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If you think of it like this.
With your focuser draw tube racked out till it clears the inside of the OTA
Plus
Needed Backfocus with thoughts into if I change my setup in the future can .... for your imaging plane aka
where the first flange/adaptor butts to draw tube ----> Image plane is your back focus.
As for your focuser, I put a 3" in for strength and to carry big loads It also allowed for a non obstructed OAG pick up. So a 2.5" would have been a minimum i would have thought. Keep in mind as prices keep dropping and new sensors come out the 11K chipset no longer is inhibitive to own... So design for a 11K chipset then you will have maximum flexibility without having to stuff around.
As for a 4 point adjustment you can do it but it isn't the best. other wise we would have 4 legs on our tripods. Adjustments become difficult with 4 points of support. So historically we have all used 3 points because geometrically its just easier to adjust
Brendan
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