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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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