Thread: 24" Nasmyth
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Old 29-04-2007, 05:24 PM
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Orion is offline
Join Date: Jan 2005
Location: Swansea N.S.W.
Posts: 1,107
I think this was the article courtesy from Night Sky Bintel.............

Precious Metal Clay, PMC, was developed and patented in the 1990’s by
Mitsubishi Materials of Japan and has been used since then to create handmade
items of jewelery. Microscopic particles of silver are mixed with a
moist binder to create a material that has the feel and working properties of
modeling clay. Using simple tools, objects are easily given shape, texture,
and character. After air-drying, the objects are heated to temperatures
approaching the melting point of the metal, where the particles fuse together
to make a dense, fully metallic object. Fired PMC work can be polished,
soldered, enameled, and worn as jewelery like any other silver item.
By substituting powdered Zerodur for eighty percent of the silver, test mirrors
have been made using the same process. Astronomical mirrors produced in
this manner are self-reflective coated and require only a very thin dielectric
coating to be made 98% reflective.
By utilising a pressure mould and injecting the PMC/Zerodur/silver slurry
into the mould under high temperature and pressure, a very accurate, thinwalled,
honeycomb mirror is produced. The surface accuracy, when removed
from the mould, is of a very high order and requires minimal ‘finishing’ to
produce a useable mirror.
The problem of ‘print-through’, which has always been the bugbear of
honeycomb mirrors, is avoided because of the added thermal stability due to
the Zerodur content and the open back of the honeycomb. Cost compares
extremely well with conventional mirror production. PMC mirrors are one
third the weight of a similar-sized glass mirror and are subject to less than a
tenth of the flexure. Thermal stability is almost as good as Zerodur.
PMC Glass Corporation has made a number of 1.45metre mirrors available
for evaluation by amateur astronomical societies under different climatic
conditions. Two of the mirrors are allocated to Australia. The first has arrived
and will be installed in a large Dobsonian-style mount. The second is yet to
be allocated to an astronomical society, but will be fitted into an equatorial
mount for comparison purposes.
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