and magnification should be such that the
                                    > apparent size of the coma in the eyepiece is kept as small as possible,
                                    > preferably no larger than about 2 degrees in size.
                                    > (b) Vision should be averted by about 30 deg from the fovea.
                                    > (c) The exit pupil at the eyepiece should be 3-5 mm in diameter.
                                    > (d) Scattered light should be reduced to a minimum to minimise the
                                    intensity
                                    > of the background sky.
                                    >
                                    > The subject of the apparent intensity of the background sky is also
                                    crucial
                                    >...
                                    > So if we take a real example, what would be the ideal scope to view the
                                    > remnant cloud of C/2010 X1? If we consider that the cloud has a minor
                                    axial
                                    > dimension of 4' (from the Oct 23 image of Sostero et al.) then the ideal
                                    > magnification to use would be 30x (i.e. 2deg/4'). So to fit that
                                    through a
                                    > 5mm pupil, the entrance aperture would need to be 150 mm across.
                                    >...
                                    > We can now see that JJGS's setup of 77x200mm was not so far from the
                                    ideal,
                                    > which would have been say 40x150mm. However, using a 400mm scope would
                                    > push the apparent size subtended by the coma well past the 2-degree
                                    criterion.
                                    >
                                    > Many comet observers have a range of scopes / binoculars of various
                                    size and
                                    > so the key is to select the instrument which is best suited to each
                                    target.
                                    >
                                    > Richard

                                    - Well, Richard, I ( almost ) can't add a single line to your extensive
                                    exposition. I only can give some additional information related with my
                                    Oct. 21 observation of C/2010 X1. Knowing from theory and experience
                                    this "near optimal" considerations, ( and based on my previous Oct. 9
                                    observation ), I've tried to observe the remnant with 25x100 binoculars,
                                    without positive results. I have also 40 and 56 mmm eyepieces, giving 50
                                    and 36x with my 20 cm SCT ( focal length: 2000 mm ). Both setups (
                                    25x100 binoculars and SCT at very low magnifications ) give a too high
                                    apparent intensity of the background sky, not allowing the observation
                                    of such a diffuse object.
                                    Besides, on this kind of situations, there is another very important
                                    factor : the brightness and distribution of the nearby field stars.
                                    Taking all this into account, on my Oct. 21.00 UT, a higher
                                    magnification ( like 77x ) was also needed for the detailed detection of
                                    the motion, the study of the morphology of the "tail-like" remnant and
                                    the sunward coma-like near-circular area with a slightly higher degree
                                    of condensation.


                                    5 - Thanks also to Jakub Cerny ... I obviously agree with the C/2010 X1
                                    section of his text to a great extent :


                                    >
                                    > 1) Statement that the JJ's observations from 21.10. is real but the 9.10.
                                    > one is not. For me it is pretty hard to believe that someone would construct
                                    > a dust cloud with "tail" extending in p.a. which was later been confirmed on
                                    > CCDs without knowin that. If that observation was not real then JJ must have
                                    > really luck in imagination ;-). I also think that while the comet was on
                                    > 9.10. more distant, the difference between cloud and comet nucleus expected
                                    > position should be smaller!
                                    >
                                    > We should not forget that the recovery of dust cloud using CCD was made
                                    > after big effort inspired by JJ's visual observations. TO me it looks
                                    > simply, that it was not recovered on 9.10. due people did not even tried to
                                    > find it with small instruments.


                                    6 - ... But I can't agree with other sections :


                                    > 3)rd issue does not have anything with C/2010 X1 but for me it is most
                                    > important. The case of "bright" visual comets C/2011 A3 and C/2011 M1.
                                    > Honestly, always when I recieve a visual observations from J.J. then I have
                                    > on my mind something like "oh my god, he went crazy" with those magnitudes.
                                    > Somehow, JJ's magnitudes are always something like 2 mag brighter than other
                                    > observers. The reason is unknown, it can be anything started with bad
                                    > methodology to some specifications of JJ's sight or conditions.
                                    >
                                    > ...
                                    >
                                    > The other comet - C/2011 M1 seems to me very similar to comet from past -
                                    > 177P/Barnard ! Extremely large, diffuse coma with low surface brightness and
                                    > almost no sign of central condensation.
                                    >
                                    > ...
                                    >
                                    > Alan you speaking about that you never had more then 0.5 mag difference to
                                    > other observers, but I know al ot of historical issues when there was
                                    > terrible differences in comet magnitudes! Always it was caused by large,
                                    > faint diffuse coma.
                                    >


                                    - Jakub, you've got exactly the point in this last paragraph : the
                                    "great" referred differences in the m1 estimates arise in the
                                    observations of comets showing large and diffuse comae with faint outer
                                    regions. I will be more explicit : ... especially for such kind of
                                    comets located at low altitude, like C/2011 A3 in my recent observation :

                                    C/2011 A3 (Gibbs):
                                    2011 Oct. 15.79 UT: m1=9.1, Dia.=8', DC=2, Tail: 0.6 deg. in PA 80 deg,
                                    20 cm SCT (77x), Juan Jose Gonzalez ( Aralla, alt. 1360 m., Leon, N.
                                    Spain ) [ Altitude: 15 deg. Mountain location, very clear sky.].

                                    I will rewrite now Jakub's text with some of my own words : Honestly,
                                    when I make a "bright" visual estimate like the C/2011 A3 recent one, I
                                    have on my mind something like "some people in the cometary community
                                    will think that I'm wrong ...".
                                    But, as previously and frequently said, as a physicist and observer, I
                                    must report the estimates exactly as they are.

                                    Here are some recent estimates from Comet_Obs site:

                                    C/2011 A3 (Gibbs):
                                    Oct. 15.79 UT, m1=9.1, Dia.=8', DC=2, 20cm SCT (77x), J. J. Gonzalez
                                    (Spain);
                                    Oct. 7.41, m1=[10.4, 40.0cm L (144x), Seiichi Yoshida (Japan);
                                    Oct. 02.76, m1=10.6, Dia.=2.3’, DC=3, 25 cm reflector (100x), S. Szabo
                                    (Hungary);
                                    Sep. 24.83, m1=9.7, Dia.=6', DC=2/, 20 cm SCT (77x), J.J.Gonzalez;
                                    Sep. 24.78, m1=11.0, Dia.=5'.0, DC=2/, 35 cm reflector (107x), J.Cerny
                                    (Czech Republic);
                                    Sep. 19.84, m1=9.7, Dia.=6', DC=2/, 20 cm SCT (77x), J.J.Gonzalez;
                                    Sep. 3.83, m1=10.9, Dia=2.2', DC=3, 20 cmL (100x), M.Paradowski ( Poland);
                                    Sep. 03.81, m1=11.2, Dia.=3'.8, DC=3/, 35 cm reflector (107x), J.Cerny;
                                    Aug. 28.94, m1=10.8:, Dia=2.5', DC=2, 22 cmL (160x), M.Goiato (Brasil);
                                    Aug. 28.83, m1=12.0, Dia.=2'.6, DC=4, 35 cm L (107x), J.Cerny;
                                    Aug. 26.88, m1=10.3, Dia.=6', DC=2/, 20 cm SCT (77x), J.J.Gonzalez;
                                    Aug. 26.84, m1=[11.3, Dia.=3'.5, 25x100 binoculars, J.Cerny;
                                    Aug. 25.84, m1=11.5, Dia.=3'.5, DC=3, 35 cm L (107x), J.Cerny.

                                    In such cases ( as C/2011 A3 ) of comets located at low altitude, as we
                                    knows, the elevation of the observing site, and local atmospheric
                                    conditions, have a great influence.

