What makes glass better than pastics for optical mirrors?
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?
Aravis
on November 30th, -0001
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).
Solitarygrowth
on November 30th, -0001
*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.
Connie
on November 30th, -0001
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.
Tad
on July 6th, 2011
Carbon fiber reinforced plastic (CFRP) is being used on the Webb space telescope. Nasa has contracted the people here in Tucson, Az to build it:
http://www.compositemirrors.com/
They have also built a 1 meter telescope out of carbon fiber reinforced plastic that weighs a total of 175 pounds (complete Optical tube assembly). These types of mirrors hold tolerance as good as quartz or borosilicate and apparently have one tenth of the thermal expansion and contraction of a glass mirror. Once the mold or “Mandrel” has been built for a particular mirror the mirror can be replicated many times on the same mandrel.
I am assuming adaptive optics will also be used on the Webb but I am not sure. But it is clear that the technology exists with current materials to make a large mirror that is up to NASA’s standards.
Clearly it is because of the thickness and rigidity of carbon fiber composite that this is possible. I have worked out a rough cost excluding labor for such a mirror of 1 Meter. It is about $2000.00 American just for the Carbon fiber and epoxy. A solid piece of billet steel would most likely be needed for the mold cavity which would be very expensive unless alternative methods were used for producing it. Layer by layer would be laid down in order to reduce movement of the composite as it cured, and a thickness at least as thick as most glass mirrors would probably be needed. Somewhere on the order of 2 inches thickness I am assuming. I have no idea how they polish and finish the mirror but they are claiming that very accurate surfaces are obtainable.
I would imagine if a person was ambitious enough and have composite craft skills from the aerospace industry etc., it would be possible to duplicate what they have done on a smaller level without too much difficulty.
Tad