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Originally Posted by SheldonE
Thanks for the responses, if I am reading the it right, the main issue is printing accuracy.
I'm not sure what you mean here, but there are a few plastic we can print now including nylon and poly-carbonate. What mechanical properties are we specifically looking for?
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Hehe what you need to find out is how much a disk of the material will sag under its own weight, and how it deforms with temperature when used as a telescope mirror. It is a function of the mechanical properties of the material (young's modulus, density, the thickness of the disk, and the spacing of the supports).
The tolerance is very small - less than 0.1 wavelength, or 50 nanometers. Even a glass mirrors made from say 25mm plate glass, say 30 cm in diameter, will sag under its own weight more than this if supported on just 3 points. Localised thermal expansion with differential temperatures across the disk of just a few degrees C can also cause hills and valleys to form in a mirror that also exceed this tolerance.
No plastic I have ever heard of is dimensionally stable to that kind of tolerance - not even close.
The surface smoothness required is of the order of 0.01 - 0.001 wavelength, 5 to 0.5 nanometres. Optical plastics used in camera lenses and spectacles can meet this by being cast in polished moulds, however its not something you can do as a home hobbyist.
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Plastic is significantly cheaper than glass, just seeing what materials have been used and if plastics have even been considered.
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They've been tried many times before - and rejected very quickly. You're far from the first to think of it.
If you are trying to make a "quickie mirror", the easiest is know of is to cut a disk of aluminised mylar (as used by florists to wrap flowers) and support it as a membrane by it's edge so that it forms the top across a cup. Using a modest hand pump, suck the air out of the cup and atmospheric pressure will deform the glass into a curved surface. The result is good enough to work as a solar barbecue, but not for forming an image.
You could also use thin disk of a rigid plastic (acrylic, perspex, whatever) or even glass say 1-3 mm thick, like the stuff used for picture frames, it will give a better result than mylar, but again it will but won't be good enough to form a decent image useful for astronomy. Thin disks will also sag under their own weight, and perhaps this will be a good way for you to find out why this doesn't work for making telescope mirrors.