                                    Another "problematic, extremely diluted and fuzzy comet" is C/2011 M1 :

                                    C/2011 M1 (LINEAR):
                                    Sep. 25.19 UT, m1=9.5, Dia.=5', DC=2, 20 cm SCT (77x) J.J.Gonzalez (Spain).
                                    Sep. 07.18, m1=9.2, Dia.=8', DC=2, 20 cm SCT (77x), J.J.Gonzalez.
                                    Sep. 03.18, m1=9.2, Dia.=7', DC=2, 20 cm SCT (77x), J.J.Gonzalez.
                                    Aug. 27.09, m1=10.4, Dia.=6'.4, DC=2, 35 cm L (64x), J. Cerny (Czech
                                    Republic).
                                    Aug. 26.89, m1=9.5, Dia.=6', DC=2, 20 cm SCT (77x), J.J.Gonzalez.
                                    Aug. 26.86, m1=10.2, Dia.=5', DC=2, 23,5 cm SCT (77x), C. Labordena (Spain).
                                    Aug. 24.82, m1=13.0:, Dia.=2', DC=2, 50,8 cm L (117x). S. Szabo (Hungary).
                                    Aug. 20.83, m1=10.6, Dia=2.7', DC=3, 25x100 B, M. Paradowski (Poland).
                                    Aug. 19.86, m1=12.2 , Dia=1', DC=1, 32 cm L (120x), U. Pilz (Germany).

                                    I must explain here something about the geographic and atmospheric
                                    conditions of the Cantabrian Mountains, or "Cordillera Cantabrica". The
                                    most simple geographic data and images can be found on the Wikipedia.
                                    They extend for more than approximately 300 km across N. Spain, along
                                    the coast of the Cantabrian Sea. The northern slope of the range rises
                                    abruptly from near sea level. The descent of the southern slope to the
                                    high plateaux of Leon and Castilla is more gradual. Many peaks are over
                                    2000 m high, but the greatest altitudes are attained in the central
                                    section, on the borders of Asturias, Leon, Cantabria and Palencia. In
                                    this section rises the higher massif, "Picos de Europa", one of the
                                    greatest mountain limestone karsts of the world, topping at 2650 m. The
                                    Cantabrian Mountains make a sharp divide between "Green Spain" to the
                                    north, and the dry central plateau of the Iberian Peninsula. The north
                                    facing slopes receive frequently heavy cyclonic rainfall from the
                                    Cantabrian Sea, whereas the southern slopes are dryer, in rain shadow.
                                    The east - west alignment of the ridge, nearly parallel and close to the
                                    sea, explains some of the special atmospheric conditions, very favorable
                                    on clear days for the astronomical observations at low altitude towards
                                    the eastern and western horizons.

                                    Let's now come back to the comets ...

                                    - Jakub and friends, my estimates AREN'T ALWAYS something like 2 mag
                                    brighter than other observers.

                                    A very good ( among many other ) recent example with very large coma is
                                    103P/Hartley near maximum brightness:

                                    103P/Hartley:
                                    2010
                                    Oct. 22.15, 4.3, & 50' (J. Cerny, Senohraby, Czech Republic, 0.05-m
                                    refractor);
                                    21.36, 4.8, 40' (M. Goiato, Aracatuba, Brazil, 8x25 binoculars);
                                    21.31, 5.0, 30' (W. Souza, Sao Paulo, Brazil, 7x50 binoculars);
                                    20.98, 5.5, 50' (G. Pappa, Mascalucia, Italy, 12x50 binoculars);
                                    20.19, 4.7, 45' (J. J. Gonzalez, Leon, Spain, 4x30 monocular);
                                    19.45, 4.9, 45' (C. W. Hergenrother, Tucson, Arizona, 6x30 monocular);
                                    18.10, 4.6, 50' (J. J. Gonzalez, Leon, Spain, naked eye);
                                    16.92, 4.8, 40' (T. Scarmato, Calabria, Italy, 7x50 binoculars);
                                    15.00, 4.7, 60' (J. J. Gonzalez, Leon, Spain, naked eye);
                                    14.12, 5.0, 40' (M. Goiato, Brazil, 8x25 binoculars);
                                    14.11, 5.1, 50' (P. Horalek, Ustupky, Czech republic, naked eye);
                                    13.93, 5.0, 50' (J. J. Gonzalez, Leon, Spain, 4x30 monocular);
                                    13.93, 5.1, 45' (M. Lehky, Hradec Kralove, Czech Republic, naked eye);
                                    13.92, 5.0, & 45' (J. Cerny, Senohraby, Czech Republic, naked eye);
                                    13.86, 5.3, 60' (V. Nevski, Vitebsk, Belarus, naked eye);
                                    13.38, 5.5, 30' (C. W. Hergenrother, Tucson, Arizona, 10x50 binoculars);
                                    12.93, 5.1, 45' (M. Lehky, Hradec Kralove, Czech Republic, naked eye);
                                    12.93, 5.3, 35' (P. Horalek, Ustupky, Czech republic, naked eye);
                                    11.95, 5.3, 40' (M. Lehky, Hradec Kralove, Czech Republic, naked eye);
                                    11.91, 5.2, & 45' (J. Cerny, Senohraby, Czech Republic, 0.05-m refractor);
                                    11.34, 5.8, 30' (C. S. Morris, Fillmore, CA, U.S.A., 10x50 binoculars);
                                    10.93, 5.4, 30' (M. Lehky, Hradec Kralove, Czech Republic, naked eye);
                                    10.86, 5.6, 45' (P. Horalek, Ustupky, Czech republic, 11x70 binoculars);
                                    10.80, 5.9, & 25' (B. H. Granslo, Nittedal, Norway, 7x50 binoculars);
                                    10.80, 5.2, 30' (T. Karhula, Virsbo, Sweden, naked eye);
                                    10.79, 5.5, 40' (K. Hornoch, Ondrejov, Czech Republic, naked eye);

                                    ( selected from the ICQ site ). The data are self-explaining ...

                                    - Let's comment the second section of the paragraph :
                                    "Somehow, JJ's magnitudes are always something like 2 mag brighter than
                                    other observers. The reason is unknown, it can be anything started with
                                    bad methodology to some specifications of JJ's sight or conditions".
                                    Bad methodology ? quoting a well-known dialogue : This can't be serious
                                    ... I've dedicated many thousands of hours on hundreds of nights to the
                                    improvement of the methodology of observation, making estimates of all
                                    kind of comets ( bright and faint, very diffuse and near stellar, ...)
                                    with all kind of instruments ( naked eye, low magnification monoculars,
                                    little and big binoculars, telescopes ... ) ...

                                    Some specifications of my sight ? Well ... from childhood, I know that I
                                    have good far vision ( but not exceptional ).

                                    - But ... yes, I can comment an interesting detail about my eyes. After
                                    all those thousands of hours at the eyepiece, my left eye is slightly
                                    more trained for the observation of diffuse comets, and the right eye
                                    for the observation of high-DC faint comets.
                                    So, I can understand well the problems of some visual astronomers when
                                    observing ( or not observing ... ) large and diffuse comet's comae.


