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Frank,
I have thought about what you said and still don�t under stand it. So I set
up this morning and preformed this little test. Please explain to me where I
went wrong in my design for the test. First I want to describe the setup for
this experiment. I arranged this so that I could take the shot though the
open door of my office. The sextant used was my Cassens & Plath. I believe
this instrument to be accurate. I set a stand on a tool cart and locked the
sextant into it. Then I measured the angle between the top of the spire and
the edge of the roof on a building that is about 7,000 feet away. This
distance was measured from the town-zoning map. All the measurements were
done with my 4-power scope. With the sextant still locked in position I
released the scope and shimmed the bottom of the post with a 1/16� thick
spacer. This put a noticeable misalignment in the scope. I also did this a
second time with the spacer at the top of the post and saw no difference in
the measured angle either way. Next I put the sextant at a 45-degree angle
and set the index arm so that the roof was an unbroken line on the horizon
glass. I repeated the use of shims to misalign the scope both up and down
relative to the frame. I still see an unbroken line on the horizon glass.
Unfortunately its overcast and I can�t see any other target that is further
away. I am sure the effect of a misalignment increases as the angle does,
but I expected at least a small jog in the image.
All the best,
Robert Gainer



>From: Frank Reed 
>Reply-To: Navigation Mailing List 
>To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
>Subject: Re: Sextant Scope Parallelism (was Re : SNO-T Sextant)
>Date: Thu, 12 Aug 2004 23:31:10 EDT
>
>Robert G, you wrote:
>"That’s where you got me. If the sextant is vertical you are measuring
>the
>correct angle. If the scope is pointing off center by a small amount then
>you see the sun a little displaced from the center of the scope. But
>isn’t
>the sun still in the center of the horizon mirror and therefore in the
>right
>place for the measurement.�  If you insist on keeping the sun in the center
>of the scope I don’t think that you can get the sun to the bottom of the
>arc, it will wander off center as you get to the bottom.�  Can you force it
>both to the center of the scope and the bottom of the arc at the same time
>if the scope is out of Parallelism?"
>
>The purpose of "swinging" the sextant is to ensure that you are holding it
>vertically. So let's set that aside. Let's suppose you have ensured that
>the
>sextant is exactly and precisely vertical by swinging or even by mounting
>the
>instrument on a pole. With that done, you can still rotate the sextant
>about its
>vertical axis so that the Sun and the spot on the horizon beneath will pan
>from left to right across the field of view of the telescope. If the
>telescope is
>not parallel to the sextant frame, the contact between horizon and Sun will
>change as you pan, typically only by a few tenths of a minute of arc. In
>practice, you would almost never notice this effect with a standard Sun
>sight,
>except at noon, because the Sun is moving so fast. The best way to test for
>this
>alignment issue is by bringing two star images together. Observed angles
>between
>stars do change (because of changing refraction as the stars travel across
>the sky), but the change occurs very slowly and would not be obervable for
>several minutes. So you bring two stars together in the field of view of
>your
>sextant. Usually you will do this somewhere near the center of the field of
>view.
>Now rock the sextant back and forth so that the superimposed star images
>shift
>from left to right. If they do not remain superimposed across the whole
>field
>of view, the telescope is not parallel to the frame. When shooting lunars,
>where a couple of tenths of a minute of arc can be a big deal, this
>alignment
>error can lead to surprising errors. Speaking from experience, getting the
>scope
>parallel goes a long way towards making the sights more accurate.
>
>Frank R
>[ ] Mystic, Connecticut
>[X] Chicago, Illinois

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