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Subject: Re: Another question from the peanut gallery
From: Brian Whatcott (betwys@XXX.XXX)
Date: Thu Oct 17 2002 - 08:08:36 EDT
The favored method for astronomical telescope mirrors is front
surface aluminizing, with or without a protective coat.
Without periodic washing with detergent flakes and distilled water,
the period to require a new reflecting film on such a mirror can be
four years.
If the front face has been coated, this coating must first be removed.
Four years is just too short a life for a sextant. And the mirror
requirement is different. Rather than squeezing the last percent or two
over that 90% plus reflectance, you would prefer long life and
robustness.
So I suggest that accepting low intensity double reflections, and
acknowledging that one uses filters to cut down light transmission
through a sextant as often as not, going with the maker's choice
is probably the right thing to do.
Brian W
At 10:28 PM 10/16/02, you wrote:
>Thanks to all for leads on re-silvering of mirrors.
>
>Now another question (because questions beget more questions):
>
>The general consensus, both on this forum and in navigation texts, is that
>front-silvered mirrors are the preferred option as they provide superior
>performance to that of rear-silvered mirrors.
>
>I am the proud owner of a C.Plath as, I am sure, are many other list
>members. In looking at my Plath, it is apparent that the mirrors are not
>front silvered but are silvered on the rear. Yet C.Plath was (is) the
>Rolls Royce of sextants. That they did not go the whole nine yards by
>silvering their mirrors on the front surface seems incongruous to their
>high status in the sextant world.
>
>Does anyone know why they may not have chosen to go with the front
>silvered mirrors?
>
>Robert
Brian Whatcott
Altus OK Eureka!
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