Thanks for all the comments and for the repro Doug. I would really like to find out if the band is coming from the stacking process or from something else. I found a similar issue with my M27 image which makes me think it's something in the optical system - perhaps some strange reflection although I'm not sure how you get a straight line like this if it is a reflection?
http://www.iceinspace.com.au/forum/s...ad.php?t=46123