Why is a mirror in a telescope made from glass?
Most of the reason is that the glass is low expansion glass and the cooling problems of a large mass are horrendous to deal with, one of the earlist methods of doing this was to slow the cooling rate ( to stop cracking ), using cow dung, (Edmund Halley Astronomer Royal, used this method ) aluminium, alas has expansion problems when set, but not all mirrors are glass, some are actually Mercury, spun to a high speed, causing it to form a parabolic depression, the weight is only a problem in space mirrors such as Hubble, the coatings, are rarely aluminium, (because of corrosion effects, of this metal, which causes a white powdering effect), mostly they are rare metallic coatings, which are used, because they do not “rust”
