I guess I'm opening another never ending story about the SCTs but I've searched for my specific question but didn't find the concluding answer for me.
Since I've attached a 2"Crayford to my SCT, I end up focusing by turning the focus knob CCW for a rough focus and to the fine tuning. In my naive approach, I though that it would suffice to collimate the optics when defocusing and making the doughnut in CCW direction.
To my surprise, when close to focus (doesn't deserve this term), the stars became comets. When I checked collimation by turning the focus knob CW, it was clear why: total decollimation! I'm not sure why this happens specifically but I was surprised to observe the bad stars when following the CCW focusing instruction as the mirror tilt should be the same while collimation in CCW direction.
I fixed it by defocusing in both directions, while I have to say that the CCW doughnut is perfectly symmetric and the CW doughnut has a very (!) slight shift of the shadow.
Why didn't collimation work by only defocusing in CCW directions?
How perfect can collimation be in both directions? I would guess that the mirror flop will make it difficult/impossible(?) to achieve perfect collimation in both directions.
About my procedure: used star Capella to do the collimation. Last night it was very high in the sky (altitude 82 deg). I attached my mono CMOS without any reducer directly to the Crayford. To assess collimation, I used sharpcap with circle cross hair (helps a lot).
Edited by barnold84, 28 February 2021 - 02:58 AM.