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My CDK17 has pretty stable focus also. Its carbon fibre body seems to be the most important factor there. I do see the scope won't achieve sharp focus until temp difference between ambient and mirror is 1C or less.
I also notice the AP RHA with its crown glass mirror and vented primary with fans holds focus really well also despite temp shifts. There is some but its pretty minor.
I have not noticed the CDK17 to shift focus with temperature changes.
Imaging at long focal length shows up the bad seeing better is probably the correct statement. It also makes achieving sharp focus very hard as the focus images with no change to focus will appear in focus at one instant and out of focus the next just from the seeing when its bad.
Greg.
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