This is just a plain post about something I just found out:

My previous way to focus my RC10 was to take a bright star in the middle of the field and adjust the focus until the diffraction pattern is a cross and not a double-cross.  This usually takes only a minute, it is more efficient and more accurate than any autofocus algorithm. BUT: Often in one corner of the image double-crossed diffraction patterns of brighter stars and egg-shaped stars  remained. This mostly is because of image tilt I am unable or too lazy to correct. Tilt values then are typically at 15 - 20%.

Today I  found that if I move the bright star from the center to that corner of the  image and focus on that position, I am getting perfect focus everywhere. Tilt values remain below 3%.

This may not work for other RC or similar telescopes, but I think it is certainly worth a try if you experience a similar notoriously out-of-focus corner of your images.
Edited ...
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