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    Re: Sextants with Polarizing filters
    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.
    
    
    

       
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