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You can get around the field curvature problem ( or at least lessen it) by using an aperture mask, if you masked it down to say 14" or maybe 13" bring the F ratio up to say f2 or f3 ,much less curvature.
I am currently building a 8" Wright/Schmidt using a F3.75 mirror, match this with a large format camera and you get one hell of a lens :-)) super fast with a large field of view.
You shouldn't have a problem getting focus, just allow the for the height of the focuser in your calculations.
I have no problem getting focus with a 8" F3 Wright/Schmidt, although I haven't tried with any form of CCD on this scope, Hmm! something to try next time the scope is out maybe
David.
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