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comatic aberration treatment?

Apr 8, 2014, 09:03
Post: #1
Question comatic aberration treatment?
Does anyone know, if there's a way to reduce comatic aberration in gimp? There are 'star-rounding' plugins for photoshop that achieve this effect for star field images, there's an astronomy plugin kit for gimp as well, is it up to date? I have gimp 2.8 now, looking to switch to 2.9 as soon as ubuntu 14.04 comes out (I currently have 32b version of 12.04, and I understand that gimp 2.9 is not available for that). The image I'm working on atm has a landscape element, which may (or may not) be a problem for removing coma?

Help appreciated!!!
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Apr 8, 2014, 09:31 (This post was last modified: Apr 8, 2014 09:33 by fluffybunny2.)
Post: #2
RE: comatic aberration treatment?
(Apr 8, 2014 09:03)kurja Wrote:  Does anyone know, if there's a way to reduce comatic aberration in gimp? There are 'star-rounding' plugins for photoshop that achieve this effect for star field images, there's an astronomy plugin kit for gimp as well, is it up to date? I have gimp 2.8 now, looking to switch to 2.9 as soon as ubuntu 14.04 comes out (I currently have 32b version of 12.04, and I understand that gimp 2.9 is not available for that). The image I'm working on atm has a landscape element, which may (or may not) be a problem for removing coma?

Help appreciated!!!
http://kcd.sourceforge.net/fix-ca.php
http://www.hennigbuam.de/georg/gimp.html
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Apr 8, 2014, 10:45
Post: #3
RE: comatic aberration treatment?
Fix-CA sounds nice but is for correcting chromatic aberration, not comatic, unless I'm reading something wrong?

Star-rounding function in astronomy plugin is stated to be useful for correcting tracking errors, which is a different issue. I don't know if it could be applied to fix coma problems though, but that's why I'm asking here, right Wink I've used it before once or twice and as I recall it only corrected in one direction and thus would not really work for this?
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Apr 8, 2014, 13:44
Post: #4
RE: comatic aberration treatment?
paynekj - especially this being an image manipulation software forum I probably wasn't trying to 'reduce an aberration' on a comet orbiting the Sun a million kilometers from us Wink So, it's about the optical aberration, not comets except for the name; comatic aberration is thus called because it makes point light sources look 'veiled', much like a comet in the sky. Sorry if I wasn't clear enough with my question.

ofnuts, coma is rarely seen or at least is rarely much of a problem in photography because it usually isn't very apparent unless you're photographing a point light source close to the image field's edge with a wide aperture lens. In astrophotography however it's a very common problem, for those same reasons! In particular reflective optical systems suffer greatly from it but it's seen with refractive systems as well, including normal camera objectives.

I'll see what I can do with the tool in astronomy plugin. If you have any suggestions, I'd love to hear them.
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Apr 8, 2014, 13:49
Post: #5
RE: comatic aberration treatment?
Attached an example of what I'm trying to fix, cropped from top left corner of the image


Attached File(s) Thumbnail(s)
   
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Apr 9, 2014, 13:27
Post: #6
RE: comatic aberration treatment?
Did some experimenting with your supplied example & came up with a few things to try:

(from best to worst method 1-3)
1. Colors > Levels...
- Click the black eye dropper on this filter/plugin & either click the grey area or click&drag around the canvas until you get a result your happy with.

2. Colors > Brightness-Contrast...
- drag the "contrast" slider to the right until it looks good

3. Colors > Threshold...
- Drag the black arrow to the left until it reaches the right side of the histogram where the peak ends.


* In all above ways, it does clean it up but you do lose some of the similar colored stars.
- So consider duplicating the image layer first, then apply the filter, then make the top layer a layer mask & erase the parts that are too dark to preserve the stars.
- or just select the aberration part,
try Foregroundselect foreground select (after clicking the points all around, you can add to & erase from the selection before you push enter to finalize it)
grow selection by 2, feather the selection
"Select > Grow..."
"Select > Feather..."
Then apply any of the 1,2.3 ways mentioned above
   

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