You could try using astrotoaster and stack live from your dslr. Have a look at Howie's youtube
https://www.google.com.au/url?sa=t&s...N8ZFOD4xbU2nov
He uses a dslr and has some vignetting. Toaster has built in stuff to balance out the vignetting and gradients. Is like a built in flattener - kind of. But it's surprising how well it works.