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    Re: sextant index error measurement
    From: Richard M Pisko
    Date: 2006 Nov 5, 22:49 -0700

    On Sat, 04 Nov 2006 00:02:33 -0800, Paul Hirose wrote:
    
    > 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 rather like the Kern DKM2-A three line "splitting the white
    interval" method; but the Wild T2 used a "continuance of lines"
    to good effect also.  Both are one second instruments.  I'm not
    sure which is more precise as far as repeating the same
    micrometer reading for any given setting of the circles.
    
    
    Similarly, in WWI, the infantry/artillery rangefinder by a
    Glasgow firm called Barr & Stroud, used a target board with
    index marks separated by 1.0 meter to check the calibration of
    the infinity reading.  Their "Artificial Infinity Adjuster" box
    used two black disks of about six inches diameter with a white
    vertical line on each spaced at one meter on a white background.
    When the rangefinder was set at infinity, and the target was at
    least 100 yards away, the view was of a lower half disk at the
    left end, an upper half disk at the right end, and a merged full
    disk with a continuous white line in the center.
    
    (I forgot to mention the Barr and Stroud split an image over a
    horizontal dividing line; the top was from the left end, the
    bottom was from the right end.  The right eye saw combined
    image, the left eye saw the range scale; if the inter-pupilary
    distance on the rangefinder matched that of the observer.)
    
    The inside scale gives readings from 250 yards to 20,000 yards
    plus five or so unmarked ticks before the asterisk sign.  I
    forgot the details so had to look.
    
    I see that Barr and Stroud's instruction booklet says to use the
    vertical edge of the moon or a bright star if possible.  There
    is an astigmatizer lever to make a star easier to view for the
    infinity setting . . . or perhaps to make a spot of light into a
    streak to range target light sources at night.
    
    Wild used a similar target board at 0.8 meter for the very
    similar rangefinder used by the German army in WWII, if I recall
    correctly.
    
    --
    Richard, 2006-Nov-05 9:20:49 PM GMT - 6 hours for summer
    
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