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Easy chromatic aberration fix in GIMP?

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#1 1983cowboy

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Posted 08 February 2018 - 12:59 PM

I have a pretty simple question that I hope has a simple answer.

When I process my astrophotos, I've been using Nebulosity to stack and do basic processing like level-stretching and curves - and afterward, I'll toss them into GIMP (2.8, I believe) and do some more basic contrasty, curve-y, level-y stuff too.

 

But as I was looking back recently at some exposures shot with an older William Optics ZenithStar 80 (non-Apo) I see that there are some blue/violet halos from chromatic aberration around the brighter stars. In most cases, these are pics are already flattened as JPGs. I'm not sure where the original RAW stacks are.

By the way, the attached file is just a cropped screen shot of the original JPG, not the whole shootin' match.

 

Anyway, is there a simple method in GIMP to get rid of those violet halos? I'll admit that I've seen an example or two on YouTube, but they don't look so simple to me. I've never used layers or masks or anything like that in the past. I'm sure I'm just missing something in there.

 

Remember, the question is regarding a simple, free solution that does not include buying an APO! (As much as I would like to...) 

 

Thanks all!

Attached Thumbnails

  • Screen Shot 2018-02-08 at 12.55.56 PM.png


#2 SDTopensied

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Posted 08 February 2018 - 01:48 PM

In PixInsight, the routine involves creating a mask with doughnuts to overlay on the stars with the opacity of the doughnuts set to a level that will allow you to move the histogram and or modify the saturation on the halo without affecting the rest of the image.

 

Conceptually, it's not that complicated, but it's not a trivial undertaking...more tedious than anything else.  If you can work with masks in Gimp, you should be able to accomplish the same thing.

 

In a nutshell, you need to create a mask that protects everything but the halo of a given star so you can stretch the halo without stretching the rest of the image.

 

-Steve



#3 calypsob

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Posted 08 February 2018 - 02:24 PM

Im not sure how gimp would handle CA, but raw therapee is also free and does have a pretty good CA removal tool. https://rawpedia.raw...atic_Aberration



#4 jiawen

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Posted 09 February 2018 - 11:38 AM

Darla's Purple Fringe removal script works natively in GIMP and is free. It's not super-powerful, but for free, it's not bad.




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