Beginner using gimp - please help me get rid of that awful green tint! - Forums

Topic starter

Oct 7, 2021 · Edited: Oct 7, 2021

Hello all.

I'm using Deepsky stacker and Gimp.
Using dark and flat calibration frames.
This happens with both my imaging cameras: Canon EOS 800D and Omegon 1200C guide camera.
As I histogram stretch, the image becomes greener and greener.
Don't know if this is coming from the cameras, or from the street lights right beside me?...
I've been dealing with this in gimp, using the black level eyedropper in the color/levels tool.
I've also tried creating a subtraction layer, bucket filled with what looks like the dominant background color.
I've also tried creating a subtraction layer, from a blurred out version of the stretched image (only on star fields).
All these methods sort of work. Sometimes the background becomes black, but the starts remain green. Never quite pleased with the result.
So, what is the best scientific approach - besides choosing light unpolluted locations and moonless nights?

Link to folder containing the DSS result image and my efforts

Regards,
A.Salvador

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andrea tasselli

Frequent contributor

Oct 7, 2021

The best way around is to avoid both DSS and Gimp. The background is green because you have some form of light poluution and possibly broadband light pollution too. This said it happens to all of us living under less than prestine skies. My suggestion to you is to bite the bullet, prepare to learn a new approach and start using Siril (totally free ) for all of your AP needs. You might want eventually to do some colour retouching/enhancement in Gimp or what have you later one once the image is fully processed and stretched. It would also help us to help you if you could post the affected images somewhere (google drive or whatever) so we could get a better understanding of the issues at stake.

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Alexandre Salvador

Topic starter

Oct 7, 2021

andrea tasselli:
The best way around is to avoid both DSS and Gimp. The background is green because you have some form of light poluution and possibly broadband light pollution too. This said it happens to all of us living under less than prestine skies. My suggestion to you is to bite the bullet, prepare to learn a new approach and start using Siril (totally free ) for all of your AP needs. You might want eventually to do some colour retouching/enhancement in Gimp or what have you later one once the image is fully processed and stretched. It would also help us to help you if you could post the affected images somewhere (google drive or whatever) so we could get a better understanding of the issues at stake.

Thank you @andrea tasselli.
I've edited my post to include a link to an image, hope it works.

andrea tasselli

Frequent contributor

Oct 7, 2021

Not sure what you wanted to do with shooting Capella but here you go. In IRIS you would just select a piece of background empty of stars and issue to command "black". That would be enough. That is the simplest way to achive the result you want. Unfortunately your background isn't flat so there is residual skew in the colour planes. Be as it may here is there results (beware: you are HUGELY oversampling!):

Alexandre Salvador

Topic starter

Oct 7, 2021

andrea tasselli:
Not sure what you wanted to do with shooting Capella but here you go. In IRIS you would just select a piece of background empty of stars and issue to command "black". That would be enough. That is the simplest way to achive the result you want. Unfortunately your background isn't flat so there is residual skew in the colour planes. Be as it may here is there results (beware: you are HUGELY oversampling!):

Thank you for taking your time to help me.
I like to shoot and collect named stars

Anyway, thought it would make a simple test subject for my background issue.

"Unfortunately your background isn't flat". By "not flat", do you mean "noisy"? How can I make it flatter?
"beware: you are HUGELY oversampling!" Could you explain what you mean by "oversampling"?

BR

andrea tasselli

Frequent contributor

Oct 7, 2021

No, "flat" means when the background values (RGB) are the same everywhere. In your case the corners are noticeably brighter than the rest. This affects how the automatic calibration works in giving you a gray background in most programs. Plus one there is a noticeable skew in how the colour planes are rendered. I sort of fixed by flattening the image in Pixinsight but some left-overs are still there.

Oversampling means that your pixel scale (also called image resolution) is (much) larger than the optimum one given by 3 parameters: the camera, the lens and the sky conditions (as in turbolent rather than calm). Pushing the image scale beyond the optimum means wasting both time and photons gaining nothing in the exchange. I don't know what gear were you using for the shots so I can't comment any further.

