[jakob]

Thanks for the helpful hints.

I did some tests on Canopus. Manual focus, F8, 2sec. the size of the star is 3x5 pix, at F38 15sec. 2x16 pix.
The 'Doughnuts' in the earlier shots are 10x10 pix!

btw Autofocus on the Moon works, but if it cant find focus, the infinity is not good enough for stars.

I will try a Hartman mask, then shoot a star trail and adjust the focus. But this would be hard as the focussing ring is not very smooth and has some play as well.