If all you have are JPEG files, or other files that have already been gamma-corrected, you can still align and stack multiple images. Proceed just as with raw files or linear TIFFs, but leave out de-Bayerization and gamma correction.
You can't subtract dark frames in the normal manner, if the images have been gamma-corrected, because the dark current and the signal no longer add linearly. But the situation is not hopeless.
In some of my early experiments, I scaled a JPEG dark frame to half its original brightness (using the Curves adjustment in Photoshop) and then subtracted it from an astronomical image; results were imperfect but much better than if no dark frame had been subtracted.
Then I found BlackFrame NR, a free software utility (from www.mediachance. com) that subtracts dark frames correctly from JPEGs. Apparently, it undoes the gamma correction and some of the JPEG compression artifacts, performs the subtraction, and re-encodes the result as a JPEG file. Intended for terrestrial photographers working in dim light, it is also useful to astronomers.
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