Re: [ATM] Mirror Silvering

[David Weinshenker <daze39@earthlink.net>]

Che Ers wrote:
> I had a piece of glass silvered from a commercial mirror-maker,
> [as an experiment], but the coating began to corrode almost immediately.
> So how does the silvering done on telescope mirrors escape corrosion,
> even for some time?

As noted, telescope mirrors are normally aluminized these days -
the silver coating does begin to corrode almost immediately
(especially if there is sulfur in the air) - a close fitting
cover over the mirror was sometimes used to protect it between
uses, but the coating would need to be renewed perhaps several
times a year depending on conditions. (At least this could be
done by a simple chemical process; unlike the older mirrors of
solid tin/copper alloy, it wasn't necessary to re-polish (and
therefore, re-figure!) the shaped optical surface of the glass
substrate.

Now, of course, vacuum-evaporated aluminum is commonly used in
place of silver - it has the same advantage that the reflective
layer can be renewed without re-polishing the figured surface,
but it is rather more durable than silver (and will generally
stay good for at least several years).

-dave w
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