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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Hello and a question about polarizer sun filters
From: Wayne Hilliard
Date: 2016 Dec 6, 17:44 -0500
From: Wayne Hilliard
Date: 2016 Dec 6, 17:44 -0500
Geoffrey, I can only presume that the manufacturer used the proper material as these were widely used in the Navy etc. What other materials were used in sextants for shades.
Also I just realized that the book that was just delivered today was written by you! The Long Term Almanac! Looks to be very interesting!
Wayne Hilliard
On Tue, Dec 6, 2016 at 4:58 AM, Geoffrey Kolbe <NoReply_GeoffreyKolbe@fer3.com> wrote:
On 5 December 2016 at 23:13, Wayne Hilliard <NoReply_Hilliard@fer3.com> wrote:
I guess I have 2 questions. Is this type of filter safe to use and assuming that these are the original filters could they have degraded with age.
I would doubt that....I have contacted the Robert White Company in Boston and have gotten pricing on overhauling this sextant and replacing the filters with new polarizers. But would like to have input from the folks on here about this subject.
Simple plastic sheet polarisers will (when crossed) give an extinction ratio of about 1:500. At the other end of the expense scale, birefringent polarisers (such as Glan-Taylor) will give about 1:106. In my experience is messing about with filters, you want about105 reduction in intensity for the sun to be comfortable to look at. The lesson is that cheap sheet crossed polarisers on their own are not safe to look at the sun.Then there is the problem of whether the polariser is truly 'neutral' across the whole spectrum and is only effective in the visible, say, but lets through the infra red.Geoffrey Kolbe
