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My CDK 17 with reducer is F4.45 and 2000mm. But it weighs 49kg plus imaging gear, plus plus and is expensive.
There are a number of astrographs appearing on the market. Ceravolo 12 inch has a corrector so it does F4.9. IDK have an F3.8 astrograph, so does Officina Stellare. Both very expensive scopes but probably pretty nice.
F5 is about the fastest you'd want to go for an easily managed scope. Faster than that requires an extra level of precision of flexure, camera squareness, focus etc.
Greg.
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