That's really nice looking nebulosity and lovely colours Beren in a great FOV. I know you are still in the testing phase so look out for when you start pushing deeper because there is plenty of nebuloisty in that frame!
All you need is to do flats, or are you already doing these just not for this shot? Even basic snaps of a piece of white paper illuimnated with a white light in front of the scope will help in this case, if you don't want to do real flats. If you have already tried flats, sorry for stating the obvious
. The moon has of course accentuated the vignetting so it may be hardly noticable in moderate exposures like this with no moon?
I agree, I would have thought you should be able to happily go colder..?
Nice
Mike