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dkamen

Oct 7, 2021

Alexandre Salvador:
Hello all.

I'm using Deepsky stacker and Gimp.
Using dark and flat calibration frames.
This happens with both my imaging cameras: Canon EOS 800D and Omegon 1200C guide camera.
As I histogram stretch, the image becomes greener and greener.
Don't know if this is coming from the cameras, or from the street lights right beside me?...
I've been dealing with this in gimp, using the black level eyedropper in the color/levels tool.
I've also tried creating a subtraction layer, bucket filled with what looks like the dominant background color.
I've also tried creating a subtraction layer, from a blurred out version of the stretched image (only on star fields).
All these methods sort of work. Sometimes the background becomes black, but the starts remain green. Never quite pleased with the result.
So, what is the best scientific approach - besides choosing light unpolluted locations and moonless nights?

Link to folder containing the DSS result image and my efforts

Regards,
A.Salvador

Hi,

Your problem is not the black point, it is color balance. And it is multiplicative, not additive so subtraction won't fix it.

Try Colors -> Auto -> White Balance

If that doesn't work, Undo and try adjusting the white balance manualy (Colors -> Color Balance)

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Eddie Bagwell

Oct 8, 2021

I used GIMP when I started AP back in January up until last month, and I found that there is no solution for the "Green" in GIMP. I recently upgraded to PixInsight and it has a one click process called SCNR that fixes it. PI isn't free but it improved the quality of my images, cuts my processing time and once you get your processes saved as a project, (IMHO) it's easier than GIMP. Just my experience. Good Luck.

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Kullas

Dec 2, 2021

Seeing that you are using DSS and Gimp you are wanting to keep it on thefree side for now. I use Siril and it works well for me. Its free. This is the vid to help me through the workflow https://youtu.be/hIhgS3bvOy0

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Tim McCollum

Dec 2, 2021 · Edited: Dec 2, 2021

It took me 2 seconds to get rid of the green in PI using the SCNR tool.

rhedden

Dec 2, 2021

I use Deep Sky Stacker, GIMP, Siril, and several other programs for processing. Without asking you to buy any software, here's my advice.

The first thing I would do with your image is to adjust the Curves in GIMP to reduce the green and get the histogram looking balanced. Open Curves, select Green, and drag the curves around until the colors seem balanced and the green hue is minimized. This processing can probably be done in Siril as well, but I have historically done it in GIMP.

Now comes the fun part. From GIMP, export the image as a 16-bit TIF, and open it in Siril. Under Image Processing, run the Remove Green Noise script. Save the result as a TIF. It will look black & white in Siril, but you can re-open it in GIMP and I think you'll be quite happy with the results. I just love this feature of Siril…. it's so fast and so effective.

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Alexandre Salvador

Topic starter

Dec 2, 2021

Thank you all for your valuable input, recently @Kullas, @Tim McCollum and @rhedden.
As you've guessed, I do wish to keep it in the free side for now. If you take a look at my gallery, you'll see that I'm not really equipped to shoot DSO's, partly because of my slow telescope with Alt-Az mount, but mainly because of my under privileged location, with a strictly eastern view and buildings just opposite.
If I had a proper backyard with most of the horizon visible, I'd start my spending with an equatorial mount, and software later.

So my focus has been star clusters and star fields.

However I've got some nice pointers from you. I took a dislike to Siril, because it wouldn't let me progress without lights + darks + flats + offsets every time. But now I see that I can start from where DSS left off, so I'll give another try.

Thank you again all that have been taking the time to reply.

regards
A.salvador

Kullas

Dec 2, 2021 · Edited: Dec 2, 2021

You can go to the siril website and download the script for just the darks there is a vid on youtube for adding scripts. But calabration frames do help and they are simple to take.
its all a learning curve at first but it does get easier with practice.

