Hi,
I have some old photos of comets taken on film and to the first digital cameras I didn't have correct flat frames. I have used AstroImagesJ to make synthetic flat frames. But didn't work very well when the shape of the vignetting was too complicated. Then I took a new look at Gimp and what it can do. Found it to be very advanced and relative easy to correct the vignetting.
I added a new page on my Gimp tutorial how to do this:
http://www.astrofriend.eu/astronomy/...ignetting.html
Merry Christmas to you all !
/Lars