There has been some discussion of how to deal with pentaprisms with damaged
silvering. It appears that there are no more spare prisms available.
I would comment that if the glass is undamaged, which is likely the case, it's
easy to strip the silvering and re-silver a prism.
Stripping: Simply immerse the prism in Nitric Acid until clean. This is the
standard approach, and will strip the paint and the silvering, and will not
harm the glass.
Resilvering. There are two approaches, true silvering and aluminizing. It's
likely that the pentaprisms are aluminized, not silvered, but for repair either
approach will work. Protect surfaces not to be silvered with varnish, which is
later removed by soaking in acetone.
Silvering. With metallic silver deposited from solution, the traditional
silvering process. This can be done one prism at a time in one's kitchen or
basement using "Brashear's process", and is documented in many places, such as
in 1960's editions of The Handbook of Chemistry and Physics, and old books on
making telescopes. I don't recall the details, but Silver Nitrate and Nitric
Acid are involved. The process is quite simple and robust, but absolutely
clean surfaces are required. Thus the use of Nitric Acid to clean the glass.
More recent books omit chemical silvering processes, because vacuum
aluminization pretty well displaced all use of silvering.
Aluminizing. One sends the optical goods off to a company that does it for
you, unless you have enough aluminizing work to do to make purchase of the
equipment worthwhile. Amateur telescope makers (ATMs) make their own mirrors,
and then send the mirrors off to be aluminized. Look in the back of magazines
like "Sky & Telescope" for the ads. There are probably newsgroups and
reflectors for ATMs. Some ATM books have the receipe for silvering as well.
It's cheapest to have a bunch of prisms metallized at once. It isn't terribly
expensive in any case.
Back paint. Whichever kind of metal is chosen, especially if it's silver, it
must be protected with a coat of black paint. Suitable paints are sold to
Glass & Mirror shops. If one has aluminizing done commercially, one can have
the coater do the painting as well.
Anyway, don't throw those old prisms out. Collect them and re-silver them.
Joe Gwinn
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