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

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Re: Sextant precision
From: Robert Gainer
Date: 2004 Oct 2, 14:23 +0000

```George said,
�Mostly, quintants were made for marine surveying, for measuring horizontal
sextant angles. Not much advantage over a sextant for lunar distances, which
as far as I know were only ever tabulated out to 120 degrees�.

George,
My instrument is indeed intended to be used for surveying and navigation. It
looks like a standard issue Navy sextant except with a longer engraved arc.

George went on to say,
�But I wonder whether Robert's "Brandis" was actually useable over its full
range to 162 degrees. Two things happened at large angles:

1. The available slot in the vertical field-of-view would shrink until at
some angle, at which the light was tangential to the index mirror, the
letter-box shape of the field of view would shrink to zero vertical
aperture�.

George,
This is true and it happens at about 140 degrees. I think the reason that
they engraved the arc past this point is twofold. First the vernier needs
something past the point of the reading on the arc to be able to select the
minuets and parts from the vernier. This adds 13 degrees and 20 minuets to
the engraved marks on the limb for this vernier. So if you wanted to read up
to 140 degrees you need the arc engraved to at least 160 degrees and 20
minuets.

Second, if you add a penta prism to the system you can now read the arc to
about 162 degrees and still have the extra 13 degrees and 20 minuets of
engraving on the arc for the vernier readings. This will measure an angle of
greater then 200 degrees in use.

The Freiberger sextant that I have can also be used for surveying
(horizontal sights) with a detachable penta prism to extend the measuring
range to 215 degrees. This is almost double the 120 degree arc of the
sextant. This sextant is said by the manufacturer to have less then 20
seconds of error in the instrument.

My Husun is graduated to 125 degrees just like the Freiberger but is
supposed to have no error on any measurement until you get to 120 degrees.
For day-to-day use that makes it as good as my Cassens & Plath which has �no
error for practical use� according to the paperwork with the instrument.
They clam the error is less then 9 seconds of arc.

I have used a Freiberger in Africa with the penta prism option that was sold
by the manufacturer.  It was a pain to use and it was better to just turn
around and shoot facing the other way with out the prism attached. I never
understood why you would need to measure more then 180 degrees under any
circumstances.
All the best,
Robert Gainer

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Get ready for school! Find articles, homework help and more in the Back to
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```
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