While going through some NGC104 shots I too the other week I notice for the first time some strange spots on the stars. This was using the ED80 and FR. Image is a stack of 240sec@ ISO400. The first one is a crop of the top left hand corner of the second shot, prior to curves. The second shot is the whole image but with very heavy curves.
Strange bit is, I took other 240 sec shots, but at ISO800, the same night of another area of the sky and there are not dark spots
Any ideas?