Thanks for looking at the image.
I tried Darktable but it only wants raw files for the chromatic abbertion tool, I am exporting the composite as png.
Gimp 2.10 has a script fix-ca which appears to shift the colours left, right, up, down so won't fix my
colour fringes. Or at least I couldnt get it to work the correct way.
The appearance of the chromatic aberration is probably mostly due to the processing rather than my lens.
The fringes are all around the stars and occur in different colours.
There is a nice warning on the dialog when you select Image Processing->RGB Compositing
WARNING: You should not perform a LRBG combination with linear images. Process images before.
So I stretched the L R B G images before combining them to make the final LRGB composite.
I am not sure what the best method of stretching should be, as it probably has to be applied
similarly to all of the frames L R G B, used in the final composited image.
I used the Histogram transformation tool (eg. Autostretch by clicking the cogs, also tried dragging the midtones slider) on one frame, eg R_stacked.fits and then jotted down the Midtones: and Shadows: settings.
I saved this frame as R_stretched.fits
And then applied the same values for Midtones: and Shadows: for the other frames needed for the composite image
and saved them accordingly as;
Then I loaded these stretched images into the RGB Compositing tool Image Processing->RGB Compositing and went from there.
I tried to follow the documentation as best I could.
What method's do others use?
P.S. I looked at the lights and stacked frames and the red channel is a bit out of focus