                                    7 - Dave Herald addresses a very interesting and controversial question :


                                    > There is another issue to keep in mind with visual observations. It is one thing to compare estimates of total magnitude – as there the object is clearly visible ‘to all’. But when you get into the area of whether or not a visual observer could actually see an object, you run into issues of credibility that can too easily be treated as personal. The fundamental problem is that an observer’s assertion that they can see an object fails the test of repeatability until others can also see it. That is, until others can see it, the situation reduces to one of whether or not you believe the observer – or (perhaps more correctly) whether you accept the attribution given by the observer to what they saw. In this regard, getting an image of an object is far more satisfactory, as ‘everyone’ can dissect the image in detail and for independent conclusions.
                                    >
                                    > Net result is (in my opinion) that while visual observations of objects that are ‘generally’ visible are valuable and reliable, visual observations involving the detection of objects that others can’t see are properly subject treated with scepticism – purely on the basis of the lack of independent ‘repeatability’ or other forms of independent verification.
                                    >
                                    >
                                    > Dave Herald


                                    - I agree with most of your exposition, Dave. In this thread, C/2010 X1
                                    remnant has been observed both visually and CCD-imaged.

                                    But ... in some cases, we only have some single observations. And yes,
                                    we must treat them with specticism. But they can be probably true if the
                                    observer is a reliable one. We have some well known historical cases in
                                    Astronomy, both positive and negative.
                                    An interesting case ( especially for us, spaniards ) is the discovery of
                                    the atmosphere of Titan. Searching in Google for "discovery of the
                                    atmosphere of Titan" gives two dates : 1908 ( visually by Comas Solá ),
                                    1944 ( spectropically by Kuiper ). There has been a long debate about
                                    this question ...


                                    8 - The mail from Nicolas Biver is short but interesting, as usual.

                                    This is a well addressed text :


                                    > ...
                                    >
                                    > I have also another concern about a kind of "race for the brightest estimate" of any comets: it may not be true (hopefully) but I feel like there is a trend to look for being the one that will "see" the comet beeing the brightest (I recently observed C/2009 P1 barely brighter than 7.5, but several are reported it brighter than 7). It may be a bias due to method or star catalog selection, and also a trend not so confirmed in full ICQ dataset...
                                    > ...
                                    >


                                    - Nicolas, I've had the same feeling many times from the beginning of
                                    the World Wide Web in the mid-1990s, and the related instant information
                                    availability. Let's hope that this possible "race for the brightest
                                    estimate" will stay at the lowest possible level, as incompatible with
                                    the correct methodology.


                                    9 - The longer and denser mail comes from Alan Hale, deserving detailed
                                    answers.


                                    > ...
                                    > First off, of course, there is now no doubt that a faint dust stream remnant
                                    > of C/2010 X1 does indeed continue to exist. I was certainly skeptical of
                                    > this, up until the time I saw Rolando Ligustri's image that he took on the
                                    > 22nd; but that image, and the other images that he and other observers have
                                    > taken since then, clearly demonstrate that this dust stream is there.
                                    > ...
                                    > Armed with this knowledge, and also the knowledge of what precisely to look
                                    > for, I made another attempt on the morning of October 24. I did in fact seem
                                    > to see a shapeless but extended "something" in about the expected position,
                                    > and moreover this "something" seemed to move westward at the expected rate.
                                    > This was an extreme borderline observation ...
                                    > ...
                                    > For what it's worth, I did attempt a brightness measurement by an extreme
                                    > defocusing of stars, and would say somewhere around m1 ~12: I agree with
                                    > John, though, that this is essentially meaningless -- for one thing, there
                                    > is no coma, and that is usually what is meant by a comet's brightness -- and
                                    > at best it refers to a shapeless, ill-defined portion of the tail remnant.
                                    > ...


                                    - Alan, I'm glad that you could observe the C/2010 X1 remnant. From my
                                    side, the only "problem" was your extreme skepticism towards my visual
                                    observations and estimates.

                                    Let's make now a thought experiment : we interchange our "roles" in this
                                    C/2010 X1 "affair". So, you would have made two observations of the
                                    comet on Oct. 9 and 21 with a 20 cm SCT, and I would have tried it with
                                    a 41 cm telescope, without positive results. What would have been my
                                    reaction ? I never would have doubts about your observations. Instead, I
                                    would have thought about the causes of my negative result : objective
                                    difficulty of observing such a diffuse object, different starfields on
                                    different nights, atmospheric conditions, personal equation of the
                                    observer, magnification, and so on.

                                    This would have been the same for me towards any other well-known
                                    observer of our cometary community. Like you have said, "I have high
                                    respect for all of you".

                                    ( Well, maybe I'm not such a "well-known observer" after all for some
                                    members of our community ... I obviously respect that ... In any case,
                                    I'll continue observing comets, as happy as always, till age put an end
                                    ... ).

                                    Let me express another related comment ( previously told ) about my
                                    feelings after such "difficult" observations like the C/2010 X1 one. I
                                    don't think then about some kind of objective personal achievement.
                                    Mainly, I feel privileged to be in the correct mountain place and time
                                    for observing the comet to which I've dedicated my efforts.


                                    > ...
                                    > For what it's worth, I did attempt a brightness measurement by an extreme
                                    > defocusing of stars, and would say somewhere around m1 ~12: I agree with
                                    > John, though, that this is essentially meaningless -- for one thing, there
                                    > is no coma, and that is usually what is meant by a comet's brightness -- and
                                    > at best it refers to a shapeless, ill-defined portion of the tail remnant.
                                    > ...


                                    - I can't agree on the "essentially meaningless" nature of the magnitude
                                    estimate on this case. From CBET 2876 : " ... The amount of the dust
                                    ejecta in the cloud of C/2010 X1 cannot be determined because of the
                                    lack of photometry. A very crude estimate of the total cross-sectional
                                    area of the dust particles is only provided by the visual magnitude 10.2
                                    ... the result is about 480 km2. The mass of the cloud comes out to be
                                    on the order of 10^(12) grams."


                                    > ...
                                    > Now, having reported all this, I would like to comment on some visual
                                    > cometary observations, not only of C/2010 X1 but some other comets, and in
                                    > general. While Juan Gonzalez has been a bit on the "hot seat" with his
                                    > reports of C/2010 X1 -- and I freely admit that I have been among his
                                    > doubters, and I will get to that in a moment -- my comments are directed
                                    > more towards the current community of comet observers in general. I will
                                    > occasionaly use the word "you," but, except in the specific case of the
                                    > recent observations of C/2010 X1, I am not referring to any observer in
                                    > particular.
                                    >
                                    > Before proceeding, I wish to reiterate something I said in an earlier post,
                                    > i.e., at no time do I mean anything personal about any individual. I
                                    > consider everyone here friends and colleagues, and I have high respect for
                                    > all of you. I am speaking strictly of the scientific observations involved,
                                    > as I would like to believe that the visual brightness (and morphology)
                                    > reports of comets continue to have scientific value. That requires, though,
                                    > that they be done correctly at all times.
                                    > ...


                                    - When you say "...While Juan Gonzalez has been a bit on the "hot seat"
                                    with his reports of C/2010 X1 ... my comments are directed more towards
                                    the current community of comet observers in general", I obviously agree
                                    with you, Alan, ... but you are referring to me ( directly or indirectly
                                    ) in many sections of your text : regarding C/2010 X1, C/2011 M1, C/2011
                                    A3, and the final sections.

                                    Well, in any case, for me like for you, "... At no time do I mean
                                    anything personal about any individual. I consider everyone here friends
                                    and colleagues, and I have high respect for all of you".


