NavList:

Here we go, the last tests until the weather and moon cooperate.  I hope.
Sun-screen costs for Cel Nav have just exceeded sun block costs for sailing.

Prescription and glasses are probably 6-7 years old.  At the time I
purchased the glasses, it was my first pair since high school (vision
corrected itself to 20/20 when I had it checked at 40).  I recall at the
time I picked them up they noted there was a lot of (astigmatism?)
correction, so if that bothered me bring them back.  I did have some dialed
out for my prescription sunglasses, but have gotten use to it although it
was a shock to the system initially.

Even with the corrections, if I look critically a circle it will be slightly
elongated along the Y axis with my left eye, the opposite with my right eye.

CONCLUSION

The eyeglasses appear to null out most of the vertical vs horizontal shift
within the man/machine ability to measure.  They also appear to bring the
sun IE closer to 0 to 0.1' off the arc linear IE (.7' to 1.7' over linear in
vertical position, and 1.5' to 2.5' over linear in horizontal position,
which is the recommended position for sun IE checks to minimize the
refraction George so correctly points out.  All sun IE checks cannot be made
with the sun at 70+d elevation.)

An obvious question:  If the glasses help that much, why not use them?

1  They help to some extent, but do not totally eliminate the problem, so
personal error corrections are still called for.

2.  The eye relief (distance the scope can be held from the eye and still
give a good view) of the scope is marginal with my glasses and facial
structure.  Other scopes I have tested are not useable with eyeglasses.  The
scope must be pressed against the glasses and the lens pressured in towards
the eye.

3.  I would have to fabricate a rubber device to keep the scope from
scratching the glass lens as the standard rubber eyecup cannot be used due
to eye relief.

4.  Even in static conditions (sitting in a chair) it is difficult to align
the scope, eye and glasses.  It would be almost unworkable for me in rough
seas.  Since a personal correction is called for in either case (bare eye or
with eyeglasses) I would prefer to use the bare eye and correct
mathematically.

Which brings us back to the initial question, how do I develop a personal
error constant--err--starting point?

My eyes are flawed.  As Red might suggest, they should be taken out back and
shot.  A waste of time IMHO; it seems they are already shot ;-)

Bill

HORIZONTAL and VERTICAL SHIFT WITH EYE GLASSES
Astra IIB, Star Scope

13 July, 2006
Left eye 16:25 to 17:01 EDT
Hc 53d to 6d

Key: 
VON   (vertical on the arc)
VOF   (vertical off the arc)
HUDON (horizontal on the arc)
HUDOF (horizontal of the arc)
HU     handle up
HD     handle down
sigma (standard deviation)


LEFT EYE
Vertical   Vertical   Horiz     Horiz
On Arc     Off arc    On Arc    Off Arc

31.8       28.6       31.5 HU   28.3 HU
31.8       29.6       31.6 HU   28.8 HU
31.6       28.8       31.8 HU   28.4 HU
31.4       28.7       31.6 HU   28.6 HU
31.6       29.8       31.4 HU   28.6 HU
31.7       29.8       31.6 HU   28.8 HU
----       ----       31.6 HD   28.7 HD
----       ----       31.6 HD   28.4 HD
----       ----       31.7 HD   29.0 HD
----       ----       31.7 HD   28.9 HD
----       ----       31.8 HD   28.7 HD
----       ----       31.6 HD   28.8 HD

Mean    Standard Deviation
VON       31.65' 0.15'
VOF       28.72' 0.098'
60-VOF    31.28'  -----
HUDON     31.625' 0.11'
HUDOF     28.67'  0.21'
60-HUDOF  31.33'  -----

TARGET 4SD:     63.2'
Vertical 4SD:   62.93
Horizontal 4SD: 62.96
Vertical IE:    0.077' On
Horizontal IE:  0.15' On


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