M16 Ha, 9 x 10 min, ASI1600MM-C at Gain 0, Offset 10, dark calibrated, cosmetic correction, dizzle integration (@ x2). Only resized back down to "normal" frame size, no cropping, just jpeg compression at 70% to fit forum size. So, you can pixel peep to see the main problem with it...
..which is shown nicely in these images:
The stars seem to point in to the centre, so I believe this means I need more space between the field flattener and the CCD. If so, that's a pleasant surprise since I had thought the reverse and was starting to puzzle as to what I could remove.