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Telescopes using mirrors - glass appears to be always used?

When constructing the mirrored surfaces for telescopes, I've read of the painstaking efforts involved in producing the glass to exacting dimensions/surface roughness, and then depositing the reflective coating on the surface. Manufacturer's are at/near the limits on the size of glass that can be produced, resulting in some other approaches used to work around the limitationssuch at the telescope that uses multiple hexagonal mirrors (name?) instead of a single large mirror.My question is (and it's probably obvious, but I'm not seeing it), why must glass be used as the substrate? Could not many other materials be used (which don't have the manufacturing limitions that glass possesses), and then the surface mirrored? I would think telescopes of huge appatures could be produced, without the tremendous time and expense of working with glass. Again, I'm sure I'm missing the obvious reason(s) that's not done! I appreciate your time for reading/responding!


it reacts to CO2 2NaHCO3 → Na2CO3 + H2O + CO2 at 200°C and Na2CO3 → Na2O + CO2 at 1000°C so it is basically a regular CO2 extinguisher but can be used in electrical fires or burning fat
You can buy mylar plastic tht has a reflective coating. It is bendable and would never shatter like glass.

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