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    Re: Sextant mirrors
    From: Nicol�s de Hilster
    Date: 2007 Oct 09, 14:38 +0200

    > 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.
    > ===================
    > Then George added in 3388
    > 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.
    And now I add a bit too:
    I played around with my David White & Co 1941 US Navy quintant. I 
    removed the telescope and observed a high contrast image through the 
    index-mirror only. In this way I saw two ghost images, one on either 
    side of the main image (something Van Breen already mentioned in 1662 
    when he described his spiegelboog [Dutch for mirror-staff] in his book 
    Stiermans Gemack). Now when I put the telescope back in place and 
    observe the same image through the horizon mirror and index mirror, so 
    using double reflection, the two ghost images seem to have disappeared 
    or at least have become too faint to distinguish. So it is the 
    combination of two mirrors that makes the multiple images disappear for 
    the eye (so they do not exactly coincide as George said). When using the 
    spiegelboog one has to deal with those annoying multiple images (and so 
    had Robert Hooke with his single reflecting instrument in 1666).
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