Long focus mirrors?

I might be interested in making a very long focus mirror (maybe f/20 or more), for either a folded or Cass/Newt (or even Gregorian) type system. Does anyone have any experience with these? Can the optics remain spherical in the Cass/Greg systems? Also how do you test such mirrors- will classical tests work at high focal ratios?

Thanks.
Jess Tauber

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The difference between the best fit parabola and a spherical surface on an 8" F20 mirror is only 0.011 waves error. You could never reliably measure this, let alone work to it.
A good sphere should do.

clear skies,
dan

Hi Jess,

Rick Schaffer wrote a wildcard article 006.1 in the Reviews section under advanced articles that details a long focus unobstructed newt using three long focus spherical mirrors. Similar to the Schiefspiegler but supposedly producing less abberitions. I've considered build a 6 inch F18 because I like planetary observations. I've done two 6 inch F10 mirrors on a machine and they turned out quite well. I hope to complete some existing projects to start on the Stevick-Paul before Mars comes around again.


Good luck Dave