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Cervit Flat

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Posted by Robert May on August 27, 2001 18:45:19 UTC

You probably got that thing for a song! Something like that is not to be let out of your possession if you can help it!
I have a 12.5" flat that I picked up for a song years ago and consider it to be worth about 3x the price of a equavilent mirror (several thousand dollars at present). I talked with a retired optician and, while I forget the exact value that he stated after testing it (better than 1/10th wave) it was over $10K in value. It used to sit in a drawer in my desk but it now has it's own cloth lined box to protect it from damage.
I'd play with it to find out exactly how good it is by doing Foucault tests in telescope mirrors in the autocollimation (the flat returns the light back to the scope) to find out how decent the piece is but I would suspect that it's better than 1/10th wave as an autocollimation flat. The 12" size is nice as it will allow for large size optics to be tested - you can directly test 12" and through some inventive offsetting of the center of the flat, test larger pieces. It is probably the best way to do testing of full optical systems as you get a double pass on the light through the system which means that the errors are double what is real.
FWIW, Cervit is an extra low expansion glass ceramic and it's not cheap at all to begin with. Go check out prices on small pieces of the stuff.

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