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Test Results With Thin Mirrors

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Posted by Tim Brezina on March 11, 2004 20:03:31 UTC

I seem to be getting ahead of myself, but I recently ordered a 10" X 3/4" mirror blank for the purpose of constructing an f/6 newt. Then I started to worry about the possible problems I could encounter when testing a thin mirror.

If the mirror is placed in the customary testing position (i.e., standing up vertical and resting on it's edge, maybe leaning back a bit) is there any chance that such a mirror could be flexed or stressed by its own weight and thus lead to inaccurate Ronchi and Foucault test results?

I suppose I could lay the mirror on it's back and conduct the testing from 10 feet above (i.e., twice the focal length), but that does complicate matters.

Another (crazy) alternative that occurred to me is to hold the mirror in the traditional testing position with suction cups, instead of having the mirror rest on its edge.

Actually, I have no idea if I need to worry about it, although I did start to wonder how the very large and very thin mirrors are tested without placing stress on the mirror. Perhaps I should just relax and have a homebrew? Thanks in advance for your help and advice. Great forum...

Tim B.
New Orleans

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