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Hi Roger,
An 8 inch f10 would be a nice instrument - no visible coma, dark field, small diagonal etc. You will easily get a 1 degree field. The only advantage (to me) in going to F12 would be the possibility of leaving the mirror spherical, if you are making it yourself. I would have to do the figures but I think an 8 inch F10 needs parabolizing.
I am fairly sure Roger Davis (from Bintel) once built a long focus newt around F9. You could PM him about what he thought of it.
Rod.
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