do you think it could be fixed pattern noise (eg from darks or flats) being smeared out by guiding drift? If your polar alignment was rough, drift could do this if you are using the same noisy dark frame throughout. looks a bit like the FPN in the image attached to -
http://www.iceinspace.com.au/forum/s...44#post1091044
If it is FPN, solution is to dither while gathering images - if it is getting in through the dark, could possibly help to get more dark data...