Mirrors

[JJDOBBER79]

Hi diy enthusiasts.
Please indulge me in a radical idea I have had. The high cost of a telescope mirror is due to the large chunk of glass. Why do we use glass for this? Is it because it can be highly polished. I am uneducated in this field so bear with me. Has anyone ever tried aluminizing a different material. Steel can be highly polished as can aluminium. And lightweight. And durable. What about a smooth plastic. Perhaps from a 3d printer? Shaping a steel or aluminium blank could be done on a lathe in no time. Polishing can also be done on a lathe.

[mswhin63]

Glass especially telescope glass like Pyrex has low expansion properties, metal expand and deform significantly over temperature variations.

[JJDOBBER79]

Does steel/aluminium return to its shape when cooled? I would think that it would cool in minutes not hours like glass.

[pdalek]

The first telescope mirrors were metal. Soon switched to glass - much easier to make.
The old Amateur Telescope Making series of books described making metal mirrors.
Lots of military mirrors are beryllium - nasty poisonous stuff - not for home use. Beryllium is used for the James Webb Space Telescope mirrors.
Other metals have been used in military or space applications where performance outweighs cost.
I think the mirrors for the MAGIC Cherenkov telescopes are aluminium.

Metal is generally difficult to polish well - random crystals polish at different rates, also work hardens in patches. High thermal expansion is a problem.

Several liquid metal - mercury - have been made, I think one is still operating in Canada.

Some new designs using carbon composite, e.g.
http://www.compositemirrors.com/

[cometcatcher]

I tried to make a metal mirror once using the same techniques as glass grinding. The going was extremely slow! Much slower than working with glass. I gave up and never finished the metal mirror as I could have made a dozen glass ones in the same time.