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Bill N wrote:

> One of the questions someone asked in trying to diagnose Bill's
> varying IE measurements was "does the position of the tangency
> vary" between left & right eyes, or between horizontal and vertical
> orientations.  Bill interpreted that as questioning whether the
> tangency was actually touching, or had some overlap or gap, but
> I interpreted the question differently.

> Are all the measurements being made with the objects brought to
> tangency *in the center of the optical field*, rather than off to one
> side or the other, or toward the higher or lower part?  To a first order
> it should not matter, but if the mirrors are not perfectly flat then it
> makes a difference, and I think side error might also vary across the
> optical field.

Excellent question. I have kept the the tangent point in the center of the
scope in all cases. It occurred to me I might center the directly viewed
image and hope lower and upper tangent points (on the arc and off the arc
would suffer to the same extent.  Unfortunately I have run out of sun the
past few days so have not been able to run tests to determine if that makes
a significant difference.

From my original cardboard sextant, and tripod mounted tests with the Astra
a year ago, I became aware quite aware that moving off center (especially
vertically) will change alignment noticeably.

Any thoughts on the advantages/disadvantages of either position?
>
> Like George, I have a hard time understanding how anything in the
> optical path after the horizon mirror can change whether the two
> images seem to be touching.  If they appear to be touching, then
> they emerge from the horizon mirror in parallel.  What then can
> separate them?  Even flaws in telescope or eyes should not be
> able to do that -- at least if the horizon mirror and index mirror are
> flat.

I'm still chewing on that.  If the two images of one star are perfectly
superimposed, then there is no side error.  In my experiments one star image
was parked slightly above or below the other where any side error is readily
observable if the sextant is perfectly vertical. A tripod and level helps
here. I need try the experiment with a images superimposed, although I have
difficulty with that, especially determining side errors when superimposed
as the images have "tails."  Once again, a lack of clear skies is a current
limitation at the moment.

As other members have reported, there does seem to be a shift in side
alignment between star/planet and sun observations. How much focus change vs
possible chromatic aberration factor in is unknown to me.

Thanks

Bill


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