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Re: [comets-ml] Re: Comet 2011 L4, Where Are The Mag Estimates ? + Poor Skies in March.

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  • Michael Boschat
    I used 7x50 binoculars last night as it cleared unexpectedly and I de-focused the field. I observed the comet at about 0040 UT and had Delta Andromeda (3.4)
    Message 1 of 17 , Mar 29, 2013
      I used 7x50 binoculars last night as it cleared unexpectedly and I
      de-focused
      the field. I observed the comet at about 0040 UT and had Delta Andromeda
      (3.4) and Pi Andromeda (4.3) in the field. To me the comet was fainter than
      Delta and very close to Pi, I gave it a mag. of +4.0. I observed from
      our apartment roof and there was lots of light pollution to be expected
      from Halifax and the comet was in the direction of our container pier.
      The tail was about 1/2 degrees long - to me.


      Clear skies
      -----------
      Michael Boschat
      Halifax Center- Royal Astronomical Society of Canada
      web page: http://www.chebucto.ns.ca/~aa063
    • Ellen Papenburg
      Hmmm, I too find the estimates too generous , because with the best imagination I only m=4.4 the most (more like 4.5) on Friday (Sorry, that is 3:00 UT
      Message 2 of 17 , Apr 1, 2013
        Hmmm, I too find the estimates too "generous", because with the best
        imagination I only m=4.4 the most (more like 4.5) on Friday (Sorry, that
        is 3:00 UT Saturday March 30) with the 15x70...

        So because I have a lower magnitude estimate than the rest of you I
        wonder... what methods are you all using? Haven't done this for a while
        so I might be "rusty", but you are supposed to use the same part of your
        retina and scope for looking at the comet and the two comparison stars
        basically all at about the same height in this case (lucky) and totally
        putting them quite a lot out of focus.

        So I wouldn't mind if our experts would shed some light on this.... for
        the sake of reaching as much accuracy as possible.

        We want to get this right!

        Thanks.

        Ellen - SW Ontario, Canada


        On 27/03/2013 9:16 AM, jbortle@... wrote:
        >
        > I must say that the number of reliable, or in fact any sort of, magnitude
        > determinations for this comet have been most disappointingly meager since
        > it entered the northern hemisphere's skies. This situation is in dramatic
        > contrast to the volume of southern hemisphere data that preceded
        > perihelion.
        > The lack of visual observations has been especially notable over the past
        > week, or so, when almost nothing in the way of reports are apparent
        > here, or
        > in the CometObs file.
        >
        > I would, however, again like to caution those who do attempt visual
        > photometric observations to employ an extra degree of care with C/2011
        > L4 so as
        > not to integrate the beginnings of this comet's bright tail into their
        > extra-focal comparisons. I feel a number may have done this recently.
        > I viewed
        > the comet last evening with large binoculars and noted the coma to be
        > significantly fainter than might have been otherwise anticipated from
        > some of
        > last week's reported sighting by others. I would note that my
        > comparison stars
        > were also nearly at the same altitude as the comet, negating virtually
        > any
        > need for arbitrary extinction corrections to be appplied.
        >
        > Given that the coma appears all but star-like (only around 3'-4' in
        > diameter) to the unaided eye and not in any way separate from the
        > tail, I am
        > particularly troubled by the apparent brightness of some of the reported
        > determinations made without instrumentation.
        >
        > J.Bortle
        >
        >
        >
        >
        > [Non-text portions of this message have been removed]
        >
        >



        [Non-text portions of this message have been removed]
      • cnj999
        Ellen, I share your concern as, just like yourself, I have almost consistently obtained somewhat lower magnitudes than those reported by many other observers
        Message 3 of 17 , Apr 2, 2013
          Ellen, I share your concern as, just like yourself, I have almost consistently obtained somewhat lower magnitudes than those reported by many other observers in recent weeks.

          I note that I have been employing the traditional Sidgwick, or In-Out method only, as both the Bobrovnikoff and Beyer methods (the out-out and way-out methods) will result in the observer integrating the bright inner tail of the comet into the extra-focal image, producing an excessive brightness estimate for the comet.

