Re: [ATM] New to the mailing list

[Mark Holm <mdholm@telerama.com>]

I won't quite agree with Bob May's reply.  Glass doesn't quite have all
the properties one could wish for, but it does have the most critical
ones.  Every other material anybody has tried, and there have been a lot
of tries, has come up short in some rather serious regard.

I understand some of the spy satellites use beryllium mirrors.  Be is
light and strong.  I think they have to nickel plate it to get it to
take a good polish.  Don't know what sort of reflection enhancing
coatings they use.  Probably something expensive!  Be does have some
drawbacks.  #1 is that it is badly toxic.  Not at all the sort of thing
you would want to work with in a home workshop.

The first telescope mirrors were made from a family of metal alloys
known as speculum.  If you do a little reading of the trials the old
timers went through, you will easily see why they dropped speculum like
a hot potato as soon as glass mirrors became practical.

The most promising recent non glass mirrors, for those not on the NSA's
budget, have been made as part of a NASA research project by Peter Chen.
 He got tantalizingly close to high quality with good stiffness and very
light weight.  It wasn't a tremendously easy process though.  Not as bad
as beryllium, perhaps, but still a lot more work than glass.

Pyrex makes darned nice mirrors at a pretty reasonable price/performance
point.  Plate glass is a bit cheaper and more available outside North
America and Europe, but not quite as nice in thermal properties.  Both
have to be quite well annealed (as do all good optical glasses) and that
drives the price up.

Even better thermal properties can be had from Astro-Sital, Zerodur,
Cer-Vit and a few similar materials.  There is a significant price bump.
 Astro-Sital is the cheapest, because the Russians work cheaper than the
others.  Fused silica is highly regarded for high precision, high
performance mirrors, but there is another price bump on that.

Mark Holm
mdholm@telerama.com


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