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Subject: Re: Re-Silvering
From: Dr. Geoffrey Kolbe (geof@XXX.XXX)
Date: Wed Oct 16 2002 - 03:34:09 EDT
Robert, I have just had the mirrors on my sextant "re-silvered." There are
a couple of things you should think about.
First, most sextant mirrors of any vintage are silvered on the back face of
the glass. This can cause ghost images due to reflection off the front face
of the glass, particularly from the index mirror where the celestial body
is at at high angle. I had the mirrors aluminium coated on the front with a
quartz overcoating to give the aluminium some rub resistance.
Secondly, (if you go this route) you should consider having the glass
surface re-polished before the coating is put on. It is difficult to get
the glass really clean without a re-polish and the aluminium tends to flake
off. This is quite cheap as specialists in this sort of work will have a
machine called a Lap-Master on which mirrors are polished flat to a
fraction of a wavelength. I had mine polished flat to half a wavelength, as
I am making a small 20x telescope to go on the sextant. Before polishing,
the mirrors were flat to about three wavelengths, which is fine for a low
power telescope.
Yours aye,
Geoffrey Kolbe.
>Does anyone know of any firms that still re-silver sextant mirrors? I am
looking for outfits or people on the continental US and Canada. The only
firm that I knew of was Maryland Precision Nautical out of Baltimore but
the main owner passed away last summer.
>
>Thanks,
>
>Robert
>
Border Barrels Ltd., Newcastleton, Roxburghshire, TD9 0SN, Scotland.
Tel. +44 (0)13873 76253 Fax. +44 (0)13873 76214.
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