                                    > ...
                                    > Let's look at Juan's two recent reports of C/2010 X1. The first one, on
                                    > October 9, specifically refers to a "coma;" we now know that there is no
                                    > "coma" for this comet, and anything that might have been visible was not at
                                    > the ephemeris position, and Juan makes no mention of this. Furthermore, he
                                    > remarks that there was zodiacal light present (with which I will concur,
                                    > with my own unsuccessful observation attempt around that same time), and,
                                    > considering the extremely low surface brightness of the "tail remnant," any
                                    > kind of background lighting should have wiped it out completely. (Recall
                                    > that all the CCD images at that time showed nothing, or at best, an
                                    > extremely faint and uncertain fragment that may or may not have been real.)
                                    > And while I accept at face value Juan's comments about the clarity of the
                                    > atmosphere down to the horizon, at an altitude of 23 degrees you are still
                                    > looking through over 2.5 air masses, and you are going to be encountering a
                                    > non-trivial amount of extinction.
                                    > ...


                                    - Here is my estimate, Alan :

                                    C/2010 X1 :
                                    2011 Oct. 9.20 UT: m1=10.7, Dia.=6', DC=1/, Tail: 0.2 deg. in PA 310
                                    deg, 20 cm SCT (77x), Juan Jose Gonzalez ( Alto del Castro - Aralla,
                                    alt. 1720 m., Leon, N. Spain ).
                                    [ The coma appears large and very diffuse, without central condensation.
                                    Observation made from mountain location under very good seeing
                                    conditions. Nearby field stars checked in DSS. Zodiacal light. Altitude:
                                    23 deg.].

                                    Yes, I used the word "coma", only referred to the morphological
                                    similarity between the observed object and a "normal" DC=1/ comet. As in
                                    my Oct. 21 estimate, this "coma" ( located in the sunward extreme of the
                                    remnant ) was a "near-circular area with a slightly higher degree of
                                    condensation".


                                    > ...
                                    > The upshot here is that I must remain very skeptical of the October 9 report
                                    > as, all things considered, it is inconsistent with what the CCD images (both
                                    > then and now) have revealed.
                                    > ...
                                    >


                                    - Sorry, but obviously I can't agree on your skepticism and
                                    interpretation of the images.

                                    Supplementary information ( from a previous post ) : For me this case
                                    wasn't problematic or near the threshold of visual detection. It was a
                                    clear observation ( the atmospheric conditions in the Cantabrian
                                    Mountains were near optimal down to the line of horizon ) made in the
                                    course of a short observing session ( 1 hour approx. ) between moonset
                                    and the beginning of the morning astronomical twilight. Under such very
                                    good conditions, and besides C/2010 X1, my plan was to observe another
                                    four comets ( C/2010 G2, C/2011 S2, 49P, and 45P at low altitude ), so I
                                    only could dedicate approx. 10 min to the observation and magnitude
                                    estimate of C/2010 X1, at low magnification as necessary. I didn't check
                                    for the motion, given the short time-interval and the apparent lack of
                                    central condensation in such a large ( 6' ) coma. The geometric center
                                    of this coma was close to the MPC ephemeris position, with an error bar
                                    of ± 4'.


                                    > ...
                                    > The second observation, on October 21, does seem at face value to be
                                    > reasonably consistent with what the later images showed, although I note
                                    > that he reported one of his "bright areas" to be at the ephemeris position.
                                    > ...
                                    >


                                    - This is my another estimate :

                                    C/2010 X1 :
                                    2011 Oct. 21.00 UT: m1=10.2, Dia.=7', DC=1/, Tail: 0.4 deg. in PA 310
                                    deg, 20 cm SCT (77x), Juan Jose Gonzalez ( Alto del Castro - Aralla,
                                    alt. 1720 m ).
                                    [ The comet's remnant appears visually like a very diffuse tail-like
                                    morphology, observable for 0.4 deg along the major axis, showing two
                                    near-circular areas of similar brightness with a slightly higher degree
                                    of condensation and 7' of diameter. One of this areas, with geometric
                                    center located approximately at R.A.=8h00m.5, Decl.=+28o26'.0 ( Oct.
                                    21.00 UT ), was close to the ephemeris position. The other area's center
                                    was located at R.A.=7h59m.5, Decl.=+28o34'.5. The motion of both areas
                                    was followed for 40 minutes, being consistent with the
                                    ephemeris.Observation made before moonrise, from mountain location with
                                    very good seeing, under slightly better conditions than twelve days ago
                                    (Oct.9). Nearby field stars checked in DSS. Altitude: 16 deg.].

                                    I said then, clearly : One of this areas ... was close to the ephemeris
                                    position. ( NOT at the ephemeris position ).


                                    > ............................ The October 21 report is much less clear to me;
                                    > I do have some problems with it, which I've discussed, but on the other hand
                                    > it is consistent enough with the CCD images that I cannot simply dismiss it.
                                    > ...


                                    - Yes, Alan, my Oct. 21.00 UT report is consistent enough with the
                                    POSTERIOR CCD images.

                                    ( I wish to sincerely thank here Nick Howes comment : "Your visual
                                    observations are what inspired our team to keep trying to image Elenin").


                                    > ...
                                    > While I am not infallible, with this kind of track record it is
                                    > inconceivable to me that I could be looking *directly* at a 9th- or
                                    > 10th-magnitude comet, and *not* see it. Yet, if I am to believe some of the
                                    > reports that have been submitted in recent years, that is precisely the
                                    > case.
                                    > ...
                                    >


                                    - Alan, I have been looking *directly* at some 9th- or 10th-magnitude (
                                    or similar ) comets, and I *didn't* see them ...

                                    Some of them were large and diffuse comets that I've been observing some
                                    days before, at decreasing altitude, when approaching perihelion.
                                    In other cases, also related to diffuse comets, there were various
                                    factors : atmospheric conditions, elevation of the site, ...



                                    > ...
                                    > There are several examples I can cite, but I will focus on two recent
                                    > ones.
                                    >
                                    > We've already discussed C/2011 M1 to an extent. Some of you were
                                    > consistently reporting this comet to be as bright as m1 ~9-10, with a large,
                                    > diffuse coma, and yet, despite numerous search attempts, I never saw it. And
                                    > in this case, the CCD images backed me up ...
                                    > ...
                                    > Another example that I have not seen discussed here yet is C/2011 A3. Again,
                                    > several of you have reported this comet as being bright at m1 ~9-10 and with
                                    > a large coma, and, again, despite numerous attempts I have failed to see it.
                                    > (Other observers have privately informed me that they, too, have failed to
                                    > see it.) Once again, the CCD images back me up ...
                                    > ...
                                    >


                                    - C/2011 A3 and C/2011 M1 visual observations have been extensively
                                    debated before.

                                    Obviously, like you, I'm not infallible in my observations, but
                                    sincerely, is for me impossible to think that more than four well-known
                                    observers ( as J. Cerny, M. Goiato, C. Labordena, M. Paradowsky, U.
                                    Pilz, S. Szabo, ... ) can be wrong on each comet ( A3, M1 ).
                                    CCD images aren't ALWAYS an evidence against a visual observation, as
                                    the recent C/2010 X1 case demonstrate, and has been exposed somewhere
                                    before.


                                    > ...
                                    >
                                    > These are *not* cases of "visual photometry vs. CCD photometry;" these are
                                    > cases of CCD images showing what is and what isn't there. It is very
                                    > disingenuous to cite the CCD images as positive evidence of the C/2010 X1
                                    > observations, and then turn around and dismiss the CCD images of C/2011 M1
                                    > and C/2011 A3 which reveal that those two comets were much, much fainter
                                    > than the visual reports at the times in question.
                                    > ...
                                    >


                                    - Well, here is a clear disagreement. These ARE cases of the general
                                    "Visual vs. CCD" debate. Regarding the C/2010 X1, C/2011 A3 and C/2011
                                    M1 references, we obviously interpret them in different ways.


