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Glass Not Metal

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Posted by Robert May on September 20, 2001 18:50:41 UTC

The aluminum coatings can last for decades if it's properly overcoated with SiO or other coating. Then there's also the Clausing's Beryl coatings which last for many decades (many coatings are still good from the first days that the company started in the '50s).
As to the surface finish, you need to have a clean scratchfree surface that is accurate to a few millionths of an inch. Light is about 22 millionths of an inch for wavelenth and you want to have the surface accurate to better than a quarter wave for decent results. Note that this accuracy is over the entire surface and for smaller scale, you want to be better than 1:100th of a wave for surface roughness.
Another item is that the amount of stressing of the mirror substrate, with the cocommitment of bending as a result, is a very real problem as a bent mirror due to a temp diff. across the thickness of the substrate can quickly overwhelm the accuracy limits. Low expansion glass (pyrex) is usually used in mirrors for just this reason as the simple solution is to use something that doesn't move with temp changes won't stress the substrate out of shape.

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