Suggestions for color correction in underexposed milky way photo

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The111 New Member • Posts: 6
Suggestions for color correction in underexposed milky way photo

See raw image here.

Note that I have a dozen other sequence images for noise reduction (via median stacking), and some other longer exposures for the foreground. But for the purposes of this post, I am mostly curious in getting the colors of the sky accurate. When I increase the exposure in ACR, I end up with a green tint, and trying to fix that just introduces more inaccuracies.

Over literally dozens (if not hundreds) of hours in the 5 months since I shot the photo, I have tried numerous techniques in ACR, mucking with the temperature, tint, curves, and every other knob I can find. I've come up with many results that look "cool" in some way or another, but none of them I think are very accurate to a properly color balanced night sky (as described in this article). I am open to advice (and .xmp files to accompany such advice if anybody is able to perform some magic that I was unable to). Thanks so much in advance for taking a look.

DavidWright2010 Veteran Member • Posts: 3,300
Re: Suggestions for color correction in underexposed milky way photo

You might have more luck getting help if you linked to a tif file of the image.

David

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Alen K
Alen K Senior Member • Posts: 1,381
Re: Suggestions for color correction in underexposed milky way photo

Your problem appears to be caused by sky glow. The greenish hue is caused by light added to the scene from the air itself (a phenomenon called airglow) or (probably not in this case) from man-made light sources (light pollution).

In the case of airglow, some people choose to leave it in the image, particularly if it shows some structure (bands) as it often does, since it is after all a natural phenomenon.

However, to remove it or the glow from light pollution requires in a general sense, subtraction of this extra light from the scene. You will not fix it using levels or curves (a point made in Clark’s article). There are some techniques for removing skyglow that work in a general photo-processing program like Photoshop, but the most effective methods are only found in astro-specific programs such as Siril and PixInsight.

sharkmelley
sharkmelley Senior Member • Posts: 2,706
Re: Suggestions for color correction in underexposed milky way photo

The article you linked says that you need to use tools that subtract the light pollution.  This is best done while the data are linear.

One way to do that is to use ACR to raw convert your image as neutrally as possible then change the mode in Photoshop to 32-bit (which is linear) to perform the subtraction of the greenish tint.

Mark

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OP The111 New Member • Posts: 6
Re: Suggestions for color correction in underexposed milky way photo

DavidWright2010 wrote:

You might have more luck getting help if you linked to a tif file of the image.

David

Weird, I cannot seem to find an edit option for my OP. Here is a tif download as well. Thanks for the suggestion!

sharkmelley
sharkmelley Senior Member • Posts: 2,706
Re: Suggestions for color correction in underexposed milky way photo

The111 wrote:

Weird, I cannot seem to find an edit option for my OP.

You can't edit a post to which someone has already replied.

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R Holt Junior Member • Posts: 29
Re: Suggestions for color correction in underexposed milky way photo
2

Are you looking for colors like this? If so, using ACR, place a color sampler in a dark area and use the curve tool to match the rgb values to a reference image.

DavidWright2010 Veteran Member • Posts: 3,300
Re: Suggestions for color correction in underexposed milky way photo

The111 wrote:

DavidWright2010 wrote:

You might have more luck getting help if you linked to a tif file of the image.

David

Weird, I cannot seem to find an edit option for my OP. Here is a tif download as well. Thanks for the suggestion!

Sorry - I should have said '16 bit tiff'.

No matter, I opened the raw file and made 2 tiffs - one for the sky and one for the foreground,, processed each to taste and merged:

You really needed a brighter foreground image.

David

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OP The111 New Member • Posts: 6
Re: Suggestions for color correction in underexposed milky way photo

DavidWright2010 wrote:

Sorry - I should have said '16 bit tiff'.

No matter, I opened the raw file and made 2 tiffs - one for the sky and one for the foreground,, processed each to taste and merged:

You really needed a brighter foreground image.

Thanks so much David for taking a look. Do you mind describing how you processed the sky? I like how yours turned out, and I'm just looking for new ideas at this point.

As mentioned in the OP, I do have a much better exposure for the foreground. And I have a dozen other sky images to stack for noise reduction. I have some pretty good looking final products in terms of clarity/noise, I just still feel like the sky colors are off in most attempts.