                                    > ...
                                    > There are two other issues I would like to touch upon. Back when they were
                                    > active, I was constantly cross-checking my observations against those of
                                    > John Bortle and Charles Morris, and I was almost always within a
                                    > half-magnitude or so of their reports. This doesn't necessarily mean that
                                    > any particular individual is "right," and, yes, we all should "call them as
                                    > we see them," but if everyone involved is doing it correctly, we should all
                                    > be seeing something fairly similar to each other. After all, if the depth of
                                    > a lake is 10 meters at a certain point, then all measurements of the depth
                                    > at that point should be somewhere close to 10 meters. Similarly, if a
                                    > comet's "true" brightness is magnitude 10.0, then all measurements should be
                                    > somewhat close to that (say, between magnitude 9.5 and 10.5).
                                    > ...
                                    >


                                    - First of all, my memory and greetings to Charles, valued friend.

                                    Alan, I remember well those times, after the beginning of the WWW in the
                                    mid-1990s. And I agree on the utility of that kind of cross-checking
                                    observations among experienced observers.

                                    But ... the problem of the m1 estimates and coma diameter of large and
                                    diffuse comets was and is always present.

                                    Only as a single example ( not a rule ... ) :

                                    45P/Honda-Mrkos-Pajdusakova:
                                    1996
                                    Feb. 14.39 UT: m1=7.5:, Dia.~25', DC=0...10x50 B....Charles Morris
                                    (Lockwood Valley, CA) [Just a brightening of the sky background ...
                                    difficult to estimate.]
                                    Feb. 16.42 UT: m1=8.3, Dia.=10', DC=0-1...10x50 B....Alan Hale
                                    (Cloudcroft, NM).
                                    ( From Charles website ... ).

                                    Another question ... What really means "comet's "true" brightness" ? In
                                    Physics and Astronomy we don't know the "true" value, we only have
                                    observed values ( and we can estimate errors ... ).

                                    As previously exposed, this is especially evident for the diffuse comets ...


                                    > ...
                                    > And then, there is the topic of limiting cometary magnitudes for telescopes
                                    > of various apertures. I have been utilizing the 41 cm telescope for over two
                                    > decades now, and can state with reasonable confidence that its limiting
                                    > magnitude for comets is around 14.5. (I'm referring to "cometary" comets,
                                    > not completely stellar ones, which I can obviously detect fainter.) Over the
                                    > years, observers I know with similar-sized telescopes have reported
                                    > something similar, and those with smaller or larger telescopes have reported
                                    > limiting magnitudes that are consistent with this.
                                    >
                                    > And yet nowadays, I am seeing some of you, including some of you with
                                    > telescopes significantly smaller than mine, routinely reporting comet
                                    > magnitudes up to a full magnitude -- sometimes even more -- fainter than the
                                    > limit of my telescope. How is this possible? ...
                                    > ...


                                    - The limiting "cometary" magnitude for the 20 cm SCT in my observations
                                    is close to 14.0-14.3 ( Henden photometry as reference ) for DC=5
                                    comets, under optimal atmospheric conditions.


                                    > ...
                                    > A question I would ask is, do you ever look for a comet and *not* see it? If
                                    > you're constantly looking for comets near the limit of your telescope, there
                                    > should be quite a few of them that you never see. If you somehow are able to
                                    > "see" every comet you attempt, then I submit that something is wrong. You
                                    > all know how many comets I have visually observed over the years, but you
                                    > may not know that there are over 200 other comets that I have looked for,
                                    > and *not* seen.
                                    > ...
                                    >


                                    - Yes, Alan, I frequently look for a comet and I don't see it. The
                                    limiting star magnitude for my 20 cm SCT is close to 15.3 ( Henden ), in
                                    good agreement with Roger Clark's data on "Visual Astronomy of the Deep
                                    Sky". Quite oftenly, I make negative attempts on comets on the 13.5 -
                                    15.0 range, depending upon particular conditions and situations.

                                    I'm not constantly looking for comets near the limit of my telescope. I
                                    know my limitations, and I know the limitations of
                                    my telescope. Don't have here the exact number, but in a good
                                    aproximation there are over 100 comets that I have looked for,
                                    and not seen.


                                    > ...
                                    > I guess I am reminded of the old story of the "boy who cried 'wolf.'" If you
                                    > consistently report 9th-magnitude comets that aren't there, report comet
                                    > brightnesses that are three magnitudes different from what I'm seeing, and
                                    > report seeing comets with a small telescope that are far too faint for me to
                                    > see with my larger telescope, then you will have to pardon my skepticism
                                    > when it comes to reports of extremely unusual cometary phenomena like what
                                    > we've recently seen with C/2010 X1. Even if you got it right this time . . .
                                    > ...
                                    >


                                    - Alan, sincerely I don't know if this text is more or less addressed to
                                    me ... But well, let's write for clarifying some questions :

                                    a ) I DON'T report 9-th magnitude comet's that aren't there ( like
                                    C/2011 A3, C/2011 M1, ... ). Some observers, depending upon particular
                                    conditions and situations, can observe these kind of diffuse comets, and
                                    others ( sometimes including myself ) can't observe them.

                                    b) I understand perfectly the normal skepticism regarding some reports
                                    about unusual cometary phenomena like C/2010 X1 post-perihelion
                                    evolution. This must be the scientific attitude.

                                    c) and d) You are putting yourself as as a kind of "standard" for the
                                    comparison of other observers with comments like "brightnesses that are
                                    three magnitudes different from what I'm seeing" and "seeing comets with
                                    a small telescope that are far too faint for me to see with my larger
                                    telescope". Is this really a "completely" scientific attitude ?


                                    > ...
                                    > The enthusiasm for visual comet observations that I am seeing now is
                                    > refreshing to see, and again, I really would like to think that the data we
                                    > obtain is scientifically useful. (It might even help in resolving the
                                    > "visual photometry vs. CCD photometry" issue, although I sometimes think
                                    > that's an unsolvable problem.) But it can only be useful if it's done right.
                                    > I implore everyone here -- veterans included, and that includes me -- to be
                                    > careful and cautious in obtaining these observations, and to see to it that
                                    > we are making the most reliable observations that we can possibly make.
                                    > ...


                                    - Fortunately, we are in complete agreement on this.


                                    10 - Finally, and sincerely, I hope that all these "C/2010 X1 threads"
                                    have been ( in some ways ) useful for the cometary community.


