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Why glass for primary mirrors?
My question is why primary mirrors seem to be always made from glass? Since a mirror eventually needs to be coated with a reflector anyway it's obviously not for any inherent optical qualities. So why aren't more versatile substrates used - I would have thought there would be suitable plastics and resins around that could be lighter, easier to work with (lower temperature casting, chemical solubility for finishing, ...), and just as able to hold there end shape.
So why does everyone still start off with a glass blank to fabricate a mirror?
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