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    Re: measuring sextant instrument error
    From: Bill Murdoch
    Date: 2000 Sep 13, 5:51 PM

    Paul Hirose and Dick Winchurch, thanks.
    
    I now think I understand how the instrument error in a homemade octant could
    be measured.  It is much easier than I had thought.
    
    I got stuck on trying to measure the angle between the sight line to the
    horizon and the sight line to the body.  That would be hard to do because the
    intersection of the two lines occurs behind the octant and moves as the index
    mirror moves.  To make the intersection of the two sight lines occur in the
    middle of a measuring circle, the octant would have to be relocated each time
    the index arm was moved for a measurement.  That would not be an easy task.
    
    The way you describe is so much better.  Fix the octant body with its index
    mirror in the center of a circle with the perimeter of the circle marked in
    minutes of arc.  That puts the sight line from the index mirror to the body
    on a circle radius and puts the horizon sight line on a line offset from, but
    parallel to, a circle radius.  Setting the octant to zero determines the
    location of both the horizon target and the body target on the perimeter of
    the circle for an octant reading of zero.  At zero the targets are separated
    by the distance between the telescope and the index mirror.  During the
    calibration neither the octant frame nor the horizon target is moved.  The
    body target is moved along the calibrated perimeter of the circle, and its
    position is measured by moving the index arm of the octant and reading the
    scale on the arc of the octant.  A comparison of the change in the position
    of the body target on the circle perimeter with the arc reading would give
    the instrument error.
    
    I think I can do it in my garage like this:  (1) Fix a block of wood on the
    floor with a 1/4" hole drilled in the wood.  (2) Strike an arc just over 100
    degrees in length with a radius of 6,875.5 mm centered on the hole.  (3)
    Stand a steel tape measure on edge along the arc so that each 1 minute of arc
    is 2 mm on the tape.  Replace the pivot bolt in the octant with a longer bolt
    and place the octant over the block of wood so that the bolt slips into the
    hole.  That places the index mirror over the center of the circle.  (4) Read
    the steel tape scale in the horizon glass for reference.  That reading
    becomes the horizon.  (5) With the index arm set to 0 degrees read the steel
    tape scale at the spot that is superimposed over the horizon reading.  That
    begins the calibration.  (6) Move the index arm and note the scale reading
    viewed in the index mirror that is now superimposed over the horizon reading.
     If the scale can be read to a half millimeter, the sextant error can be
    measured to 0.25 minutes of arc.  That is four times better than my usual
    shooting accuracy and I think good enough for checking a homemade instrument.
    
    Think that would that work ?
    
    Bill Murdoch
    

       
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