I'm working on a similar configuration Martin in that I am setting up my ASI6200 with a Canon EF200mm f2.8 L USM lens. As Greg alluded to, I am finding that this camera is very unforgiving of lens aberrations so it is proving difficult to get the back focus and tilt adjustments dialed in. I did have to stop down the lens to f5.6 before I got anything like reasonable stars in the corners of the image so it is important to do this before proceeding further.
For what it is worth, here are the details of my configuration thus far to see if this helps.
I have purchased the
Astromechanics adapter that they have designed for the Canon lens. These adapters have the inbuilt controllers to adjust the f stop and focus of the lens. They also have an inbuilt tilt adjuster which also provides the option of adjusting the back spacing. Having software control of the aperture and focus mechanism is a real bonus and I have found that the auto focus routine works well with SGP Pro. In my case, the lens comes to focus near the left hand edge of the infinity mark.
The Astromechanics adapter that I am using is designed to give the correct flange to sensor distance of 44mm for the Canon lens. The components in the image train are; adapter 11.5mm, ZWO 2" EFW 20mm, ASI 6200 camera 12.5mm. As indicated above, the adapter can be adjusted to increase the spacing and adjust tilt. I have wound out the tilt adjuster to give me an extra 1mm spacing to allow for my Astronomik 50mm filters which have an optical thickness of 1mm. I am not entirely happy with winding the tilt adjuster out this much and am thinking of having a spacer made to pack out the adapter by 1mm.
My testing last night is promising though it is quite apparent that there is much tilt to be adjusted which I will approach very carefully as it is very easy to make things much worse very quickly.
Here is a link to a sample sub from last nights testing if you want to have a peek. I have an additional problem with star trailing as well but this should be unrelated to getting the back spacing and tilt correct.
Your experience with the difference between the QHY600 and the ASI6200 is interesting in terms of not being able to reach focus with the correct back spacing distance. What is different between the QHY and ASI6200 set up? Are the filters different and therefore a different optical thickness? I see that the back focus consumed by the QHY600 is 18mm compared to 12.5mm for the ASI6200 so I'm assuming that your adapters in the image train allowed for this? I would also be interested to know which lens you are using that has a back spacing requirement of 55mm.
Clear skies,
Rodney