That vignetting is severe, which scope do you have? I can post you up some spacers if you need them.
In the meantime I'd crop the images and just process the centre and avoid the issue for now.
There are a few flats for QHY8 discussions on IIS:
I'll see if I can find the others.
http://www.iceinspace.com.au/forum/s...ad.php?t=50012
Oh, and give it a clean!