[AZ-Observing] Re: 17/P Holmes Annular or Not?
- From: Jeremy Perez <jeremy@xxxxxxxxxxxxxx>
- To: az-observing@xxxxxxxxxxxxx
- Date: Sun, 28 Oct 2007 20:47:20 -0700
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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