I was just reading about some very large observatories being planned or under construction, and how these huge mirrors require up to 6 months to cool, or the glass will crack. So I have to ask the question, why use glass at all? My understanding of mirrors (for reflecting telescopes) is that they put the reflective coating on top of the glass, and it's that reflective coating that does all the reflection. This is different than a normal household mirror, which puts the reflective coating underneath the glass for protection, so then of course you need something transparent like glass. But I'm asking about telescopes, which put the coating on top. So why not just use steel, or some other stalwart material, and put the reflective coating on that?