Re: [APML] Gradient and Vignette removal software

[Matt BenDaniel <matt@starmatt.com>]

Chris,

I'm of the opinion that deep sky AP on film cannot be optimally AV'ed by any automated procedure. As Jerry mentioned, this is primarily due to vignetting and differential reciprocity failure, but also film is subject to uneven illumination from sky glow, humidity, development, etc. I believe that an automated method could bring an image closer to being "flat" (in the sense of a flat field), but that a capable human can always improve on the process by manual tweeks. BTW typically the wider the field of view, the more difficult it is to do the AV.

For example, here is an image that I think is well AV'ed:
http://world.std.com/~mattb/gallery/astro/mwrising.html

Here is the hand-drawn AV layer for that image:
http://world.std.com/~mattb/articles/02041413av.jpg

Matt

At 01:10 AM 6/27/2002 -0600, John C. Mirtle wrote:
>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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--
Matt BenDaniel
matt@starmatt.com
http://starmatt.com


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