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I think Mark (Satchmo) has a pretty good handle on how difficult it is to make large, fast mirrors. The best really fast 20" mirror I've ever viewed with was one of his (f/3.7 from memory). The folded design is fine for imaging where secondary obstruction is not as critical but otherwise you have to pick your poison, a large obstruction or a very long focal length yielding a tiny field of view.
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