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

That's a great analysis Tom! It's funny, I had done the same thing,  
but by measuring from another photographer's image (I was waiting for  
his permission to post the image). That rough analysis confirms what  
you noted -- that the annular appearance is an illusion. I guess that  
plateau in brightness fools the eye and brain into seeing weaker  
luminosity perhaps because we expect the increase in brightness to be  
more steady? It is a fascinating illusion though.

I've got those measurements (minus the reference image) charted here:

http://www.perezmedia.net/beltofvenus/archives/000745.html

Jeremy

On Oct 28, 2007, at 2:22 PM, 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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