Help removing shadow noise from night sky

Hey all,

I'm trying to process a shot of the Milky Way, except I didn't know what I was doing and I guess my original shot pool wasn't large enough, and they were all taken ISO 3200+. The composite image looks nice enough, but I have unacceptable amounts of shadow noise, and I don't know how to process it out. Any hints?

I'm talking about the blue-ish, green-ish and purple-ish hues you see in the shadows there in that 100% crop, that just appear as random splotches of color. The closest I've got is through heavy de-noising but I have to push it up so far it blurs the stars themselves too, and leaves some splotches behind anyway. I tried something like a reverse high pass filter process (the kind you use to smoothen skin) but it didn't really work.

You didn't say what you're using for editing, but here's an attempt using Photoshop Elements. All I did was add a Levels adjustment layer and push the black point way up to the right. Since all you want in this image is either black or white, that works pretty well, I think.

Nah, actually I tried that too. It seems the milky way isn't that well defined so while that works OK in the outer areas, it makes the core look weird. Also, the space between stars being pitch-black seems rather unnatural. Thanks for trying though .

Interesting! Thanks for the feedback. Hope you get it figured out so we can see your image.

I thought Rimmer's job came out quite well, considering what there was to work with. The biggest problem with your shot was the "blotch" of noise on the left side that pretty much wipes out the fainter actual stars from showing through.

The question is, did you take other photos of the scene so that you could stake/blend them for noise subtraction? If not, well, maybe a combination of what Rimmer did with some cloning to fill in that "empty space"?

I'm not an astrophotographer, so I'm just taking stabs in the dark...

I'd just select the color ranges you don't want or that you want to reduce, then just paint those colors out with a mask and some brushing.

I'd normally do this in Aperture 3, which allows you to spot pick a color, and then you can reduce the saturation of just that color channel... then just use the brush tool to paint the less saturated (or darkened) effect into the areas of the image where you want it.

An example is attached.. I basically painted in the color removal on the left side only, so you can see the difference.

tonylong wrote in post #14671756
I thought Rimmer's job came out quite well, considering what there was to work with. The biggest problem with your shot was the "blotch" of noise on the left side that pretty much wipes out the fainter actual stars from showing through.

The question is, did you take other photos of the scene so that you could stake/blend them for noise subtraction? If not, well, maybe a combination of what Rimmer did with some cloning to fill in that "empty space"?

The lighter patch there you mean? That's actually a cloud passing through (though it does have its own noise issues, but since it blocks the stars I can probably paint over/ blur/ denoise like its 1999).

Here's the thing: that image you see is already the composite of 7 exposures and one dark frame (merged with DSS), and one pass of Noise Ninja. However, I didn't know about DSS when I took those shots, so they're all between 10-20 seconds, f/2.8 and ISO 3200. Had I known about that technique beforehand, I'd just have taken some 30 5-second shots at lower ISOs, but now it's too late to go back because I'm about 10,000 miles away from that place .

I see what you did there...


@Todd: that's already a pretty good step, thanks! You've taken out the blue hues, I assume by spot picking one of the reddish areas you could do the same to the reds. Some of the stars are colored though; will that technique only impact the colors as dark as (or close to) the one you picked, or the entire channel including the highlights?


edit: I'll try to post a WIP of the entire shot just to give you guys an idea.

I'd just select the color ranges you don't want or that you want to reduce, then just paint those colors out with a mask and some brushing.

I think we all knows what it feels like to take a shot and then later discover it is near usable. I think maybe this is not salvageable, this probably should not have been shot at ISO 3200, and there seems to be a bit of coma from the lens, but here's a quick stab anyways.

I've never used noise ninja, but most noise software lets you filter out chroma noise and luminance noise at separate strengths. In this case, the image needs reductions of both types. Even after that, there's still a lot of color splotches. That's the first image.

You can further tweak it by desaturating the red blue and magenta chanels a bit and increasing the contrast to get some more definition to the stars, but even then I don't know if I would can the image usable.

Here's my attempt.

I used noiseware to do some noise reduction, I separated the chroma and the luminance NR so that I could use the 2 at different opacities (I tend to go overboard w/ the noise reduction and then use the opacity to scale back to taste and/or mask out parts I want to keep detail).

I also darkened it a little with curves but put an anchor up at the bright corner to keep the stars bright.

armis wrote in post #14672501
@Todd: that's already a pretty good step, thanks! You've taken out the blue hues, I assume by spot picking one of the reddish areas you could do the same to the reds. Some of the stars are colored though; will that technique only impact the colors as dark as (or close to) the one you picked, or the entire channel including the highlights?


edit: I'll try to post a WIP of the entire shot just to give you guys an idea.

Yes, you can just keep adding spot colors to remove as you need. I only did one (blue). This can be set with sensitivity, so that only the problem color is removed, and of course, if you're brushing it in, it's easy to control.

john29 wrote in post #14672538
I'd just select the color ranges you don't want or that you want to reduce, then just paint those colors out with a mask and some brushing.

What, is there an echo in here?

I fiddled with it a little more and think I've got something decent. Desaturation in the areas where the star field is less dense, a little more noise reduction followed by an overlay of uniform noise (got the idea off somewhere - surprisingly, it does make the gradients look smoother) and some more random attempts at stuff. Will post new crop in a few.

armis wrote in post #14673426
I fiddled with it a little more and think I've got something decent. Desaturation in the areas where the star field is less dense, a little more noise reduction followed by an overlay of uniform noise (got the idea off somewhere - surprisingly, it does make the gradients look smoother) and some more random attempts at stuff. Will post new crop in a few.

A little late, but here goes:

Looks good, much better.

Thanks! You can see the full shot over here .