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Re: 17P recent visual/CCD appearance

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  • cnj999
    ... Lehman ... February. ... a ... the c.cond. ... observation on ... general ... Expanding on Alfredo s post, I would note that while the condensation
    Message 1 of 7 , Mar 9, 2008
      --- In comets-ml@yahoogroups.com, Alfredo Pereira <alfxyz@...> wrote:
      >
      > --
      > Greetings All,
      >
      > Recently Richard Miles called attention here to CCD images by Fred
      Lehman
      > (Davie, Florida) highlighting the changing morphology of 17P during
      February.
      > I also mentioned a change in visual appearance. Richard suggested
      a
      > geometry/projection effect which is supported by the centering of
      the c.cond.
      > plus the changes in visual appearance.
      >
      > Well, geometry/projections change with time, and my visual
      observation on
      > March 5.86 UT hinted at the coma becoming more extended in the
      general
      > westwards direction as viewed in 14x100 binoculars. I sent this on
      > March 5th evening to the ICQ and Astrosite Groningen (where the
      > full descriptive note is posted).
      >

      Expanding on Alfredo's post, I would note that while
      the "condensation" currently occupies a generally more central
      location within the coma than previously, it's placement is still
      decidedly eccentric (toward about PA300 degrees) relative to the
      position of the pseudo-nucleus. It has also transformed from a highly
      elongated mass to a more circular one, undoubtedly due to our
      changing perspective. However, this condensation does remain rather
      noticeably offset from the coma's actual center in recent deep
      images, toward the direction of the most ill-defined portion of the
      outer coma.

      Perhaps exemplifying my original comment that the softer or "open"
      side of the coma, seen best earlier in the apparition, did not
      actually represent a rudimentary tail in the normal sense, is that
      feature's current presentation. On March 5th, the most diffuse
      portion of the coma's perimeter was centered roughly around PA 300
      degrees, whereas the anti-solar direction, that expected for a normal
      ion tail, was closer to PA075 degrees. Even a rather heavy dust tail
      would not readily be expected to lag the ERV by so much, suggesting
      instead, its origins in an explosive or eruptive discharge occurring
      in a preferential direction.

      Finally, I would add that the present apparent diameter of this
      object on deep images seems to be about 75'. This is significantly
      larger than some have reported it to be visually since the beginning
      of the latest moonless period. This would necessarily compromise the
      accuracy of m1 being reported recently if the differences become too
      great.

      JBortle
    • Alfredo Pereira
      ... Indeed, John. It s quite compelling evidence for both directionality and abnormally high initial impulse. ... I would just add that for the few
      Message 2 of 7 , Mar 9, 2008
        >cnj999 wrote:
        > On March 5th, the most diffuse
        > portion of the coma's perimeter was centered roughly around PA 300
        > degrees, whereas the anti-solar direction, that expected for a normal
        > ion tail, was closer to PA075 degrees. Even a rather heavy dust tail
        > would not readily be expected to lag the ERV by so much, suggesting
        > instead, its origins in an explosive or eruptive discharge occurring
        > in a preferential direction.

        Indeed, John. It's quite compelling evidence for both directionality and
        abnormally high initial impulse.

        > Finally, I would add that the present apparent diameter of this
        > object on deep images seems to be about 75'. This is significantly
        > larger than some have reported it to be visually since the beginning
        > of the latest moonless period. This would necessarily compromise the
        > accuracy of m1 being reported recently if the differences become too
        > great.

        I would just add that for the few observations that have been posted since
        last Full Moon, in most if not all instances outside the 60'-80' range,
        the observer noted either poor conditions or other factors affecting the
        observation.

        I for one, could observe only on 6 occasions this dark run, and recorded
        the coma between 60-80' in the 5x22 monocular (similar in 14x100B), except
        on one of the nights when it was on top of a 6th mag star, and I couldn't
        make out more than 36' ! I simply took note of the fact that the star
        seriously hampered the estimate. A mistake would be to make undue effort
        to see the larger coma I knew was probably still there, but given the
        temporary appulse to the star I could not see.

        I trust that observers will still report what they see with their
        instruments/conditions, regardless of what CCD images (or other observations,
        including the observer's own estimates made the previous night) show.

        There are many reasons why the coma as seen visually would differ somewhat
        from CCD images. Besides, opera glasses, monoculars, binoculars, will all
        show different morphologies/diameters. The CCD appearance too is dependent
        on chip, exposure, processing..

        But yes, if an observer is aware of and alert to potential biasing problems,
        it is a seasoned observer, is confident and understands what he/she is doing,
        then it can be helpful to know what CCD images are showing. In that way one
        can get a feeling for potential problems like inadequate sky/instrumentation
        if the observed diameter greatly differs from concurrent CCD images. But the
        final word is always: record what you see, not what you expect to see.

        Further, from m1 estimates done at several occasions by defocusing stars to
        the extremes of 17P diameter uncertainty, I have found somewhat less impact
        on m1 (+-0.2 mag in the worse occasions) than what I expected, given the
        coma's low DC. But one thing is what I expect, another is what I get by
        actual careful observation.

