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

After posting that chart to my site, I was contacted by another  
observer who pointed me to a couple images where the annular  
structure is definitely not an illusion, and sampling the images  
showed that the ring was definitely brighter. However, those images  
also had been strongly processed to bring out subtle structures  
around the pseudonucleus. I believe the processing generated a false  
annulus. I know that unsharp masking in photoshop emphasizes edges by  
further darkening the dark side of the edge and lightening the light  
side of the edge. That would be the perfect recipe for processing a  
false ring into the comet. I don't know enough about wavelet  
processing to know if it works in a similar way, but I suspect it does.

So I generated a fake comet in Photoshop that possesses the same  
basic characteristic as 17P / Holmes and then ran wide-radius unsharp  
masking on it. The results were that it added a false ring to the  
outside edge. So, I think it's something to keep in mind. Here is a  
link to my post showing the results of that test:

http://www.perezmedia.net/beltofvenus/archives/000746.html

Let me know if I've missed something important in my line of thinking  
there.

Jeremy
http://beltofvenus.perezmedia.net

On Oct 28, 2007, at 4:14 PM, Jeremy Perez wrote:

> 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
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