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> Here is where it gets sticky.  We have been using a special case (mirrors
> parallel and one object).  If the point the horizon mirror is looking at on
> the index mirror is not on the axis of rotation of the index mirror, things
> go to hell in a handbasket when you try to measure the angular distance
> between the horizon and a body or two bodies. Note a similar loss of
> precision occurs if the index-arm is eccentric.
>
> I can easily see this with my cardboard sextant. It is initially calibrated
> by rotating the index mirror until it appears as a sliver through the sight
> tube when reflected by the horizon mirror.  Then, looking through the sight
> tube, a horizontal marker line is placed on the front-silvered horizon
> mirror (actually polished stainless steel, with air where the glass would be
> on a rear-silvered horizon mirror) right in the middle of the reflected
> index-mirror image.  If I line up the horizon or an object between the
> horizon and air (glass) through the sight tube ON THE MARKER LINE, and then
> shift the sextant up or down so the images are significantly above or below
> the marker line when viewed through the site tube, they will no longer be
> aligned.  Since it is a sight tube, no blame can be placed on the optics ;-)
>
> This is why it is so important to center the objects being viewed through a
> real sextant on the horizon mirror.  I also suspect the optical quality of
> scopes is such that using the edges introduce distortion as well.

George

I am coming around to your way of thinking on sextant mirrors.  While my
facts are straight, my equations are wrong.  I am confusing good design and
input factors, and believing a cause-and-effect relationship that does not
exist.

In the example of looking at another a portion of the horizon mirror on my
cardboard sextant, I am changing the geometry of the light path so things go
south.  I have to look at the horizon mirror at the same angle is was set up
for in the initial calibration.

I have done some overlay drawings with the pivot point in the center and at
the lower end of the index mirror. As you stated, things seem to work out
fine in either scenario.  In the case of wanting the "point" to be at the
axis of the index mirror, this is a design consideration.  When I located
the pivot point at the bottom end and placed the mirror at an angle to
observe a high body, the line of sight from the scope to the horizon mirror
to the index mirror was such that it missed the index mirror entirely.  So
it is optimal to have the "point" at or near the axis so one can see the
index mirror while still using the center of the horizon mirror.

As the quip goes, I have come to the proper conclusions for all the wrong
reasons.

Thanks for serving as a sounding board.

Bill

   

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