So I'm looking for suggestions on why this happens and maybe if there's a setting to fix this.
I just get a new mono setup, and this is the first I have seen of this artifact:
1) It only appears in a DSS stack, not in ASTAP or Siril stacks.
2) It doesn't appear in single lights, darks, biases, flats, master dark, master flat or master bias.
3) It gradually appears the more light subs you add for stacking, so if you stack half the subs, it's half as pronounced.
4) It only appears in Ha data, not in LRGB data.
5) My current reducer WO Flat6aiii and my current reflector Apertura 72EDR don't mix - stars in corners have significant coma.
6) My attempts to change stacking method or kappa value in DSS produced no change.
This is an image from Siril Histogram viewing mode - it's an extreme stretch that's meant to show the entire range of data at the same time.
These are stacks of about 3 hours of 4 min subs of Ha data.