Back to forums

Neutral Background Color (Black)

Processing Background Extraction Deep Sky
16 replies•664 views
Anderl avatar
Anderl avatar
Hello fellow Astro Friends, 

i am at this point 4 years? into taking and processing deep sky pictures as an hobby, a thing that is a constant trouble for me is archiving an optically pleasing background. 

my processing is an almost boring process. after using spcc I try my best not to mess with the color balance of the image, but the steps of stretching, saturation etc. are almost always led to an unpleasant background color. my pictures tend to get an either green or blue color cast as soon as I push my data.
right now I am dealing with it by using color masks in photoshop with the aim of only reducing the background saturation. 

do you guys have any tips on how to better deal with that problem? is there anything I can do earlier in the process within pixinsight? 

cs
anderl
Engaging
Fibo1 avatar
Fibo1 avatar
Hi Anderl,

can you please list your process sequence steps which you apply? Else, it is difficult to help you. 

CS

Hubert
Anderl avatar
Anderl avatar
Hi Anderl,

can you please list your process sequence steps which you apply? Else, it is difficult to help you. 

CS

Hubert

Servus Hubert,

- stacking
- gradient correction
- blur x
- channel combination
- spcc
- star x
- stretching of starless/stars 
- color saturation 
- recombination of stars and starless
- final adjustments

Thats more or less my workflow

cs
anderl
andrea tasselli avatar
andrea tasselli avatar
Without a practical example is hard to tell.
Fibo1 avatar
Fibo1 avatar
Servus Anderl (aus Österreich??),

just a question: you do stretching after star x, thus you do stretching twice: one time for starless version and one time for the stars??

My steps are (OSC frames from my DSLR):

- stacking
- channel extraction
- linear fit
- channel combination 
- dynamic crop
- gradient correction 
- script "autocolor" from PixInsight Austria (I use it instead of SPCC, as it is very fast and the results are pleasing me)
- SCNR 
- NoiseXterminator
- script "DeLinear" from PixInsight Austria (works fine for me in almost all cases so far)
- Starnet2  -> CurvesTransformation for the starless image plus gradually ColorSaturation, a little bit of CurvesTransformation also for the stars
- recombination of stars and starless
- final adjustments in Photoshop Lightroom

with this workflow I never ever had any issues of color shifting so far

CS Hubert
Helpful Engaging
Anderl avatar
Anderl avatar
andrea tasselli:
Without a practical example is hard to tell.

Hey andrea,


https://astrob.in/08fji5/0/

The picture had a noticeable green color cast in the background area as well as in the weaker nebula parts. This is the picture after i reduced the background saturation in photoshop. 
Servus Anderl (aus Österreich??),

just a question: you do stretching after star x, thus you do stretching twice: one time for starless version and one time for the stars??

My steps are (OSC frames from my DSLR):

- stacking
- channel extraction
- linear fit
- channel combination 
- dynamic crop
- gradient correction 
- script "autocolor" from PixInsight Austria (I use it instead of SPCC, as it is very fast and the results are pleasing me)
- SCNR 
- NoiseXterminator
- script "DeLinear" from PixInsight Austria (works fine for me in almost all cases so far)
- Starnet2  -> CurvesTransformation for the starless image plus gradually ColorSaturation, a little bit of CurvesTransformation also for the stars
- recombination of stars and starless
- final adjustments in Photoshop Lightroom

with this workflow I never ever had any issues of color shifting so far

CS Hubert

Thx for sharing your workflow hubert!
almost Austria 😄 lower Bavaria near passau. 



am i wrong if i assume that i shouldn’t use scnr after spcc?
andrea tasselli avatar
andrea tasselli avatar
Anderl:
Hey andrea,


https://astrob.in/08fji5/0/

The picture had a noticeable green color cast in the background area as well as in the weaker nebula parts. This is the picture after i reduced the background saturation in photoshop.


*I can't see much of a green cast in the BG or in fact anywhere else. Except for the green halos around most of the stars that is. What I was looking for is an example before correction in PS is made.
Anderl avatar
Anderl avatar
andrea tasselli:
Anderl:
Hey andrea,


https://astrob.in/08fji5/0/

The picture had a noticeable green color cast in the background area as well as in the weaker nebula parts. This is the picture after i reduced the background saturation in photoshop.


*I can't see much of a green cast in the BG or in fact anywhere else. Except for the green halos around most of the stars that is. What I was looking for is an example before correction in PS is made.

Thx for taking look andrea. I sadly don’t have such an example rn. I will post one as soon as i have time to process something. 

