Re: [APML] Gradient and Vignette removal software

[Jerry Lodriguss <jml@astropix.com>]

Hi John, Chris,

This method might work for very short exposures, but I'm not sure it will 
for long deep-sky exposures, unless your flat field frame is the same 
exposure length as the real deep-sky exposure.

The long exposure will suffer from differential reciprocity failure due to 
the vignetting, whereas the short exposure won't, or it will be different.

This method also won't work if any kind of brightness gradient is present 
in the sky during the real exposure.

It might get you close though, more or less.  It's worth a try.

Jerry





At 11:19 PM 6/26/2002 -0600, you wrote:
>Chris;
>     I have been meaning to try this with my telephotos and Schmidts, but 
> haven't got around to it yet. Grab a clean white T-shirt and wrap a few 
> layers of it around the front of the Schmidt. (or telephoto)  Install 
> your filters, focus telephotos to infinity and shoot a cloudy sky, a 
> blank wall, something to give an even amount of light. Bracket exposures. 
> This should give a nice smooth image of nothing, but will show the 
> vignetting pattern. Scan the full negative, invert and apply as a 
> luminance control layer in Photoshop. At some intensity setting, I am 
> guessing that this will cancel out vignetting. Perhaps the gradient in 
> MaxIm will work better when it only has one defect to worry about.
>     I am guessing that this would work if you scan the full negative, and 
> apply the correction to a full negative scan of an image. Cropping would 
> change the results. I guess that this would be the film equivalent of a 
> flat field in CCD work. Thoughts???
>
>John Mirtle
>Calgary, Ab. Canada
>(G8 country!)
>>----- Original Message -----
>>From: <mailto:cschur@cybertrails.com>Chris Schur
>>To: <mailto:astro-photo@seds.org>APML
>>Sent: Wednesday, June 26, 2002 10:56 PM
>>Subject: [APML] Gradient and Vignette removal software
>>
>>Im looking for a simple to use piece of software that removes gradients 
>>and vignetting from my schmidt camera images with film.  Ive tried Maxim 
>>DL, and I get horrible results, and wondered if there is something I can 
>>say put a grid on and it does what it has to do to flatten the brightness 
>>of the grid.  Maybe Im overlooking some thing obvious, but doing this in 
>>photoshop is a gamble at best.
>>
>>
>>Chris Schur
>>
>>Astrophotography: 
>><http://www.psiaz.com/schur/astro/index.html>http://www.psiaz.com/schur/as 
>>tro/index.html
>>


Astronomical photography: http://www.astropix.com



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