Why are mirrors always made of glass?

[fwm891]

I think Silicon Carbide is used for your more exotic optics.

Silicon Carbide is the grinding material used to shape optics...

[Peter Drew]

I have an excellent 18" BVC (Black Vitreous Ceramic) and a 42.5" part finished blank. Better than plate glass and not as expensive as the more exotic materials.

[FraserClarke]

I have an excellent 18" BVC (Black Vitreous Ceramic) and a 42.5" part finished blank. Better than plate glass and not as expensive as the more exotic materials.

Does anyone know of a supplier these days though? The canadian source has had "we're not accepting any new orders" on there webpage for about 3 years now (at least everytime I look).

Silicon Carbide is used as a substrate for mirrors, where you need a very high stiffness/weight ratio -- and are prepared to pay for it! The Herschel primary mirror is a 3.5m SiC mirror only 3mm thick...

[stevepenny]

Aluminium Silicon is used to manufacture mirrors that are spun as part of a laser. The material is light and exhibits very low thermal expansion. In addition a reflectivity of >90% and a tolerance of 0.05 Microns can be achieved.

Steve