What Makes Glass Better Than Pastics For Optical Mirrors?
Wednesday, January 13th, 2010 at
6:23 am
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?
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Rigidity, and little expansion or contraction during temperature changes. Also glass is easy to work–to grind and polish.
Look in a regular full length mirror from a little distance so that you can see all of you without much distortion.
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Hi, a good place to start in San Diego is in Balboa park where a lady teaches weekend classes in glass blowing in an outdoor environment and she’s really cool. It takes place in a small plaza where all the art shops are at. Ask around the plaza area and someone will aim you in the right direction. It’s well worth it so good luck and tell her that that silent glass guy on a bicycle from Arizona told you about her. Have fun, you’ll never regret it! Jim
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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).
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Ken, you’ll recall that not everyone with hands participated in Hands Across America.
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