NavList:

Bill,

Some dumb suggestions, in no particular order of priority.

First, you mentioned you add a slight side error to improve star IC 
checks.  Have you tried nulling that out?

Second, have you had others try an IC check on the sun with your 
sextant and compared to yours?

Third, have you tried changing the position of tangency to try to at 
least get the SDs to agree?

Fourth, the 4x telescope for my C+P shows changes in side error 
depending upon the focus setting.  Is anything like that happening 
with yours? (I don't know this would affect the sights.

Fred

On Jul 10, 2006, at 5:47 PM, Bill wrote:

>
> 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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