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    Re: Sextant mirrors
    From: George Huxtable
    Date: 2007 Oct 9, 10:47 +0100

    Jim Martin wrote-
    
    | Issue 96 of the Nav.Found,newsletter has an article concerning
    | mirrors.In which the author suggests that in an emergency the mirrors
    | may be replaced,using any type of mirror available e.g.vanity,hand
    | etc.It is my understanding that sextant mirrors are manufactured from
    | an optically flat material,which to me means that not only the two
    | sufaces be  flat but,also the refractive index.Is this correct.I have
    | read that using ordinary glass results in multiple images.
    
    and Fred Hebard added, in 3386-
    
    Some sextant mirrors have the reflective surface on the front, but,
    traditionally, they've been on the back, I suppose in order to
    protect the reflector; they allow coating of the back and sides with
    salt- and water-resistant materials.  Yes, rear-coated mirrors give
    multiple images, but they are easy to distinguish.
    
    and Bill adds, in 3387-
    
    I use an Astra IIIB deluxe, with a traditional (split) rear-silvered horizon
    mirror and front-silvered index mirror. I have not noticed this phenomena.
    
    ===================
    
    Bill is right, and the others are wrong. We have been into this matter
    before.  The problem doesn't exist, with a normal sextant mirror. No matter
    which side the glass is coated, the thickness of the glass does not give
    rise to multiple images, not at all. The refractive index and the thickness
    of the glass make no difference whatsoever. With rear silvering, some light
    will have been reflected from the front surface, some from the back, but to
    the observer's eye the two images will EXACTLY coincide.
    
    The only factor to give rise to multiple images would be if the front and
    back surfaces were not exactly parallel, or if the glass quality was so bad
    that its refractive index varied from place to place across the mirror. For
    precise work, glass for sextant mirrors is selected and ground to avoid such
    problems. The surfaces also need to be flat so that the image stays in the
    same spot, wherever on the surface of the mirror it's reflected from.
    
    But these requirements are not hard to meet, to the sort of standard that 's
    required for the "emergency" that the article referred to. For such an
    emergency, any old bit of mirror would do, and would be a lot better than
    having no mirror at all.
    
    But I wouldn't then try to use it for lunar distances.
    
    George.
    
    contact George Huxtable at george---.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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