I am new to astronomy, and I have been noticing how expensive large glass mirrors for reflector telescopes can get. I am just curious why a quality mirror can't just be cast or stamped out of say acrylic or polycarbonate or some other cheap plastic, then be polished and coated? Is there some property of plastic that makes it less suitable for a telescope mirror?
2007-12-03 04:11:01 · 4 answers · asked by YeahWellYouKnowHowItIs 2 in Science & Mathematics ➔ Astronomy & Space
It's probably not possible to mold plastic with sufficient accuracy to use as a telescope mirror, because of the way molded plastic shrinks as it cools. That would require polishing and figuring, but plastic is too soft to polish with conventional methods. Thermoplastic resins tend to creep under load, making long-term stability an issue - store your mirror on edge and it could warp like a potato chip.
This is not to say it couldn't be done - small precision lenses are routinely cast from resin. But the driving force behind developing those technologies is the market for millions of small camera lenses. The telescope market is a lot smaller, so the economic incentive to develop the technology is weak while the technical challenges are much greater.
2007-12-03 04:34:31 · answer #1 · answered by injanier 7 · 1⤊ 0⤋
An astronomical mirror must hold its shape to within a fraction of one wavelength of light, usually 1/8 wave or better. Since the middle of the visible spectrum is .55 microns, you need a material that can hold its shape against its own weight to within .068 micron at all times and under all rotations. Most plastic is just too flexible to do that.
There is also the problem of figuring the mirror to the exact shape required. With glass, you just grind it. Plastic is much softer, and therefore harder to "figure" into the right shape. (If you're used to carving wood, carving soft butter would be a frustrating experience).
2007-12-03 04:23:53 · answer #2 · answered by Keith P 7 · 1⤊ 0⤋
*Chuckles* Plastic warps fairly easily- remember, you're working with 1/10th of the wavelength of light- we're talking angstroms! Takes very very little to have a totally unusable mirror.
The best glass to make a mirror out of is pyrex. Its thermally stable at most temperatures, thats why its used.
2007-12-03 04:26:24 · answer #3 · answered by Anonymous · 1⤊ 0⤋
Rigidity, and little expansion or contraction during temperature changes. Also glass is easy to work--to grind and polish.
2007-12-04 03:57:44 · answer #4 · answered by Mark 6 · 0⤊ 0⤋