I recently posted an image of the Bubble nebula using the ASI6200mm on a C11" EdgeHD @f/7 using the Celestron 0.7x focal reducer (https://astrob.in/4vx389/B/).    While I thought it was OK with a nice wide field of view, I was not at all happy with many of the outer stars which have a coma-like appearance.   My scope is well collimated, backfocus is spot on per Celestron recommendations, PA excellent, and no major camera tilt.  Guiding was also very good.  So, the option is to ignore it or try to address the problem.  Thus, I turned to John Hayes based on his background in optics, his experience with the large Celestron SCTs and overall helpful guy.  He had two immediate suggestions: 1) remove the reducer as he said I have run into its correction limit and 2) try some very small space adjustments on the sensor side per Niall MacNeil who was very successful in getting round stars on his C14.  I was hesitant to remove the reducer due to my usually only average or below average seeing conditions.

Well, back under my horrid sky conditions, I had only a 2 hr window to try a repeat of the Bubble in Ha @ f/10.  The result was astoundingly better despite being under 99% humidity, below average transparency and below/average seeing!  (Image A- using MMT denoise only).  Some of the outer stars have a slight egg-shape to them but this is at least a 90%+ improvement if you compare it to the 5 hr image I took @ f/7.  I may try to tweak the spacing but if that does not help, I am much happier with this version.  Of course, I need many more hours under better sky conditions and I lose some FOV but I am fine with that. 

I tried Topaz AI denoise (Image C) and compared it to PI MMT denoise side by side (Image D).   I need to play with this some more but I like what I see.

I also tried StarXTerminator (Image E) on the MMT denoise version.  Amazing tool and appears much better than Starnet++.  

Images F and G added more hours of data, showing more nebulosity.

Any feedback or suggestions are welcome.