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Help with Greory/Cass. Design Params.???

Started by Eblanken, 11/23/2003 11:23PM
Posted 11/23/2003 11:23PM Opening Post
Hello fellow ASTROMARTERS,

I have an 8 inch f/3 spherical mirror that I bought from Surplus Shed (an ASTROMART sponsor) and was thinking of trying to make a Gregorian Cassagrain scope just for the experience. My questions are, "Can I use a 75mm f/4 sperical mirror as a secondary ?" and "How bad would this design perform ?" and "What correction could I put into the system to make a "decent" performer ?"

Thanks in advance for your thoughts,

Ed

P.S. Happy Holiday Season to all.

P.P.S. Rick or Vlad, are you there ? I assume this would be "child's play" for you guys to "slam out" with your tools and experience. Don't spend any "real" time on this yet, since I'm in the "ideas flowing, but little time to build until after New Years" mode.
Posted 11/25/2003 12:48PM #1
Ed, Scott is right - you can't have an acceptable two-mirror system with two spheres. A pair of spheres would give you horrendous spherical aberration. Even with only primary spherical (and aspherised secondary) coma is still exorbitant, worse than with an f/3 parabola.

Other than aspherising the primary, one thing you might try is to get one of those 6" short tube ota's with spherical primary and negative lens corrector in front of diagonal. I think those primaries are also ~f/3 and, if so, the corrector should work as well with an 8" sphere (as long as it is placed at the same distance in front of the primary's focal plane).