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In a recent post, I stated:

"I also produced a card to sit on the mirror side of the horizon glass to
indicate dead center (vertically in normal upright sextant position).  This
idea popped up while trying to determine index-mirror line of sight to
horizon-glass line of sight distance using George's idea.  Even when the
sextant was tripod mounted, plumb, and zero-mag sight tube was level my
measurements ranged from 65mm to 67mm depending on the height of the
sextant--which was based on eyeballing the perceived vertical center of the
horizon glass in relation to a target line."

This has proved very interesting. All this time I have been operating under
the assumption that I was using the vertical (physical) center of the
horizon glass for observations.  Once my card was in place dividing the
physical horizon glass equally top and bottom I did a sanity check and moved
the index mirror out to its limits, noting its location in relation to the
card's  dividing line as it moved.  To my surprise the optical center of the
foreshortened index mirror is about 5.5 mm below the physical center. Using
George's method with the physical center the lines of sight increased from
66mm to 73mm.  Adjusting the dividing line to the optical center it
decreased to 65.5 mm.

A couple of hours of playing produced results, some supported by my earlier
thoughts, and some observations.

1. By tracing the laser path when the pointer is aligned with the scope
barrel, the scope does not look at the physical center of the horizon glass
mirror, but rather at a spot below the physical center that coincides with
the point located by viewing the foreshortened surface of the index mirror.

2.  It does matter whether the laser beam is is centered on the eye-end lens
and spot on the horizon glass the scope is looking at.  Keeping the entry
point centered and using either extreme of the horizon glass/mirror juncture
changed the point separation by >5 mm at 40 ft.

3.  A zero-mag sight tube can produce sharp spikes if the beam hits the
mirror/glass juncture dead nuts on. This is useful at shorter distances
(10-30 ft), but its value diminishes at longer distances as the spike
enlarges proportional to beam spread spread at larger distances.  Working
with a focused scope one can keep the spike sharp at greater distances, but
vertical and horizontal alignment becomes increasingly demanding.

Bill






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