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Re: Could a plexiglass mirror work for a reflector

Started by jonisaacs, 10/10/2009 08:50AM
Posted 10/10/2009 08:50AM Opening Post
"Plexiglas" is a trade name for the polymer Poly-Methyl MethAcrylate, often shortened to PMMA or Acrylic.

There are a variety of reasons it would be an ideal material for a mirror but one that has not been mentioned is it's thermal properties and homogeneity. Polymers are very sensitive to temperature differences, PMMA has a coefficient of thermal expansion that is about 20 times that of Pyrex, a small change in temperature results in large changes in dimensions. This in combination with the difficulty in making the material exactly uniform throughout the sheet would mean the mirror would not maintain it's shape.

Other issues mentioned at the stiffness and hardness, PMMA is about 1/20th the stiffness of Pyrex and something like half the density, that means a comparable thickness piece would deflect about 10 times as much under gravitational loads.

PMMA works for small, non critical optics but for telescope mirrors where fractions of a wavelength of light are critical, I wouldn't recommend it.

Posted 10/10/2009 02:25PM #1
Stiffen with embedded mineral fibers away from the optical side? Would create a composite structure heavier on the plastic side (usually such efforts have more stiff elements than plastic or sticky ones).

Jess Tauber
Posted 10/10/2009 03:16PM | Edited 10/11/2009 03:28AM #2
You could probably make a solar cooker like this, say a 3' diameter, f/1?
Posted 10/11/2009 05:03PM #3
Scott Kennon said:

That's the type of thing I'm talking about. I could have something underneath the mirror, like a piece of steel or sheet metal that was molded to the exact shape of the plexiglass mirror. I could super-glue them together, so that the plexiglass wouldn't be able to move, not even a fraction. I read that someone successfully got a 56" piece of plexigalss polished well enough to be 95% reflective. This guy was using it for a solar panel.

The Plexiglas will move and it will move much more than a fraction... The tolerances on a mirror are tiny, for a mirror it is measured in fractions of a wave length of light. Look at a piece of printer paper, it's about 1/10th of a millimeter thick. Then consider that the shape of a telescope mirror needs to be about accurate to about 1/1000th the thickness of that paper over it's entire surface.

Gluing a piece of metal to the back actually causes problems because they expand and contract differently as the temperature changes.