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I've built a 12" f5/ f20 Cass in the past. As Ingalls says in ATM bookII, don't!!!
A far easier solution is to make use of the excellent Powermate barlows which have come on the market to achieve the same focal length as the cass. Much less hassle and easier on the pocket.
Moving the primary in a classical Cass screws up the images and introduces abberations! Baffling the system is also a major issue. The mirrors are designed to work at ONLY one spacing to get a back focal length.
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