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    Re: Sextant design. [was: Another Davis...}
    From: George Istok
    Date: 2003 Mar 3, 21:21 -0500

    Rodney and all,
    
    Thanks for forwarding George Huxtable's message.
    
    I purchased the Davis Instruments Mark 15 used.  It appears to have been
    used very little.  All of the adjustments on the sextant appear intact, the
    index is positive with no play, and the micrometer is the same.  There is an
    error in the instrument in that the mark on the frame and the mark on the
    arm do not align the same through the arc.  The error is visible with the
    micrometer at zero and the index set at zero as a slight misalignment.  The
    error disappears with the index set at 90 degrees.  It was this error that
    prompted my first question to Davis Instruments.  It was their answer that
    the "accuracy rate" was plus or minus five degrees that prompted my question
    to this list.
    
    I am interested in the application more than the theory of navigation and
    piloting.  I sail on Lake Michigan in a 24 foot Columbia Challenger and know
    that it is nearly impossible to take accurate bearings and plot a fix along
    the coast unless it is near calm.  It is interesting to see that the same
    condition applies to CN.  I am new to CN, but it seems that, just like
    coastal piloting, one is fortunate to determine a position from a series of
    sights.  I think the goal of taking bearings, sun sights, etc. on the small
    boat is to allow one to compare the apparent actual position to the DR
    position.
    
    I think the final answer to my question about the accuracy of the Davis
    Instruments Mark 15 is not a numeric specification.  Instead, it is the
    concept that even a plastic sextant that has been made with reasonable care
    and is handled in a reasonable manner can produce data with errors much
    smaller than those introduced by the conditions in which the measurement was
    made.
    
    With that said, I want to thank everyone who responded to my original
    question.
    
    George Istok
    Senior Consultant
    Signal Hill Consultants, Inc.
    574 232 9888
    
    
    

       
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