>If you make the test sphere too long in radius it will be very difficult
>to detect zones with a knife or grating, and your trouble with air
>gradients will be greater. I think F10 at the center of curvature ( ie
>100 inches radius ) would be fine.
I read (Bob Cox, TMT number 4) that f/10 is considered "doable" but longer is better, f/20 to f/30 being ideal. I can fit 35 feet (f/21) in my basement, which has nice stagnant air. They say that a little telescope, e.g. binoculars, at the Foucault tester, focused on the sphere, makes it easy.
Bratislav: nice idea. The 10 inch mirror would be just f/21, but you've got me thinking solar/lunar telescope now; I could mask off most of the mirror and still have enough light. The 1.8 inch image would just fit in a big eyepiece.
Aart M. Olsen aart@uiuc.edu 217-333-7467 College of Veterinary Medicine Univ of Illinois at Urbana-Champaign