Bradley Watson

Dec 2, 2021

Hey Alexandre, I use GIMP as a post processing tool but also use others like Siril, APP, and ASTAP. I understand fully not wanting to spend money on software, but at some stage you will need to spend something.

There is a great AP plugin for GIMP called PyAstro which can be found at https://stargazy.weebly.com/pyastro-for-gimp.html.

It’s a little awkward to install but when installed there are some great tools there that will help you solve some of your challenges. I suggest reading the documentation for it as it explains well what you can do.

Hope this helps.
CS
Brad

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Dale Penkala

Dec 2, 2021

Bradley Watson:
Hey Alexandre, I use GIMP as a post processing tool but also use others like Siril, APP, and ASTAP. I understand fully not wanting to spend money on software, but at some stage you will need to spend something.

There is a great AP plugin for GIMP called PyAstro which can be found at https://stargazy.weebly.com/pyastro-for-gimp.html.

It’s a little awkward to install but when installed there are some great tools there that will help you solve some of your challenges. I suggest reading the documentation for it as it explains well what you can do.

Hope this helps.
CS
Brad

There are a lot of nay sayers about Gimp, but I’ve personally found it to be an extremely powerful tool and use it for pretty much all my final work after StarNet and Topaz DeNoise.

There is a thread that I started some time ago about “Plugins for Gimp” here on Astrobin and @Bradley Watson nailed one of them! I’d like to also include another very powerful plugin and I use it all the time called “G’MIC” they have a wavelets and DeNoise filters under there “repair” tab that is excellent. By the way in PyAstro there “Smart Sharpen” has some very good options and it works excellent as well.

Dale

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Dale Penkala

Dec 2, 2021

@Alexandre Salvador I know we got off topic somewhat here but in my last post I mentioned that I started a thread a while ago about Gimp Plugins. I found the thread and thought I’d post it here for you and anyone else that might use Gimp. As I stated Gimp is a powerful “FREE” tool and I use it on every image.

Here is the link: https://www.astrobin.com/forum/c/astrophotography/other/gimp-astrophotograpy-plugins/

Hope this helps!

Dale

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Bradley Watson

Dec 2, 2021

@Alexandre Salvador Yep I also use GMIC. I have been teetering on the edge of purchasing PI but keep finding ways of solving my issues in GIMP. To be absolutely honest GIMP is as @Dale Penkala says a powerful FREE tool that is always being updated. Most of the time it’s down to the user to want to learn the tool at hand and GIMP does a lot. If you browse around in AB you will find some very very good APers who use GIMP, @Ruben Barbosa is such an example so don’t give up. Learn the tools first before discarding them and if you want to move onto something like PixInsight you will actually understand what you are doing when you click a button.

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rhedden

Dec 2, 2021 · Edited: Dec 2, 2021

Another plugin for GIMP that I like is the star-rounding plugin by Georg Hennig. You will find it easily if you search on it. (You'll need to install Python on your computer for this plugin and for PyAstro.) If you have a slight tracking error that produced egg-shaped stars, this plugin can save the day. I have also played with the PyAstro plugins and G'MIC with less success, and I generally prefer to do sharpening and other noise reduction operations outside of GIMP. I like wavelet sharpening or Lucy-Richardson deconvolution in particular, but I'm not a fan of AI-driven detail enhancement at all.

Going off-topic again…. I love the noise reduction features in Topaz AI (Severe Noise mode, lowest setting) for starless images, but not so much for images with the stars in place. A lot of people avoid Topaz AI because it is known to "chew up" the stars. Removing the stars with Starnet++ first, like Dale is mentioning, is a good workaround with some caveats about artifacts that can show up. If you're working with starless images, I have found that Topaz AI is utterly fantastic when applied to narrowband data in particular, which is often very noisy in the dimmer areas like the sky background. When adding integration time is seemingly not helping the noise in the sky background, it's time to Topaz it. I have warmed up to Topaz AI since they enabled users to turn the sharpening completely to zero earlier this year. I plan to start working it into my narrowband processing workflow in the near future for noise reduction only. I think Topaz AI is gaining some traction in our community, provided it is not used to generate "fake details" or to apply excessive noise reduction that turns the image into "plastic".

