# NavList:

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

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Re: Calibrating a sextant scale
From: Bill Morris
Date: 2007 Nov 22, 20:40 -0800

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On Nov 23, 4:46 am, Alexandre E Eremenko
wrote:
> Dear Bill,
>
> 1. I am very interested in the results of your testing,
> especially for SNO-T. Do you have a factory certificate
> of this sextant to compare?

No, Alex, I don't. The handbook claims "Instrumental accuracy within
angle range 0 to 120 deg. - +/- 6".
>
> 2. If I understand your first procedure correctly,
> a collimator creates a beam of parallel light rays.
> (As you say, it is "focused at infinity").
> You measure the angle between these two beams with
> a theodolite, and then with a sextant.
> Now, how wide is this beam for a typical collimator?
> I mean, if you place your collimators only few meters away
> from the theodolite, how large is the space from which
> both these beams from the collimators are visible?
> If I understand correctly you then replace the theodolite
> by a sextant; the sextant has to be roughly on the
> same place where the theodolite was.
> But the typical distance between the two mirrors of the
> sextant is few centimeters. Will you be able to catch
> the beams from both collimators into the sextant mirrors?

The beams converge to the theodolite axis as if coming from infinity.
There will be a position not far away where the sextant mirrors can
intercept their respective beams. The axes of the collimators will be
seen to intersect at the sextant telescope.
>
> In other words, how wide are these beams?
>
It's quite easy at a distance of 2 or 3 metres using the Mk I naked
eye to find roughly where the beams intersect and then to "bring down"
one image to the other, but it gets progressively harder to find the
intersection of the crosswires as the distance increases. The image of
course gets dimmer too. I use my two Hilger and Watt Microptic
autocollimators as collimators. The free apaerture of the objective

> Another problem I see with this setting is that even
> the smallest LED light is not a point source at the
> distance of few meters.

The LEDs are simply used to provide a background illumination of the
crosswires, not as a reference image. A diffuser made from the lid of
a margarine tub or similar can be used if you wish.
>
> Alex.
>
> As I understand from the journal
> description of the arrangement in the Kew observatory,
> the collimators there are of
> very large diameter, and the distance
> to them is not few meters but something like 15 meters.

I would love to have a copy of this or the reference if it is readily
available.

Bill Morris
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