DavidWright2010 Veteran Member • Posts: 3,300
Re: Suggestions for color correction in underexposed milky way photo

The111 wrote:

DavidWright2010 wrote:

Sorry - I should have said '16 bit tiff'.

No matter, I opened the raw file and made 2 tiffs - one for the sky and one for the foreground,, processed each to taste and merged:

You really needed a brighter foreground image.

Thanks so much David for taking a look. Do you mind describing how you processed the sky? I like how yours turned out, and I'm just looking for new ideas at this point.

As mentioned in the OP, I do have a much better exposure for the foreground. And I have a dozen other sky images to stack for noise reduction. I have some pretty good looking final products in terms of clarity/noise, I just still feel like the sky colors are off in most attempts.

I just follow Roger Clark's tutorial.  It's the chapter a few lines down from the link you posted at the start. Start with a 16-bit tiff, incrementally subtract the background and stretch (via the middle slider in the levels histogram) the color. At the start  in a heavily polluted image you can subtract the entire spectrum (RGB mode), then later subtract each channel - R, G, and B - to keep the sky black (or grey).

I use Photoshop Elements (desktop version - cheap) and there are many things it won't do in 16 bit mode, but fortunately it can do this.

David

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chris gunn
chris gunn Veteran Member • Posts: 9,582
Re: Suggestions for color correction in underexposed milky way photo

DavidWright2010 wrote:

The111 wrote:

DavidWright2010 wrote:

You might have more luck getting help if you linked to a tif file of the image.

David

Weird, I cannot seem to find an edit option for my OP. Here is a tif download as well. Thanks for the suggestion!

Sorry - I should have said '16 bit tiff'.

No matter, I opened the raw file and made 2 tiffs - one for the sky and one for the foreground,, processed each to taste and merged:

You really needed a brighter foreground image.

David

The composition has potential!

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cheers!
Gunn
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CatchingTime
CatchingTime Regular Member • Posts: 202
Re: Suggestions for color correction in underexposed milky way photo

The111 wrote:

Weird, I cannot seem to find an edit option for my OP. Here is a tif download as well. Thanks for the suggestion!

Are you sure there are no edits to the TIF file you linked?

The problem with airglow is that it often does not affect the image frame uniformly, e.g. it is usually stronger near the horizon. Due to this fact, any color balance adjustment made on the entire frame affects it differently in different areas, as the one attempt posted above illustrates (the area of the upper Milky Way turned magenta on upper left side, while it decreased the green a bit in the main part of the Milky Way).

I use Clark's basic approach, but I only use Curves to neutralize any possible coloration from either pollution or airglow in the very darkest parts of the image (typically the upper reaches, though not near a corner where light falloff affects color balance). I personally like to see how the sky changes from night to night, and airglow is responsible for much of this, at least in dark places.

Everyone's mileage varies...

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CatchingTime
CatchingTime Regular Member • Posts: 202
Re: Suggestions for color correction in underexposed milky way photo

From the raw file:

By the way, it looks like your white balance was set to something other than 'Daylight' when shooting. I'm not familiar with Sony sensors, but when set to Daylight this file went very, very green (could be an effect of the Sony sensor, but I'm a Canon guy).

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OP The111 New Member • Posts: 6
Re: Suggestions for color correction in underexposed milky way photo

CatchingTime wrote:

By the way, it looks like your white balance was set to something other than 'Daylight' when shooting. I'm not familiar with Sony sensors, but when set to Daylight this file went very, very green (could be an effect of the Sony sensor, but I'm a Canon guy).

Unfortunately yeah I've always shot at AWB, and reading now I am reading why that is not a great idea. I know WB can always be corrected in post, but it's kind of hard to know what the right place to land is now. Will definitely stick with Daylight for all future outdoor shots.

Thanks for the input!

sharkmelley
sharkmelley Senior Member • Posts: 2,706
Re: Suggestions for color correction in underexposed milky way photo

The111 wrote:

Unfortunately yeah I've always shot at AWB, and reading now I am reading why that is not a great idea. I know WB can always be corrected in post, but it's kind of hard to know what the right place to land is now.

Take a raw shot (of anything you like) with a daylight white balance. When you open it in the raw converter note the temperature/tint that the raw converter uses. You can then use that same temperature/tint for the raws that you shot with AWB.

Mark

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