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Originally Posted by Merlin66
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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Stop it Merlin, your depressing me and crushing my dreams
Seriously, thanks for the advice. The sort of planetary imaging I am doing and want to keep doing runs upwards of F40 which is even beyond a 5x powermate. But I know Bird bumps his up more with extension tubes etc so maybe a good option. I've actually got a 12inch F7 newtonian OTA I made some 15 years ago, but it's a challenge for mounting... hmmm, actually maybe I could dob platform something up for that one. I like the eyepiece at the bottom end, but then I like $ too....
cheers,