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