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[Photoshop] Maintaining Color Balance without a background space reference (ie an image completely filled with nebulosity)

22 Jan, 2017 00:39
Hey all, I've been working on an f/6 Orion set, meaning that I can get M43 and only most of M42 in frame - and as a result of the exposure times, the entire image has visible banding of nebulosity.

Ive been using the color sampler tool in Photoshop to keep track of color in parts of the image, but I am somewhat limited in maintaining the background color. The closest method I've found is to sample the darkest parts of the nebulosity, but I know there's still interstellar medium there because it appears when the curves are severely stretched.

I can see some color changes image-wide but the sampler has the advantage of seeing any single change in RGB for 256 values, which is finer than I could consciously notice. Are there other methods of finding the black / neutral point? Is there way to sample an entirely different image and then apply that white balance to the Orion image?
Edited 23 Jan, 2017 21:47
23 Jan, 2017 15:39
I usually use a 32x32 or 51x51 average pixel sample (on a known white or black area) and use Photoshop > Image > Adjustments > Color Balance > Shadows (black) or Highlights (white).

I also use…

1.  Photoshop > DeepSkyColors > WhiteCal
2. Pixinsight > ColorCalibration

#2 may give you the best control, but the others work well too.
23 Jan, 2017 22:46
Had not heard of that PS plugin before - it will take some tweaking but it gives me promising results. I also had not thought of simply using a larger sample size, that will probably be simplest.
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