# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Sextant Scope Parallelism (was Re : SNO-T Sextant)
From: Frank Reed CT
Date: 2004 Aug 12, 17:58 EDT
Bruce S wrote:
"Your eye and two stars form another plane. As long as this second plane is perpendicular to the axis of the sextant, the sextant can accurately measure the great circle distance between the stars. If it is not perpendicular to the axis the angle will measure too great."

There is only one issue here: collimation of the telescope. If the telescope axis is parallel to the sextant frame, then the instrument will measure a great circle angle. It is incapable, by design, of measuring a "small circle" angle when properly adjusted.

And:
"This is a serious problem only with large angles. You might experiment by measuring the distance between two stars 110 or 120 degrees apart. Move them back and forth across the field of view and see what happens."

Yes, IF the telescope is not properly collimated (adjusted so that its axis is parallel to the frame), THEN there will be a problem. If you bring together two star images separated by a large angle (>90 degrees, e.g.) and adjust the sextant so that the images are exactly coincident on the right side of the field of view (right side here meaning the side closest to the sextant frame) and then rotate the sextant slightly so that the star images are on the left side of the field of view, the two images should still be coincident. If they're not, the telescope should be adjusted slightly. After this adjustment is done, you don't have to worry about it anymore and you can take sights anywhere in the instrument's field of view. This is a critical adjustment for lunars and it is often over-looked by people experimenting with them today. If you want accuracy at the 0.1 to 0.2 minute of arc level (very do-able), the sextant's telescope has to be parallel to the frame. The index correction will probably change after the telescope is adjusted.

By the way, I checked a more recent Bowditch and the description of this adjustment recorded by the modern editors is almost exactly the same as in manuals from 200 years ago EXCEPT that the references to the two wires in the field of view have been replaced by 'left' and 'right' in the field of view.

Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois

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