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Re: Mirrors Of Thin Laminated Glass

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Posted by greg welte/">greg welte on March 14, 1999 13:46:26 UTC

: Forgive this very naive question but is it possible to make an 8" or 10" mirror for a reflector out of an assembly of 2 6mm plate glass sheets epoxied together and the lot mounted by epoxy on a 25 or 30mm craftwood (fine chipboard) base? Will it work? Thanks.

I have wondered that myself, since the windshield in your car consists of two sheets of laminated glass, with a tough plastic sheet in-between. The windshield suffers far more abuse than any telescope mirror ever would, yet rarely de-laminates.

I read in a telescope making book that, in the 1920's, a mirror was made by a student at Yale, in which two discs of glass were cemented together, with strips of glass between. That mirror failed after a couple of years. However, epoxies did not exist in the 1920's. Further, that failure is not surprising (to me). Even if all of the glass pieces are identical in composition, they are probably not of uniform thickness. That non-uniform thickness probably contributed to the failure. (Try gluing together two pieces of wood which have been sawn on a tablesaw. The joint will not hold. You must plane the surfaces smooth.)

I contacted a mirror manufacturer recently on this subject. He said that a buddy of his laminated a sheet of Pyrex to a ceramic base, but got unbelievable astigmatism in the final mirror. However, I think that result should not be surprising. The coefficients of thermal expansion of the Pyrex and the ceramic are certainly different. That difference will create ENORMOUS stresses in the structure, as temperature changes (which it always does, unless you place the mirror into a temperature-controlled cell).

Anyway, it would seem that, if you glue sheets of identical glass together, using epoxy, that you should obtain a stable structure. I plan to undertake an experiment, because I am building a mirror- grinding machine.

That's what I know.

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