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Rob,
good on you for delving into DSOs.
The glow around NGC253 is always tricky to remve i think.
Theo is right, the flats will help with vignetting.
technically to remove the left over glow without affecting the galaxie too much, there are a couple of ways of doing it.
The one i tend to use is :
copy the image twice.
blur copy 1 in gaussian blur maximally
blur copy 2 just a bit and invert (black becomes white
copy and paste copy1 onto original (you would of course want to have tucked away a copy of the original so it is saved safely without alterations)
in layers select the mask icon, Alt click on it copy and paste into that window the copy 2
you're now pasting a blur onto the original with a mask of everything but the galaxy
select the type of blending works best and that should help
you'll need to experiment with size/intensity of mask, blending mode, blending transparency etc
but that should help remove some of the glow around the galaxy without touching the galaxy itself.
hope this helps
frank
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