# NavList:

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

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Re: sextant index error measurement
From: Bill B
Date: 2006 Nov 03, 19:31 -0500

```Jim wrote

> According to "Nicholls concise guide"vol.1,a close by object
> or straight line should not be used to determin index error,because it
> does not allow for sextant parallax and atmospheric refraction.I can
> give the textbook explanation if it is of interest.

Thank you Jim.

In the methods proposed by Frank, George, and I think Paul we are attempting
to null out parallax mathematically or empirically. I am still digesting
Paul's method.

The gist being that with the laser we are using two dots spaced to represent
the distance between the scope's line of sight and the index mirror line of
sight, so parallax is not an issue. Similar with George's lower tech method.

With the laser, find the distance between the scope's line of sight and the
index mirror line of sight (near target).  When the laser dots are the same
distance apart a goodly distance away, the rays and therefore index and
horizon mirror are for practical purposes parallel, which when viewing an
object at close to infinity translates to no parallax error.  So the setting
on the sextant is the IE.  Then for IC to paraphrase Frank, "When it is on,
take it off; when it is off put it on."

I would think refraction would be for all practical purposes identical for
both laser beams or rays from a distant celestial object.

Bill

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