                                    Best regards from your old and tired friend,


                                    Juan Jose Gonzalez


                                    --------------------------------------------------------------------------
                                  • Uwe Pilz
                                    Dear Alan, dear Jakub, the German group has a very large archive of comet images. You find images of 2011M1 here (all by Michael Jäger):
                                    Message 17 of 24 , Oct 29, 2011
                                      Dear Alan, dear Jakub,

                                      the German group has a very large archive of comet images. You find images of
                                      2011M1 here (all by Michael Jäger):

                                      http://kometen.fg-vds.de/pix/2011M1_e.htm
                                      --
                                      Uwe Pilz

                                      On Fri 28/10/11 20:19 , Alan Hale ahale@... sent:
                                      > Dear Jakub,
                                      >
                                      > One of the C/2011 M1 images is here:
                                      >
                                      > http://www.fototime.com/ftweb/bin/ft.dll/picture?PictId={FB5D92
                                      > 0C-F948-4AA7-8F78-77EEEE1A1AD6}&size=ORIG
                                      >
                                      > Unfiltered CCD, total exposure time 15 minutes.
                                      >
                                      > Two others were taken by Michael Jaeger and posted to the comet-images
                                      > group, but they seem no longer to be there. I will e-mail those to you
                                      > privately.
                                      >
                                      > The C/2011 A3 images were e-mailed to me privately, and I will have to
                                      > askpermission to forward them along.
                                      >
                                      >
                                      > Sincerely,
                                      >
                                      > Alan
                                      >
                                      >
                                      >
                                      >
                                      > ------------------------------------
                                      >
                                      > Comet Observations List: http://groups.yahoo.com/group/CometObs/Comet Images
                                      List: http://groups.yahoo.com/group/Comet-Images/
                                      > NOTICE: Material quoted or re-posted from the Comets Mailing List should be
                                      > indicated by:
                                      > Comets Mailing List [date]
                                      > http://www.yahoogroups.com/group/comets-mlYahoo! Groups Links
                                      >
                                      > To visit your group on the web, go to:
                                      > http://groups.yahoo.com/group/comets-ml/
                                      > Your email settings:
                                      > Individual Email | Traditional
                                      >
                                      > To change settings online go to:
                                      > http://groups.yahoo.com/group/comets-ml/join(Yahoo! ID required)
                                      >
                                      > To change settings via email:
                                      > comets-ml-digest@yahoogroups.com comets-ml-fullfeatured@yahoogroups.com
                                      > To unsubscribe from this group, send an email to:
                                      > comets-ml-unsubscribe@yahoogroups.com
                                      > Your use of Yahoo! Groups is subject to:
                                      > http://docs.yahoo.com/info/terms/
                                      >
                                      >
                                      >

                                      Hol Dir das Notizbuch von uni.de: Schreibe dazu einen Artikel zum aktuellen Monatsthema und mit etwas Glück wird dein Artikel veröffentlicht. Alle Infos hier: http://uni.de/redaktion/nachwuchsjournalisten-gesucht
                                    • dfischer@astro.uni-bonn.de
                                      Wow, I m deeply impressed by the enthusiasm of the debaters in the Elenin et al. issue - have there ever been postings that long and detailled? To digest them
                                      Message 18 of 24 , Oct 29, 2011
                                        Wow, I'm deeply impressed by the enthusiasm of the debaters in the Elenin
                                        et al. issue - have there ever been postings that long and detailled? To
                                        digest them all would take more time than most probably have, so: Is there
                                        a kind - and impartial - soul who would take up the task to condense it
                                        all into one (open access) article one could then point others to?

                                        The question of what the - trained, experienced amateur's - eye can do
                                        with extended low surface brightness sky objects comes up quite
                                        frequently, esp. among visual deep sky enthusiasts, and I've witnessed
                                        pretty aggressive debates between some observers and their doubters.
                                        Perhaps deepening our understanding here a bit would be a fine legacy of
                                        comet Elenin, living longer than its dusty remains ...

                                        Dan

                                        http://cosmos4u.blogspot.com/2011/10/bright-spot-emerges-on-uranus-cam.html
                                        (see 2nd graf)
                                        http://www.oculum.de/newsletter/astro/100/40/8/148.ln1in.asp#2
                                        http://cosmos4u.blogspot.com/2011/10/comet-elenin-gone-for-good-though-some.html
                                      • Alan Hale
                                        Dear Juan, and all, I think I probably agree with Juan on one thing, i.e., the folks here at comets-ml may be getting a bit tired of the discussion, and one
                                        Message 19 of 24 , Oct 29, 2011
                                          Dear Juan, and all,

                                          I think I probably agree with Juan on one thing, i.e., the folks here at
                                          comets-ml may be getting a bit tired of the discussion, and one can only say
                                          the same things so many times before its gets tiresome. So this will likely
                                          be my last post on the subject, and to keep it at a reasonable length I will
                                          address one of Juan's points, and then share another one of my boring
                                          stories for the general community to ponder.


                                          Juan, you asked what the responses would be like if the roles were to be
                                          reversed. I'm not sure that's a valid question, because the roles would not
                                          be reversed in any strict sense.

                                          At the risk of belaboring something that's already been covered: I am
                                          confident enough in my observing skills that, if I were to report that there
                                          was a 9th-magnitude comet in a certain location, then everyone in this group
                                          -- not just some people, but *everyone* -- who had the necessary sky and
                                          weather conditions, any kind of decent telescope, and any observing
                                          experience whatsoever, could turn their telescopes to that location, and see
                                          a relatively bright comet that would be somewhere near 9th magnitude. Anyone
                                          on this group with imaging capability would record a relatively bright comet
                                          that would be consistent with a visual brightness of 9th magnitude. I
                                          wouldn't have to offer any hand-waving explanations as to why experienced
                                          members of this group couldn't see anything and why long-exposure CCD images
                                          only recorded an extremely faint and barely detectable smudge of light.

                                          As far as the C/2010 X1 remnant goes, all individual considerations aside,
                                          given all the evidence up until that point that nothing of this comet had
                                          survived, this was, at face value, an extraordinary observation. If I in
                                          fact had then seen something, even if I was sure I was seeing it -- which I
                                          highly doubt, since even though I have now "observed" this remnant on three
                                          different mornings, it is such a borderline detection that, in the absence
                                          of any recent images, I'm still not 100% sure I've seen anything -- I would
                                          not have posted on this group "I got it! I got it!" At most, I would have
                                          posted something like "I think I might have something here; could those of
                                          you with imaging capability try to confirm it?" I may not have posted
                                          anything at all, but rather privately e-mailed those who I know have the
                                          capability to confirm (or dis-confirm) my observation and awaited their
                                          results before going public.

                                          In fact, the face value nature of the observation was so extraordinary that
                                          I probably would have had an attitude something like the physicists from
                                          CERN who recently reported that they apparently detected neutrinos traveling
                                          faster than lightspeed. Note that they have openly invited skepticism and
                                          scrutiny of their results.

                                          I will end this segment of my post with a plea to those of you with imaging
                                          capability: I really would like to see more images!! Especially if you took
                                          any on the mornings of October 24, 26, and/or 28, so I can compare them with
                                          what I think I detected visually.


                                          And, now, to my story . . . I have debated with myself whether or not to
                                          share this with the group, and if so, when . . . but now I think the
                                          occasion is appropriate. Members of the group can take this however they
                                          wish.

                                          There was a time, back some years ago, when a somewhat younger version of
                                          myself was zealously and enthusiastically trying to track down as many
                                          comets as I could. I was using a 41 cm reflector (not the one I have now),
                                          and "seeing" everything I looked for, and submitting all my obsevations to
                                          the Central Bureau. It turns out that some of my submitted observations were
                                          rather "interesting" and "unusual," as they involved comets that had
                                          seemingly disintegrated before the times of my observations, and that some
                                          of my other observations were not consistent with what other, more
                                          experienced, observers were reporting.

                                          I received a letter -- this was in the days before e-mail -- from none other
                                          than Brian Marsden, who pointed out these facts to me, and who politely, but
                                          nevertheless sternly, suggested that I was trying too hard to "see" comets
                                          at the limit of my telescope. He politely, but nevertheless sternly,
                                          recommended that I exercise a little more care and caution in obtaining my
                                          observations.

                                          There were essentially two ways I could have responded. My initial
                                          temptation was to write Brian back and adamantly maintain that I had indeed
                                          seen all the comets I had reported, and provide some "explanations" as to
                                          why other observers weren't seeing these comets -- and then continued on
                                          doing what I was doing.

