For this test I used a T-2 fine adjustment ring (0.5mm) to slightly extend the distance between DSLR sensor and corrector (54mm measured distance without any ring). The stars in the corners look much better than before, but I need to go to 55mm distance. Next time I’ll use the 1mm ring!
Additional I encountered for the first time some light reflections. I’m not sure about the source. There is no bright star outside the field of view in that direction… I’m a bit confused about this…
Maybe it’s time to darken the tube inside…
Of course I tried to remove the reflection (in PixInsight). I think its better now…