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    Re: sextant index error measurement
    From: Paul Hirose
    Date: 2006 Nov 04, 00:02 -0800

    I wrote:
    > To check parallax, make another measurement at half the original
    > distance. It would be best to construct another target so the sight
    > picture looks the same at the shorter distance. The results from this
    > second test will reveal any residual parallax, since IE is constant with
    > distance while parallax varies in inverse proportion.
    This residual parallax could be used to correct the measurement of the
    sight-line offset. In fact, the precise measurement method George
    Huxtable described never occurred to me. I imagined that an approximate
    measurment would be gotten by eye and ruler, then refined with the
    parallax test.
    In turn, with a precise offset value you can prepare a 3-line target
    custom-tailored to a particular sextant. If the interval between the
    white space (separating the paired lines) and the single line is just
    right, the index error test becomes independent of distance. In
    practice, if you cut the observing distance in half, the widths of the
    white space and the single line must also be cut in half to maintain
    sensitivity. Dimensional accuracy of the test pattern becomes twice as
    critical too.
    I think it would be easiest to create and print the pattern with a
    computer. The self-test patterns my inexpensive ink jet printer
    generates are amazingly crisp; I think it could do a fine job if I knew
    how to prepare a suitable drawing.
    The bisection method occurred to me because some of the theodolites I've
    handled use that principle in the optical micrometers that read the
    graduations on the circles. For example, on a Kern 1-second DKM2
    instrument you rotate the micrometer knob to center a single line in the
    narrow space between paired lines. This is more precise than simply
    putting one line atop another.
    The globe sight on a target rifle also relies on the eye's ability to
    judge centering with great accuracy.
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