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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sextants with Polarizing filters
From: George Huxtable
Date: 2006 Jan 27, 00:27 -0000
From: George Huxtable
Date: 2006 Jan 27, 00:27 -0000
Maybe there's a rather important question being missed, about polarising filters as fitted to sextants. Looking unprotected at the Sun, with or without a telescope intervening, is a dangerous thing to do. A conventional sextant shade, to be used with the Sun, is a VERY dark one. Do some observers use a pair of nearly-crossed polaroid filters, with no additional filtering, to view the Sun? Perhaps they do, if the polaroids are sufficiently good to achieve near-extinction? If that's the case, I suggest that there are two dangers which such users ought at least to be aware of. 1. Setting those angles to be nearly-crossed must be a very delicate adjustment, and a slight accident which might shift one even slightly could cause a lot of sunlight to flood in, quite unexpectedly. 2. I have heard (but don't know whether it's true) that the main cause of retinal damage is the effect of heat from the Sun actually overheating a spot in the back of the eye. I have read that good sextant shades are designed with that in mind, and that some cheaper shades have been made using colour-film transparencies, the dyes of which only look as though they are effective, but which might transmit a dangerous fraction of infra-red. With that in mind, can we be certain that a pair of crossed polaroids extinguishes heat just as well as it extinguishes light? Does it operate just as well in a very different waveband, for which it was never specifically designed? For sextant users, that seems to be a question that ought to be asked. Has it ever been tackled? Could we tackle it experimentally, using a burning-glass, a thermometer, and a pair of crossed polaroids? George. contact George Huxtable at george@huxtable.u-net.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.