Reynolds Metals

Why are large telescope reflectors made of glass instead of a sensible structural material like steel?

Primary reflectors for astronomical telescopes up to 8.4 metres diameter are made of glass. As an ex truck driver I can't imagine a more stressful job than driving an oversize, expensive brittle object like that to the top of a mountain to be installed in an observatory. Why not make it out of steel? Make the surface parabolic by spinning it until it's solidified, instead of grinding it. Then plate or vapour-deposition a layer of inert silvery metal like platinum on the surface.

Public Comments

1. Steel is not thermally stable. It changes dimensions too much with changes in temperature, and when you're dealing with tolerances measured in angstroms, that's a big deal. Edit: On the other hand, when Newton invented his reflecting telescope, the 'state of the art' in mirror making was to make it out of polished 'speculum' metal, so the idea is not without some merit.
2. Metals cannot be polished to 1/20 th of a wave accuracy.
3. In addition, the largest mirrors are no longer made in a single piece, so the truck drivers job is much easier.
4. In addition to all the above answers, metals also absorb certain electromagnetic frequencies (light) which glass can reflect. if using steel, some image information would be lost.
5. A metal surface can be polished to about 1/4 wave (about 125 nm) accuracy. This was good enough for Isaac Newton, but glass can be polished to tolerances better than 1/100 wave (5 nm). More importantly, especially for large mirrors, metal expands and contracts much more with changes in temperature than glass does, and even glass mirrors can be useless until their temperature has stabilized. Spin-casting is actually used for some large telescope mirrors, and it is theoretically possible to make a telescope using a spinning dish of liquid mercury as a mirror. A mercury telescope could only point vertically, but it's an interesting concept.
6. Steel would make it heavy and not easy at all to use. Glass is light and it catch's things very easily. I dont see why they would want to use steel, it would be heavy and it would be tough for it to catch signals.