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In my reading, I came across a way to test for index error using the
sun's diameter as the point of reference instead of the horizon.  The
procedure is (as described by Bruce Stark in "Tables for Clearing the
Lunar Distance"):
1) Set the sextant to 0D 30.0' (that is, 30' on arc) and adjust the
images so the two limbs just touch (as in a regular sight).  Record
the actual reading. Re-set the micrometer to 0D 30.0' and again
re-align the images.  Repeat this process a third time.
2) Set the sextant to -0D 30.0' (that is, 30' off arc) and again
adjust the micrometer until the limbs in each image just meets.
Record the actual reading. Re-set the micrometer to -0D 30.0' and
again re-align the images.  Repeat this process a third time.
3) Sum up the three on arc readings (no need to convert from arc minutes
to degrees) and then total the off arc readings.  Subtract the off arc
total from the on arc total.  Divide the difference by 6.  The
resulting quotient is the index error.  To determine if the error is
on or off arc, inspect the two totals (on and off arc).  If the on arc
error total is greater, the index error is on arc.  If the off arc
error total is greater, the index error is off arc.
An example:
 on | off
----|----
32.8|32.6
32.6|32.2
32.8|32.3
----|----
98.2|97.1
98.2 - 97.1 = 1.1
1.1 / 6 = 0.2 on arc
With a little effort, it becomes apparent that this same technique can
be used to remove much of the index error.  By adjusting the horizon
mirror, the amount of error relative to 30' (this is the angular
diameter of the sun) can be equalized, giving an index error of 0' or
very close to it.  I tried this with two sextants and found the
constant or offset varied from one sextant to the other.  That is,
with an Astra, setting the sextant to +/- 31.5' will cause the desired
image to appear in the 3.5x scope.  Repeating the process with a Plath
sextant the value is +/- 32.0'.  The question, then, is where is the
extra 1.5' or 2.0' coming from and why does it vary from one
instrument to the other?  I assume this is a function of either the
scopes' magnification (anybody with a 6x scope - or, better still,
both a 4x or 3.5x scope and a 6x scope - is invited to report on their
experiences) or the sextant's construction (arc radius???).
Rick
S/V One With The Wind, Baba 35
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