[AZ-Observing] Re: 17/P Holmes Annular or Not?

When I took my V band photometric image I also did a 3D profile of 
Comet Holmes.

See http://www.hposoft.com/CometHolmes.JPG

Jeff

At 14:22 -0700 10/28/2007, Tom Polakis wrote:
>In all of the views through the eyepiece, I have been convinced that 
>there is a weak, thick ring structure to the main, bright disc. 
>Properly exposed images suggest a ring as well.  In this image by 
>Jim Cormier...
>
>http://media.skyandtelescope.com/images/Holmes-1.jpg
>
>...the disc appears to be brighter from the edge inward to about 80% 
>radius, then fainter inward to 50% before becoming very bright. 
>That's a ring, right?
>
>Last night I overcame my fear of through-the-telescope imaging, and 
>put my 20D on my TV101 refractor.  An exposure of 10 seconds at ISO 
>400 was about right to begin to show the outer halo and just barely 
>not saturate the core.  That image again showed to my eyes what 
>appears to be a ring.
>
>Lacking any proper image analysis software, I used brute force in 
>Photoshop, and measured a 3x3 average pixel value in the three color 
>channels along a horizontal slice passing through the center of the 
>nebula.  They are plotted above my image of the comet here.
>
>http://members.cox.net/tpolakis/astro/holmes_photometry_20071028.jpg
>
>If it were really a ring, there would be a dip in the brightness 
>curve as you approach the center.  In fact, there is only a change 
>in slope (on the east side only) before it becomes steep again.  So 
>it appears that it is not really a ring, and there is some sort of 
>illusion at play.  It seems to have a lot to do with the very bright 
>core.  I wonder how many times I have called planetary nebulae with 
>bright central stars rings due to the same illusion.
>
>As I have already heard a couple times, "don't believe everything you see."
>
>Tom
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-- 
Jeff Hopkins
HPO SOFT
Counting Photons
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Hopkins Phoenix Observatory
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