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

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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