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Dale Penkala

Dec 2, 2021

Going off-topic again.... I love the noise reduction features in Topaz AI (Severe Noise mode, lowest setting) for starless images, but not so much for images with the stars in place. A lot of people avoid Topaz AI because it is known to "chew up" the stars. Removing the stars with Starnet++ first, like Dale is mentioning, is a good workaround with some caveats about artifacts that can show up. If you're working with starless images, I have found that Topaz AI is utterly fantastic when applied to narrowband data in particular, which is often very noisy in the dimmer areas like the sky background. When adding integration time is seemingly not helping the noise in the sky background, it's time to Topaz it. I have warmed up to Topaz AI since they enabled users to turn the sharpening completely to zero earlier this year.

ABSOLUTLY NAILED IT WITH THE WAY TO USE TOPAZ! I was concerned as well about some of the artifacts but being able to work with “starless” images makes a huge difference!

I have considered PI but just haven’t gotten that serious about it yet. I know there are some very powerful tools in it, but I’ve been quite happy with what I’ve been getting so haven’t to excited to move in that direction yet.

rhedden

Dec 3, 2021

I've downloaded the trial edition of PixInsight at least three times since 2014. I'm still using GIMP! I shared some of my raw data with PixInsight users at some point to see if they really would get better results. They came up with different renditions of my images, but not better renditions. I could recreate their versions easily using GIMP with slightly different decisions on curves/saturation. I think it's all about knowing your favorite software "inside out", and making good processing decisions. We're in a minority as GIMP users, but not necessarily at a disadvantage.

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Dale Penkala

Dec 3, 2021

I've downloaded the trial edition of PixInsight at least three times since 2014. I'm still using GIMP! I shared some of my raw data with PixInsight users at some point to see if they really would get better results. They came up with different renditions of my images, but not better renditions. I could recreate their versions easily using GIMP with slightly different decisions on curves/saturation. I think it's all about knowing your favorite software "inside out", and making good processing decisions. We're in a minority as GIMP users, but not necessarily at a disadvantage.

@rhedden agree 100%! I’ll continue to use Gimp unless I find a major positive to change to something different.

jjpoole741

Mar 3, 2022

I’ve been a little frustrated with this as well. What helped me a bit was setting my black point under Levels. Click the black point, then select a dark neutral portion of the photo. This seems to help me, but not sure it’ll work for you.

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Claudio Tenreiro

Mar 4, 2022 · Edited: Mar 4, 2022

Ok, it took me 10 seconds using GIMP to get rid of the green, no problem at all, you just must align the colour channels. Select viewing the histogram on a lateral window, then Colours - levels, and select red, green and blue and move them so they come aligned. To stretch the image (first step) colours - curve and stretch astronomical curve (pronounced arc), then go to colours - levels, and used mid tones and black to adjust.

Claudio Tenreiro

Mar 4, 2022

I forgot, these tutorials are awesome to use GIMP
https://www.youtube.com/watch?v=fSxdHrO0zfU&t=737s
https://www.youtube.com/watch?v=0h7gAwrHLco&t=25s
The first one teach you to process the image, and the second to reduce star sizes.

Dale Penkala

Mar 4, 2022

Claudio Tenreiro:
I forgot, these tutorials are awesome to use GIMP
https://www.youtube.com/watch?v=fSxdHrO0zfU&t=737s
https://www.youtube.com/watch?v=0h7gAwrHLco&t=25s
The first one teach you to process the image, and the second to reduce star sizes.

Agree totally! Ruzeen is awesome! I’ve chatted with him on a couple of occasions. He’s a pretty good guy to chat with!

Dale