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Re: [APML] PixInsight questions ( was:Two New Images)
> I think you misread the tutorial. Division is used for digital CCD/DSLR >images because they are linear, but the flat should be subtracted with film >images. I would go more with the tutorial on DBE than the flat-fielding.
Hi Gary
We usually substracted the flat with film becouse, as you said, it is nonlinear. But, vignetting isn't an additive effect, it is proportional to the amount of light that gets to the film base or ccd. So, the correct way to deal with it would be always a division. Of course that we can't apply a common division with film, becouse the media represents the intinsity in a nonlinear way, so we have to manipulate the data or the application. Wei-Hao has worked on nonlinear divisions with quite good results. My solution is based on a linearization of the film data before applying the division. This is done with a gama function (a middtones transfer curve).
The main difference in the results between substracting and a modified division is that while the background is practically the same, with division you correct better the information in brighter objects, like stars, nebulae and milky way's gradientes. Also there is a better correction in the tone... just remember Wei-Hao's MW mosaic. A good comparison between both methods can be seen in my tutorial too. It is a Orion wide field. With substraction the Barnard's Loops is dimmer and the central area is wrongly lighter.
Just a final note... if you substract the "physical" effect is that you are removing the sky... then, of course the background will be uniform. But it has nothing to do with a propper image calibration, film or ccd.
Regards,
Carlos Milovic F.
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Astro & Photo - CMF
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