Re: [APML] Gradient and Vignette removal software

["John C. Mirtle" <jmirtle@shaw.ca>]

Jerry;
    You are right - this isn't the perfect answer due to film's non-linear
response over time, and it won't touch gradients. It might be possible that
adjusting curves or contrast to the control layer might help offset the
effect of reciprocity. Or not, but I do plan to try this. This has to be
more accurate and easier that manually removing stars from an image,
removing any DSO's, applying a massive blur, inverting and applying as a
control layer which is how I do it now. With unfiltered Schmidt shots -
which Chris does - exposures are short, generally under 10 minutes. If all
goes well, I will try this out on the weekend.

John Mirtle
Calgary, Ab. Canada

----- Original Message -----
From: "Jerry Lodriguss" <jml@astropix.com>
To: <astro-photo@seds.org>
Sent: Thursday, June 27, 2002 12:48 AM
Subject: Re: [APML] Gradient and Vignette removal software


>
> 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



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