My approach is to subtract a heavily-smoothed version from the actual image to remove the inhomongeious background brightness, however the results of that were far from satisfactorily. Changing the black point of the image obviously does not work either.
Is there a way I miss to deal with such problem?
Information on the lens vignetting behaviour are not available beyond what I see on images.
An approach I can imagine to work is to subtract a frequency-filtered version of the image where I only retain the very low radial-symmetric frequencies from the original, thus keeping the high-frequency content of stars and galaxies intact. Yet I fail to find any way to do so safe than writing such plugin myself. Do I miss any approach with gimp (or another programme which I can get working on linux)?