Looking for some post-processing help. I keep getting diagonal "brush stroke" artifacts in the stretched images. The direction of the lines are not in line with apparent star motion through the image so it does not seem to be related to tracking error (i.e. there is very little horizontal or rotational change throughout the subs). Attached image is a cropped section from the NA nebula taken on 6/23/2020
Using an un-moded Canon 80D with 400mm f/5.6 canon prime telephoto and an unguided ioptron skyguider pro tracker. 80 subs @ 30 seconds, 40 offsets, 25 darks, 25 flats. stacked in DSS, stretched using curves in Gimp.
All comments appreciated!