Re: [ATM] Rotating Resin Mirror

[Tony Gondola <acgna@comcast.net>]

You'll notice that's an IR mirror, it's not designed for visual 
wavelengths which are much shorter and demand a much higher surface 
accuracy. IR mirrors of smaller size can also be molded. In theory spin 
casting any kind of resin or even molten glass should work but it's 
almost impossible in practice. The biggest problem is changes in the 
shape of the surface as the material hardens or cures. That process is 
never 100 isotropic and always seems to introduce distortions that ruin 
the surface. It's not uncommon for large telescope mirrors to be spun 
cast in glass but that's just to reduce the grinding and polishing time. 
They still have to be finished using conventional methods. In short this 
is a method that looks beautiful on paper but putting onto practice is 
something else again.

Tony


Ian R. Williams wrote:

>Has any body heard of an amateur project that uses the "spin-cast" 
>method?  A rotating cylinder of fluid takes on a parabolic surface.  If 
>you make a cylinder of resin and spin it on a mill at just the right 
>speed until it sets you should have a parabolic former.  The resin can 
>then be aluminized to make a parabolic mirror.  A number of large 
>professional telescopes have been constructed using this method, e.g.:
>
>http://www.nasm.si.edu/research/dsh/artifacts/SS-caltech2.htm
>
>Has anyone heard of this been done as an amateur project?  Surely 
>spinning resin is cheaper than grinding glass?  All the required 
>mathematics for the problem can be found at:
>
>http://www.syvum.com/cgi/online/serve.cgi/eng/fluid/fluid304.sal?0
>
>Any thoughts on the idea would be much appreciated.
>
>Ian
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