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    Re: Instrument Corrections
    From: Fred Hebard
    Date: 2003 Mar 9, 17:38 -0500

    Bruce,
    
    Thank you.  I have been researching my own question and came to the
    same conclusion.  The relevant passage from the 1962 edition of
    Bowditch was:
    
        Usually, it is preferable to make a single correction for all
        three errors, called INSTRUMENT ERROR.  Customarily, such a
        table is determined by the manufacturer and attached to the
        inside cover of the sextant case.  The sign of the error is
        reversed, so that the values given are for INSTRUMENT
        CORRECTION (I).
    
    (The three errors are prismatic error, graduation errors and centering
    error).  Elsewhere Bowditch makes the distinction between index error
    and index correction, where you reverse the sign of the error to yield
    the correction.  I did not find any passages in the 2002 edition of
    Bowditch that contradicted the above, but also found little supporting
    it.  My other two nav books, Turner and Toghill, also are relatively
    silent on the subject.
    
    The offending passage from Bauer was:
    
        So the instrument error taken from the certificate in the
        sextant box top is applied with reversed sign to sextant altitude.
    
    This clearly is incorrect.  All the certificates illustrated by Bauer
    indicate "correction" not "error," although only the old Husun
    certificate indicates error existed in its instrument.
    
    Making this change sure has straightened out my data!  I've been close
    to pulling my hair out for several months trying to get my accuracy
    close to my precision, even contemplating purchasing another sextant to
    try to narrow things down.  Now I feel confident enough to try the
    instrument out on the water (but still close to coast)!  It would help
    if we lived closer than 350 miles to salt water.
    
    By the way; John Luykx at Navtrak Nautical indicated that it might take
    about 500 observations to calibrate my sextant; I probably still need
    to get my standard deviation consistently under 0.2' before starting.
    
    Fred
    
    
    

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