        Cheers,

        Alfredo
        --
      • Brian Cudnik
        I spotted 17P with the unaided eye last Saturday night from our club s Dark Site near Columbus, Texas. It was at about 2:00UT (March 9, 2008) when I sighted
        Message 3 of 7 , Mar 10, 2008
          I spotted 17P with the unaided eye last Saturday night from our club's Dark Site near Columbus, Texas. It was at about 2:00UT (March 9, 2008) when I sighted the comet as a vague hazy spot about the size of a Full Moon. Through the binoculars, the comet appeard as a larger hazy spot, with a noticable elongated brightening near the center. I did not see the well-defined "limb" that I have seen in the past. The object was about 1.3 X 1.0 degrees in size. The limiting magnitude of the observing site was about 6.3.

          I also saw 46P/Wirtanen, C/2008 C1 (Chen-Gao) and C/2007 W1 (Boattini) using a 17.5-inch f/4.5 Dob. Curiously, I was not able to track down C/2007 T1 (McNaught) though I was looking in the right part of the sky. 29P looked like it was very close to a bright star (based on the finder charts from the comet chasing web site) so I did not attempt it...

          Alfredo Pereira <alfxyz@...> wrote:
          --
          Greetings All,

          Recently Richard Miles called attention here to CCD images by Fred Lehman
          (Davie, Florida) highlighting the changing morphology of 17P during February.
          I also mentioned a change in visual appearance. Richard suggested a
          geometry/projection effect which is supported by the centering of the c.cond.
          plus the changes in visual appearance.

          Well, geometry/projections change with time, and my visual observation on
          March 5.86 UT hinted at the coma becoming more extended in the general
          westwards direction as viewed in 14x100 binoculars. I sent this on
          March 5th evening to the ICQ and Astrosite Groningen (where the
          full descriptive note is posted).

          Now I have just seen yet another wonderful CCD image of 17P by Rolando
          Ligustri, obtained remotely from RAS in New Mexico on March 8, 0530 UT,
          and to my eyes the westwards coma is looking rather more diffuse than the
          eastward edge.

          Cheers,

          Alfredo
          --






          [Non-text portions of this message have been removed]
        • Alfredo Pereira
          ... Hello Brian, It s a pity you forgot to mention the binoculars you used for this observation. BTW, please take care with full-quotes. Not just you, but
          Message 4 of 7 , Mar 10, 2008
            >Brian Cudnik wrote:
            > Through
            > the binoculars, the comet appeard as a larger hazy spot, with a noticable
            > elongated brightening near the center. I did not see the well-defined "limb"
            > that I have seen in the past. The object was about 1.3 X 1.0 degrees in size.

            Hello Brian,

            It's a pity you forgot to mention the binoculars you used for this
            observation.

            BTW, please take care with full-quotes. Not just you, but others as well.
            People who read the daily digest, and those who read comets-ml on the web
            (like I did for several years), find full-quotes _extremely annoying_, i.e.
            it is confusing, takes up unnecessary space, makes understanding what is
            relevant more difficult, takes time, etc.

            I estimate that more than half of the time I spent checking comets-ml
            in the past few years, has been spent because some people are not
            as careful and considerate of others as they should.

            This subject has been brought up here recently by several people, and still
            we get full-quotes, with the full text being appended to the new material
            for no reason whatsoever. If you are not replying directly to someone's
            statement then do not repeat the initial text. Notice how I did for this
            very mail ? I just included the text that pertained to my remark about
            the lack of binocular specifications. It would have been an overload to
            repeat the whole mail.

            There's quite a high percentage of interesting/quality posts on this list,
            but the ease/clarity of access to this information is at stake because of
            full-quotes.

            As a final, more _cheerful_ tone, I'll simply say that I hope that those
            making full-quotes at least pay more attention when they are observing
            than when they are sending mail ;-) The same for those so eager to post
            m1 data and just roll in the numbers, yet so reluctant to discuss the
            methods used to get those numbers ;-)

            Cheers,

            Alfredo
            --
          • David Seargent
            ... From: Brian Cudnik To: comets-ml@yahoogroups.com Sent: Tuesday, March 11, 2008 8:56 AM Subject: Re: comets-ml] 17P recent visual/CCD appearance . ...
            Message 5 of 7 , Mar 10, 2008
              ----- Original Message -----
              From: Brian Cudnik
              To: comets-ml@yahoogroups.com
              Sent: Tuesday, March 11, 2008 8:56 AM
              Subject: Re: comets-ml] 17P recent visual/CCD appearance


              .



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

              Regarding the failure to see C/2007 T1, I saw this on March 3 with a 25-cm reflector at 71X and found it very diffuse and difficult, even though the total magnitude was estimated at 10.4. The coma just seems to be melting away into the bachground sky.

              David Seargent


              [Non-text portions of this message have been removed]
            • Brian Cudnik
              Forgot about the binocular type...12x50 binoculars. The yahoo mail setting I use is to include the entire message in an un-editable box and highlighting part
              Message 6 of 7 , Mar 11, 2008
                Forgot about the binocular type...12x50 binoculars.

                The yahoo mail setting I use is to include the entire message in an un-editable box and highlighting part of it is not possible...all or nothing, which is annoying. I usually keep only the relevant parts. I like the html format because it offers greater flexibility...except for selecting the amount of included text to include in my reply. This may be what others are dealing with...

                The diffuseness of comet C/2007 T1 makes sense...it was getting rather low by the time I got to it...

                Cheers...Brian





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