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    Horizon mirrors: was [NAV-L] Question about Davis Mk 25 sextant beam converger
    From: George Huxtable
    Date: 2006 May 10, 11:20 +0100

    On May 03, GregR asked-
    
    "I bought a Mk 25 sextant a couple of weeks ago,
    and am wondering about the accuracy of the beam
    converger (it's a full-horizon model vs.
    split-mirror) "
    
    I think between us Nav-L members have resolved
    GregG's problems for him, but it may be worth
    discussing again the values of what Davis call a
    "beam-converger", and others call a full-horizon
    or full-view mirror, as opposed to the traditional
    half-horizon mirror, silvered across only half its
    width.
    
    What's brought it to mind is a thoughtful note,
    (together with an abstract of the Davis patent
    about their version) in the Spring issue (No 91)
    of "The Navigator's Newsletter", by its new
    Editor, David Burch (of Starpath). That quarterly
    journal, issued to members of the Foundation for
    the Promotion of the Art of Navigation, has been
    through a very lean patch, but has recently been
    thoroughly transformed, I'm pleased to say.
    
    A few Nav-l members pop up in that journal. For
    example, Bill Murdoch (a name familiar to Nav-l
    readers) has contributed about that Davis patent.
    
    I won't copy details of that note, except David's
    "nutshell summary", which goes as follows-
    
    "Full-view mirrors make the easy sights easier,
    and the hard sights harder."
    
    That view corresponds exactly with the reports of
    the "traditional" navigators, using lunar
    distances, in the 2001 re-enactment for the BBC of
    a leg of Cook's first circumnavigation. They found
    that in daytime a high, pale, Moon was invisible
    against a milky sky using a full-view mirror, but
    showed up clearly with a split mirror. It was a
    question of contrast.
    
    For anyone interested, the foundation is run by
    Terry Carraway, and the subscription for members
    in the US is (I think) $30. The website address
    is-
    www.navigationfoundation.org
    the address is PO box 1126, Rockville, Maryland
    20850, USA.
    Phone or fax is 301-622-6448
    navigate1@comcast.net for emails.
    
    George Huxtable.
    
    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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