                                          But there was a second course of action I could have pursued. I could
                                          carefully consider what Brian had written, and carefully re-evaluate the
                                          observations I had made and submitted. Doing so would force me to have to
                                          admit that, yes, I had indeed been trying too hard to "see" comets that
                                          really weren't there (and among other things, this would require the removal
                                          of a couple of comets from my tally, which I was already keeping). Doing so
                                          would also require me to carefully redouble my efforts to ensure that any
                                          future observations I made and submitted were valid.

                                          I chose the second course of action, and wrote a letter back to Brian
                                          admitting my errors, and promising to exercise more care and caution in the
                                          future. As a result:

                                          A) I received a second letter from Brian. This one was also polite, but was
                                          not stern. Instead, it was warm and friendly, and he warmly and strongly
                                          encouraged me to continue making and submitting observations.

                                          B) I held true to my word, and from that point on did everything I could to
                                          ensure that I was making the best possible observations I could make, and no
                                          longer "seeing" comets just because I wanted to see them.

                                          C) Within a few years I had earned Brian's trust to the point where I was
                                          being asked to confirm comet discoveries for the Central Bureau.

                                          I somehow don't think these would have happened if I had decided to pursue
                                          the first course of action . . .


                                          Before closing, I would be remiss if I didn't mention that I had the good
                                          fortune to have frequent observing sessions with Charles Morris during the
                                          2 1/2 years we lived and worked near each other in southern California. We
                                          observed a lot of comets together; yes, we competed with each other, and,
                                          yes, we sometimes disagreed with and argued with each other, but I learned a
                                          tremendous amount from him, and there is no question that all those joint
                                          observing sessions in the southern California mountains during those 2 1/2
                                          years played a significant and huge role in helping to make me the
                                          experienced comet observer that I would like to think I am today.


                                          Sincerely,

                                          Alan
                                        • Josef Müller
                                          ... I took an image in the morning (at 2:45h UT) of Oktober 24. (SBIG ST-10XME - 130 mm Astrophysics f/6,4 - 4 exposures à 300 s)
                                          Message 20 of 24 , Oct 29, 2011
                                            Am 29.10.2011 19:38, schrieb Alan Hale:
                                            >
                                            > Dear Alan,
                                            >
                                            I took an image in the morning (at 2:45h UT) of Oktober 24.
                                            (SBIG ST-10XME - 130 mm Astrophysics f/6,4 - 4 exposures à 300 s)

                                            http://www.westerwald-astro.de/jjm/kometen/elenin-remnant.php?BackPath=kometen

                                            I hope I can help you!

                                            Sincerely,

                                            Josef Müller A21



                                            [Non-text portions of this message have been removed]
                                          • Alan Hale
                                            Dear Josef, Thank you! -- that does help quite a bit. Your image was taken a few hours before my observation, but the displacement of the brightest portion of
                                            Message 21 of 24 , Oct 29, 2011
                                              Dear Josef,

                                              Thank you! -- that does help quite a bit. Your image was taken a few hours
                                              before my observation, but the displacement of the brightest portion of the
                                              tail with respect to the comet's ephemeris position, as well as the location
                                              of the tail's northern "edge," is consistent with what I was seeing.


                                              Sincerely,

                                              Alan
                                            • Jakub Černý
                                              Dear Alan and all, This message is not adressed to you or JJ s or someone else, its simply to all of us (including me). Everyone have different motivation of
                                              Message 22 of 24 , Oct 29, 2011
                                                Dear Alan and all,



                                                This message is not adressed to you or JJ's or someone else, its simply to
                                                all of us (including me).



                                                Everyone have different motivation of why doing comet observations. Some
                                                people like to see their names, others want as much comets as possibile. I
                                                guess everyone have different story. Mine is that I have done my first comet
                                                light curve analysis half year before first visual estimate, with
                                                calculator, pencil and paper. That motivated me to observe comets even for
                                                me it is not the main thing. I more likely analyzing data then observing,
                                                however life at computer on seat is quite boring and I would die if I would
                                                not be in contact with dark starry nights. But still my interest are most
                                                accurate data to analyze, because without them, my work simply have no
                                                point. But after 13 years of observing of almost 100 comets (without thoose
                                                what I did not see, yes I also have some negative observations), my
                                                experience tells me that everyone doing mistakes, including very experienced
                                                observers. Where does new observers missing experience, the old one thinks
                                                that they never can be wrong.



                                                The visual observations will always contain bad observations, because it is
                                                not very accurate and depends on observer, instrument, conditions and
                                                technique! So much can do a big mess, thats why analysis can be serious only
                                                if it contains a hughe number of observations. That is also reason why,
                                                while I planning observations, I divide comets on thoose, who are bright and
                                                good visible and the others thats near or under visible limit. The second
                                                group I observing only sometimes. I know it can contain larger errors, thats
                                                why I observing theese comets for example once or twice per month. Observing
                                                brightest comets everyday is really boring.



                                                As I am working with visual and CCDs data on daily basis, I do not see
                                                anything surprising on thoose cases discussed. It is pretty normal that in
                                                cases of large diffuse comets, that the magnitude spread of visual observers
                                                can reach even 3 magnitudes. Also it is usual, that in case of condensed
                                                comets the CCDs giving usually 1 mag fainter results then visual observers,
                                                in the case of large diffuse comets, the difference is much larger. There is
                                                also extremely difference when people doing "photometry" from astrometric
                                                images. These must be done without saturated center of comets and result is
                                                large loose of whole coma. Usually the astrometric magnitudes can produce
                                                even 8 mag difference!





                                                Because theese threads consumed so much time already I deciede to do some
                                                deeper investigation. How does that observations fit the theory.



                                                C/2011 A3 (Gibbs): I have found comets with most similar activity in near
                                                same heliocentric distance (during its brightening) to be C/1998 T1, C/2003
                                                T4 and C/2005 E2 with coma diameter 190 000, 150 000 and 90 000 km. With
                                                median at 150 000.



                                                The expected visible coma diameter according to this value gives 1.2' at
                                                beginning of August and 1.1' at its end. With r ~ 2.7 AU, there is not
                                                expected strong water production and the coma should be dominated with dust
                                                with continuum peaking in red, favourable for CCD chips with best QE in red
                                                or infrared. There is nothing excceding this values on many CCDs
                                                http://astrosurf.com/cometas-obs/C2011A3/fotos.htm .



                                                C/2011 M1 (LINEAR): I have found comets with most similar level of activity
                                                in near same heliocentric distance (during its brightening) to be C/1998 U5,
                                                C/1999 Jc, C/2011 OG108 and C/2007 W1 theese comets have max. coma diameter
                                                275 000, 290 000, 100 000 and 400 000 km. with median at 282 500 km.



                                                The expected visible coma diameter acording to this value it gives 5.1' at
                                                beginning of August and 4.4' at its end. With r ~ 1 AU the comet should be
                                                gas rich, therefore with magnitude peak in green, favourable for visual
                                                observers. CCD observations confirming that inside coma continuing very
                                                small activity of comet which may feed its coma with gas
                                                (http://astrosurf.com/cometas-obs/C2011M1/fotos.htm ). However I suppose
                                                further imagining of this comet using small apertures with low f/ and try
                                                remove stars from background in order to improve S/N ratio.





                                                Conclusions are clearly for me, the visual observations of C/2011 A3 (Gibbs)
                                                with coma larger then 1' are theoreticly unreal (6' coma would need coma on
                                                diameter near 800 000 km!). However the visual observing of C/2011 M1 with
                                                large diffuse coma fitting theory well, maybe the observations excceding 5'
                                                coma diameter may cause small overestimate of its m1.



