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Alex

I understand the mechanics.

What I am getting at goes to a thread a while ago.  Vertical sun IE checks
even with the sun over 60d elevation do not match horizontal IE checks.  4SD
for vertical IE checks will be slightly over, 4 SD for horizontal checks at
or under interpolated almanac 4SD values, so clearly not a "squished" sun.

When I switch to the eye where the astigmatism is rotated approx. 90d of the
other eye, the opposite happens.  I ran all sorts of tests, including
inversion and substituting photo gels over the scope for shades.  Not the
instrument's fault.

The problem is in my eyes.  The point I am getting at here is that MY
PERCIEVED IE (not the instrument's IE) changes as I rotate the sextant from
vertical to horizontal as one might do with a lunar.  What my perceived IE
is between vertical and horizontal is anyone's guess. In this case of lunars
I would be better served using my horizontal IE value.

That is why I favor an IE check from a line, (or point source if I could do
do so well) rather than large spheres which my eye distort).  I am still
working on how to reconcile the difference between vertical sun and horizon
IE's to give me a working IC for sun and moon shots with a natural horizon.

Put another way, if I had two identical stars equidistant from the moon's
center, one below and one to the side, I will get two different angular
measurements due to my eye distorting the sphere.

Bill

> Bill,
>
>> but have no clue what is happening
>> when I tilt the sextant to do a moon IE
>> check.
>
> Why don't you just try, using the current nice weather
> and good Moon in the evenings:-)
>
> It is like the normal "swinging".
> In one position of the sext, two images of the Moon
> just touch, when you swing, they slightly separate.
> This separation becomes smaller and smaller as the Moon
> becomes fuller.
>
> With 100% full moon, swinging has no effect.
>
> Alex.

   

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