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I just release the mail from Chris, and I see I spelled your name wrong. Sorry for the mistake.
I have the 130GTX+QTCC+ASI6200MM. I tried very hard to get perfectly round stars at each of the four corners and after many nights of experimenting gave up after I got what I feel is close enough. I ended up with an optimal distance in my setup at 103.1mm. I also had 0.3mm M68 spacer rings that I used. I believe that the small pixel size of the ASI6200 and the large image area exposes any flaws in the imaging train. I believe there is a slight tilt in my system which I tried to correct with an M68 tilt adapter but this was a futile effort. With the current setup I get round stars in three corners but the fourth corner I do get triangles for stars at the edge and it is more noticeable if a larger star happens to be in this corner. With the field flattener stars are round across the image with the same spacing. I do love my 130GTX setup and the images are a joy.