[APML] RE: OT: Lunar Mosaic

[Juan Conejero <skycad@ctv.es>]

Thanks Jeff,

Photoshop's unsharp mask is basically a high-pass filter. It can enhance 
small-scale contrast to provide a "refocusing" simulation with indeed very 
good results from an aesthetical point of view. Deconvolution is a 
conceptually different process. It is a more physically justified 
procedure. In simple terms, we can consider an observed image I as the 
result of the following expression:

I = O*P + N

where I is the image we actually have, O is the real, unaltered image, P is 
a function representing changes introduced by the atmosphere, imperfect 
optics, etc., and N is an additive noise term. P is usually known as the 
instrumental function or point-spread function, PSF. The symbol * stands 
for convolution, a mathematical operation which adapts one function's shape 
to another's. Deconvolution is precisely the inverse operation: since we 
already have I, knowing P and N with extreme accuracy is what we only need 
to solve the above equation for O, which sure is the best image of our 
lives! Well, not exactly. We can model N in more or less statistically 
correct terms, but finding P is another question. Furthermore, unmodeled 
noise is a severe limit. Nothing is perfect, but deconvolution algorithms 
actually do work. Take a look at the following image we obtained a few days 
ago:

http://www.pleiades-astrophoto.com/software/experiments/deconvolution/jupiter-VC/35.jpg

It's the integration of approx. 350 video frames. Now after Van Cittert's 
deconvolution:

http://www.pleiades-astrophoto.com/software/experiments/deconvolution/jupiter-VC/35-VC.jpg

For deep-sky work there are more appropriate algorithms, such as 
Richardson-Lucy. But deconvolution is best suited for CCD images, since 
algorithms are extremely sensitive to input S/N ratio.

Hope this helps,
______________________________________
Juan Conejero, Pleiades Astrophoto
skycad@ctv.es
http://www.pleiades-astrophoto.com/en.html

At 22:14 13/03/2003 -0500, you wrote:
>Hello Juan,
>That is big and sharp.  I have a question.  What (in simple terms) is the
>difference between Photoshop sharpening/unsharp mask and deconvolution in
>other programs?  Thanks.
>
>Jeff Ball
>www.astro-photography.com


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