          To obtain a correct magnitude determination for bright comets like c/2011 L4 the extra-focal image of the comparison stars must equal that of the coma's in-focus diameter as measured across its latus rectum. Incorporting anything more than that into the extra-focal image will give a false brightness value, particularly in the case of a bright-tailed comet. This I demonstrated in conjunction with Comet Hale-Bopp in the 1990's with a long series of photometric measures employing all the accepted comparison methods along with a ranges of apertures.

          With the very small coma diameter exhibited by Comet PanSTARRS together with its very bright inner tail, accurate brightness estimates can prove a difficult thing to accomplish when using binoculars and quite impossible with just the unaided eye. I think this has been the source of many of the seemingly eccessively bright estimates, particularly those made immediately following the comet's perihelion without instrumentation.

          J.Bortle



          --- In comets-ml@yahoogroups.com, Ellen Papenburg <ellen@...> wrote:
          >
          >
          > Hmmm, I too find the estimates too "generous", because with the best
          > imagination I only m=4.4 the most (more like 4.5) on Friday (Sorry, that
          > is 3:00 UT Saturday March 30) with the 15x70...
          >
          > So because I have a lower magnitude estimate than the rest of you I
          > wonder... what methods are you all using? Haven't done this for a while
          > so I might be "rusty", but you are supposed to use the same part of your
          > retina and scope for looking at the comet and the two comparison stars
          > basically all at about the same height in this case (lucky) and totally
          > putting them quite a lot out of focus.
          >
          > So I wouldn't mind if our experts would shed some light on this.... for
          > the sake of reaching as much accuracy as possible.
          >
          > We want to get this right!
          >
          > Thanks.
          >
          > Ellen - SW Ontario, Canada
          >
          >
          > On 27/03/2013 9:16 AM, jbortle@... wrote:
          > >
          > > I must say that the number of reliable, or in fact any sort of, magnitude
          > > determinations for this comet have been most disappointingly meager since
          > > it entered the northern hemisphere's skies. This situation is in dramatic
          > > contrast to the volume of southern hemisphere data that preceded
          > > perihelion.
          > > The lack of visual observations has been especially notable over the past
          > > week, or so, when almost nothing in the way of reports are apparent
          > > here, or
          > > in the CometObs file.
          > >
          > > I would, however, again like to caution those who do attempt visual
          > > photometric observations to employ an extra degree of care with C/2011
          > > L4 so as
          > > not to integrate the beginnings of this comet's bright tail into their
          > > extra-focal comparisons. I feel a number may have done this recently.
          > > I viewed
          > > the comet last evening with large binoculars and noted the coma to be
          > > significantly fainter than might have been otherwise anticipated from
          > > some of
          > > last week's reported sighting by others. I would note that my
          > > comparison stars
          > > were also nearly at the same altitude as the comet, negating virtually
          > > any
          > > need for arbitrary extinction corrections to be appplied.
          > >
          > > Given that the coma appears all but star-like (only around 3'-4' in
          > > diameter) to the unaided eye and not in any way separate from the
          > > tail, I am
          > > particularly troubled by the apparent brightness of some of the reported
          > > determinations made without instrumentation.
          > >
          > > J.Bortle
          > >
          > >
          > >
          > >
          > > [Non-text portions of this message have been removed]
          > >
          > >
          >
          >
          >
          > [Non-text portions of this message have been removed]
          >
        • biver_nicolas
          I also just follow Ellen and John Bortle s remarks about reported (too bright) magnitudes of C/2011 L4. From a not so perfect sky 50km from Paris I made it at
          Message 4 of 17 , Apr 4, 2013
            I also just follow Ellen and John Bortle's remarks about reported (too bright) magnitudes of C/2011 L4. From a not so perfect sky 50km from Paris I made it at 4.9 on april 1.8 (probably a bit biased by low transmission), but looking at the beginning of the tail it did look at least as bright as the core of M31... but of course this is not the coma. And generally we also like having a magnitude estimate that gives an idea of its outgassing activity while dust tail especially when composed of dust emitted several days before provides some additional brightness not really representative of the comet actual activity. (17P was an extreme case several weeks after its outburst when outgassing had severly decreased and we were seeing a dust tail face on).

            Nicolas
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