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After a long series of tests, I have determined I have a problem, and don't
know how to adjust for it.  I feel like I am overlooking something obvious,
but just can't get a handle on it.

My left eye distorts a sphere, with the vertical axis longer than the
horizontal axis.  My right eye distorts a sphere with the vertical axis
shorter than the horizontal axis.

When doing IE checks with my left eye (which I use for observations although
being right-eye dominant as the left eye is sharper for some reason) and two
sun images my vertical and horizontal sextant positions yield two different
sets of results. (Before suggesting frame rigidity, I can reverse the effect
using the right eye.)

Sun Hc nominally 63d, left eye

Vertical average on the arc:             32!08'
Standard deviation:                       0!10'
Vertical average off the arc:    29!17 = 30!83'
Standard deviation:                       0!10'
4 SD from Almanac:                       63!2'
4 SD observed:                           62!9'
IE:                                       0!625' on the arc

Horizontal average on the arc:            31!65'
Standard deviation:                       0!1'
Horizontal average off the arc:  28!75 = 31!25'
Standard deviation:                       0!10'
4 SD from Almanac:                       63!2'
4 SD observed:                           62!9'
IE:                                       0!20' on the arc

Vertical minus horizontal on the arc:     0!43'
Vertical minus horizontal off the arc:   -0!42'

If I achieve tangency with the left eye and rotate the sextant (handle up or
down) the images will separate.  If I achieve tangency with the right eye
and rotate the sextant (handle up or down) the images will overlap. Again, I
don't think I can blame frame rigidity for the discrepancy.

Later tests done with sun Hc nominally 32d, left eye

Vertical average on the arc:             31!625'
Standard deviation:                       0!08'
Vertical average off the arc:   29!595 = 30!405'
Standard deviation:                       0!10'
4 SD from Almanac:                       63!2'
4 SD observed:                           62!03'
IE:                                       0!11' on the arc

Horizontal average on the arc:            31!875'
Standard deviation:                       0!10'
Horizontal average off the arc:  28!90 = 31!10'
Standard deviation:                       0!11'
4 SD from Almanac:                       63!2'
4 SD observed:                           62!975'
IE:                                       0!39' on the arc

Vertical minus horizontal on the arc:     0!25'
Vertical minus horizontal off the arc:   -0!305'

NOTE:  All sun IE checks were made without changing focus, as this will
change IE and side error with my scope.

It makes some sense to me that as the sun begins to squish marginally, the
images appear closer to a sphere to my left eye, so vertical and horizontal
observations become closer.

Using a natural horizon, flat-roof buildings, radio towers and tree lines 3+
miles away across the Wabash River valley I am observing -0!1 IE (off the
arc) +/- 0!1 minute (no filters).  So this whole thing is driving me up the
wall. My clusters are tight.

Are there some other tests the list might propose?

More to the point, is there some way to mathematically null out the eyeball
problem and possible problems with shades (shades on sun but often not on
the horizon) factor when using a sphere (sun or moon) and a natural (flat
line) horizon with my left eye?

Along the same line, if I am deriving distance from the top of an object
with its base below the water line (Bowditch table 15), should I regard the
top of the object as a point source, or is my vision causing the object to
appear taller, hence I am observing too large an angle between the top and
horizon?  The question being, will that cause me to observe a larger angle
therefore calculate my distance closer to the object; so I need to derive my
own "constants" for Bowditch T15 etc. for my eyes?

As a check, I ran a series of tests using shoreline objects of known height
whose bases were visible, and I always overshoot the angle needed to make
the trig work.

Any thoughts would be appreciated.

Bill


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