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    Re: Another question from the peanut gallery
    From: Brian Whatcott
    Date: 2002 Oct 17, 07:08 -0500

    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
    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?
    Brian Whatcott
       Altus OK                      Eureka!

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