                                                Best regards,

                                                Jakub Cerny



                                                From: comets-ml@yahoogroups.com [mailto:comets-ml@yahoogroups.com] On Behalf
                                                Of Alan Hale
                                                Sent: Saturday, October 29, 2011 7:39 PM
                                                To: comets-ml@yahoogroups.com
                                                Subject: Re: [comets-ml] C/2010 X1 remnant, and other things.





                                                Dear Juan, and all,

                                                I think I probably agree with Juan on one thing, i.e., the folks here at
                                                comets-ml may be getting a bit tired of the discussion, and one can only say
                                                the same things so many times before its gets tiresome. So this will likely
                                                be my last post on the subject, and to keep it at a reasonable length I will
                                                address one of Juan's points, and then share another one of my boring
                                                stories for the general community to ponder.

                                                Juan, you asked what the responses would be like if the roles were to be
                                                reversed. I'm not sure that's a valid question, because the roles would not
                                                be reversed in any strict sense.

                                                At the risk of belaboring something that's already been covered: I am
                                                confident enough in my observing skills that, if I were to report that there
                                                was a 9th-magnitude comet in a certain location, then everyone in this group
                                                -- not just some people, but *everyone* -- who had the necessary sky and
                                                weather conditions, any kind of decent telescope, and any observing
                                                experience whatsoever, could turn their telescopes to that location, and see
                                                a relatively bright comet that would be somewhere near 9th magnitude. Anyone
                                                on this group with imaging capability would record a relatively bright comet
                                                that would be consistent with a visual brightness of 9th magnitude. I y
                                                wouldn't have to offer any hand-waving explanations as to why experienced
                                                members of this group couldn't see anything and why long-exposure CCD images
                                                only recorded an extremely faint and barely detectable smudge of light.

                                                As far as the C/2010 X1 remnant goes, all individual considerations aside,
                                                given all the evidence up until that point that nothing of this comet had
                                                survived, this was, at face value, an extraordinary observation. If I in
                                                fact had then seen something, even if I was sure I was seeing it -- which I
                                                highly doubt, since even though I have now "observed" this remnant on three
                                                different mornings, it is such a borderline detection that, in the absence
                                                of any recent images, I'm still not 100% sure I've seen anything -- I would
                                                not have posted on this group "I got it! I got it!" At most, I would have
                                                posted something like "I think I might have something here; could those of
                                                you with imaging capability try to confirm it?" I may not have posted
                                                anything at all, but rather privately e-mailed those who I know have the
                                                capability to confirm (or dis-confirm) my observation and awaited their
                                                results before going public.

                                                In fact, the face value nature of the observation was so extraordinary that
                                                I probably would have had an attitude something like the physicists from
                                                CERN who recently reported that they apparently detected neutrinos traveling
                                                faster than lightspeed. Note that they have openly invited skepticism and
                                                scrutiny of their results.

                                                I will end this segment of my post with a plea to those of you with imaging
                                                capability: I really would like to see more images!! Especially if you took
                                                any on the mornings of October 24, 26, and/or 28, so I can compare them with
                                                what I think I detected visually.

                                                And, now, to my story . . . I have debated with myself whether or not to
                                                share this with the group, and if so, when . . . but now I think the
                                                occasion is appropriate. Members of the group can take this however they
                                                wish.

                                                There was a time, back some years ago, when a somewhat younger version of
                                                myself was zealously and enthusiastically trying to track down as many
                                                comets as I could. I was using a 41 cm reflector (not the one I have now),
                                                and "seeing" everything I looked for, and submitting all my obsevations to
                                                the Central Bureau. It turns out that some of my submitted observations were
                                                rather "interesting" and "unusual," as they involved comets that had
                                                seemingly disintegrated before the times of my observations, and that some
                                                of my other observations were not consistent with what other, more
                                                experienced, observers were reporting.

                                                I received a letter -- this was in the days before e-mail -- from none other
                                                than Brian Marsden, who pointed out these facts to me, and who politely, but
                                                nevertheless sternly, suggested that I was trying too hard to "see" comets
                                                at the limit of my telescope. He politely, but nevertheless sternly,
                                                recommended that I exercise a little more care and caution in obtaining my
                                                observations.

                                                There were essentially two ways I could have responded. My initial
                                                temptation was to write Brian back and adamantly maintain that I had indeed
                                                seen all the comets I had reported, and provide some "explanations" as to
                                                why other observers weren't seeing these comets -- and then continued on
                                                doing what I was doing.

                                                But there was a second course of action I could have pursued. I could
                                                carefully consider what Brian had written, and carefully re-evaluate the
                                                observations I had made and submitted. Doing so would force me to have to
                                                admit that, yes, I had indeed been trying too hard to "see" comets that
                                                really weren't there (and among other things, this would require the removal
                                                of a couple of comets from my tally, which I was already keeping). Doing so
                                                would also require me to carefully redouble my efforts to ensure that any
                                                future observations I made and submitted were valid.

                                                I chose the second course of action, and wrote a letter back to Brian
                                                admitting my errors, and promising to exercise more care and caution in the
                                                future. As a result:

                                                A) I received a second letter from Brian. This one was also polite, but was
                                                not stern. Instead, it was warm and friendly, and he warmly and strongly
                                                encouraged me to continue making and submitting observations.

                                                B) I held true to my word, and from that point on did everything I could to
                                                ensure that I was making the best possible observations I could make, and no
                                                longer "seeing" comets just because I wanted to see them.

                                                C) Within a few years I had earned Brian's trust to the point where I was
                                                being asked to confirm comet discoveries for the Central Bureau.

                                                I somehow don't think these would have happened if I had decided to pursue
                                                the first course of action . . .

                                                Before closing, I would be remiss if I didn't mention that I had the good
                                                fortune to have frequent observing sessions with Charles Morris during the
                                                2 1/2 years we lived and worked near each other in southern California. We
                                                observed a lot of comets together; yes, we competed with each other, and,
                                                yes, we sometimes disagreed with and argued with each other, but I learned a
                                                tremendous amount from him, and there is no question that all those joint
                                                observing sessions in the southern California mountains during those 2 1/2
                                                years played a significant and huge role in helping to make me the
                                                experienced comet observer that I would like to think I am today.

                                                Sincerely,

                                                Alan





                                                [Non-text portions of this message have been removed]
                                              • Alan Hale
                                                Dear Jakub, While I had stated earlier that I had probably made my last post on the ... As I have repeatedly stated, I am not infallible, and I am constantly
                                                Message 23 of 24 , Oct 29, 2011
                                                  Dear Jakub,

                                                  While I had stated earlier that I had probably made my last post on the
                                                  subject, I can't let this slide:

                                                  > experience tells me that everyone doing mistakes, including very experienced
                                                  > observers. Where does new observers missing experience, the old one thinks
                                                  > that they never can be wrong.
                                                  >

                                                  As I have repeatedly stated, I am not infallible, and I am constantly aware
                                                  of that when I make my observations. I posted earlier today an account of
                                                  some serious observing errors I once made, and I can post quite a few more
                                                  accounts if people would like.

                                                  But that being said, after four decades of visual comet observations, (with
                                                  a few years of CCD observing thrown in as well), and over two decades of
                                                  that chasing down comets to confirm for the Central Bureau, there is simply
                                                  *no* way that I can be looking *directly* at a 9th, 10th, 11th magnitude
                                                  comet, and *not* see it.


                                                  Sincerely,

                                                  Alan
                                                Your message has been successfully submitted and would be delivered to recipients shortly.