I think the green halo around the brighter stars is a good example for the green cast. The halo areas around the stars seems bright enough to not be affected by the masking i do in photoshop.
Fibo1 avatar
Fibo1 avatar
Anderl:
am i wrong if i assume that i shouldn’t use scnr after spcc?


SCNR removes any removing green cast. As most astronomical objects (except stars, galaxies, planets and moons) do not have any green color, removing any residual green touch makes normally sense, if there is any. I do not use it "full scale", but apply it reduced to the level of about 50 to maximum of 80%, if it seems to be necessary. According to the guidelines I follow I do apply it after color correction. Taken this in account, a replacement of my color correction process by SPCC  would mean that SCNR would then be applied after SPCC

CS

Hubert
andrea tasselli avatar
andrea tasselli avatar
SCNR should be applied to the stretched image and then only up to 50%.
andrea tasselli avatar
andrea tasselli avatar
Anderl:
Thx for taking look andrea. I sadly don’t have such an example rn. I will post one as soon as i have time to process something. 

I think the green halo around the brighter stars is a good example for the green cast. The halo areas around the stars seems bright enough to not be affected by the masking i do in photoshop.


*I think that is most likely self-inflicted in the sense that it goes with the processing. A different take on the stretching would likely lead to more acceptable results. And SCNR can be applied to the stretched starless image, with some moderation maybe in conjunction of using masks. Again, the only way to test this is by doing it differently or having someone else do it and see what is the outcome.
Anderl avatar
Anderl avatar
andrea tasselli:
Anderl:
Thx for taking look andrea. I sadly don’t have such an example rn. I will post one as soon as i have time to process something. 

I think the green halo around the brighter stars is a good example for the green cast. The halo areas around the stars seems bright enough to not be affected by the masking i do in photoshop.


*I think that is most likely self-inflicted in the sense that it goes with the processing. A different take on the stretching would likely lead to more acceptable results. And SCNR can be applied to the stretched starless image, with some moderation maybe in conjunction of using masks. Again, the only way to test this is by doing it differently or having someone else do it and see what is the outcome.

Not that i doubt that but my assumption was that one should not mess with anything throwing off the color calibration after using spcc?

i am only doing linked stretched after spcc and if i increase the saturation i also increase the overall saturation, this also includes not using scnr after spcc.
andrea tasselli avatar
andrea tasselli avatar
Once stretched you can apply SCNR without affecting the overall color balance. Moderation in color saturation in relation to the SNR is they key word here. As we used to say: "respect the light".
Wei-Hao Wang avatar
Wei-Hao Wang avatar
In the example image, there are red nebulas everywhere.  I believe you can get Ha emission from every single pixel if you go deep enough.  Under this circumstance, any algorithm will have problems finding Ha-free regions to evaluation the "true background" (there is no true-background).  So if the algorithm uses some areas with faint Ha emission as background, it will over-subtract the red, leaving a cyan tint in the "background" region.

I see such effects all the time in my images with large nebulas. I don't think this can be avoided. You have to rely on your own judgement to compensate this is post processing.  This becomes art, not science.
Well Written Helpful Insightful Concise Engaging
Anderl avatar
Anderl avatar
Wei-Hao Wang:
In the example image, there are red nebulas everywhere.  I believe you can get Ha emission from every single pixel if you go deep enough.  Under this circumstance, any algorithm will have problems finding Ha-free regions to evaluation the "true background" (there is no true-background).  So if the algorithm uses some areas with faint Ha emission as background, it will over-subtract the red, leaving a cyan tint in the "background" region.

I see such effects all the time in my images with large nebulas. I don't think this can be avoided. You have to rely on your own judgement to compensate this is post processing.  This becomes art, not science.

Thx wei-hao,

makes sense. 

cs
anderl
Riccardo Civati avatar
Riccardo Civati avatar
Hi Anderl.

I haven't seen all the answers but I have seen your workflow.
I strongly suggest you to use blur x terminator in # correct only# mode before color calibration.
Afterwards you have to use blur x terminator in # sharpen stars mode and not stars#.
Altering the PSF does not allow for correct color calibration

I also recommend you to use a DBE on the single monochrome channels and then use linear fit between R-G-B

because of the green color cast, as @andrea tasselli  says, SCNR should be used to make the background color uniform and never at 100%.

If you have a green cast only in some areas, you can create a mask with the @Adam Block  GAME script and remove the green in those areas partially

Riccardo
Helpful
alpheratz06 avatar
alpheratz06 avatar
Anderl:
Hello fellow Astro Friends, 


do you guys have any tips on how to better deal with that problem?
cs
anderl

Hello
I don't know if this can help, but, when I have a non neutral background residual on the final picture, I just clip the left side of the histogram with GIMP for instance in LRGB mode, to "push" the background to black
cheers