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    Re: Davis Instruments Mark 15 Sextant
    From: Trevor Kenchington
    Date: 2003 Feb 18, 21:50 -0400

    George Istok wrote:
    
    > Just soliciting opinions on this sextant.  Any thoughts appreciated.
    
    
    Mine is a Mark 25 but I doubt that there is much real difference from a
    Mark 15.
    
    Working on land, with a sea horizon, I am disappointed any time I get an
    LOP more than a mile from my true position. The one time I fixed my
    position in the open sea, I came out half a mile from the GPS position
    -- though I did have the advantage of the stable deck of a big sailing
    ship, plus GPS readings that allowed me to advance the morning LOP
    without errors in dead reckoning. In short, the Davis plastic sextants
    are quite accurate enough for practical use.
    
    However, you do have to adjust the mirrors before every set of
    observations, you have to check index error before and after each
    individual sight, and you have to take multiple sights and average them
    by plotting altitudes against time. If temperatures are extreme (as in
    winter here in Nova Scotia), you have to leave the sextant in the open
    air for a while before starting so that it can equilibrate to the
    temperatures that the sights will be made at. Also, the optics are not
    the best. The field of view is narrow and the light-gathering power is
    lower than I would like, which makes sights of the fainter stars
    awkward. (Efficient use of the brief period when stars and horizon are
    both visible becomes critical.)
    
    Gary Harkins has pointed out that, if you are making serious voyages, a
    Davis sextant would be useful in an abandon-ship bag. It would also be
    useful on days when spray is flying, accurate sights are impossible and
    a precious metal sextant could be easily damaged. For those of us taking
    sights more for fun than navigation, starting with a Davis will teach
    you to be very careful and precise. After working with one, using a
    "real" sextant should seem easy and efficient by comparison!
    
    
    Trevor Kenchington
    
    
    --
    Trevor J. Kenchington PhD                         Gadus{at}iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
    
                         Science Serving the Fisheries
                          http://home.istar.ca/~gadus
    
    
    

       
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