On Mon, May 3, 2021 at 11:17 PM, Khushrow Machhi wrote:
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.
My experience matches Khushrow's. I did a lot of spacing testing with the GTX+QTCC and could not quite get perfect stars in all corners but settled in at "good enough." Using Roland's method of finding focus and then checking corners after a few steps in and then out, revealed the sweet spot at 105.7mm. It's odd that my spacing is quite a bit different than Khushrow's. Spec is 102.9mm, but with a filter in the path that would be 103.9mm. So I'm at 1.8mm over spec. I also had a very small amount of tilt to contend with that I "mostly" corrected with a Gerd CTU.
FWIW- Roland stated to me (through George) that you won't get perfect stars in the corners with this setup and 3.76um pixels.
In all honesty, I do think this is good enough. This is a MASSIVE field and with tiny pixels is still a very high resolution image. Regardless of what camera you decide to use I'd jump on that GTX if you have the option to buy! It's an amazing scope.