# NavList:

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

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Re: Dipmeter: was [NAV-L] Wires, back sights and collimation
From: Alexandre Eremenko
Date: 2004 Dec 1, 22:10 -0500

```Frank,

On Wed, 1 Dec 2004, Frank Reed wrote:

> Myself, I can't think of any technical reason. Can you? I suppose simple
> conservativism of design would be an explanation...

The only explanation I can think about is that the inverting scope
is used for taking large angles. (I am not saying that they
were thinking of lunars, but probably large horizontal angles
between the objects on the shore. The arc of this sextant
permits measuring angles to 140d.
Now when taking such large angles, very small collimation
error becomes relevant.
So they did everything to reduce it.
Alex.

P.S. By the way, when taking large distances, I always
have an error which varies between 0.3 and 0.7
which I cannot explain. It occurs permanently, and I can
correct for it if I wish. But what is the reason for this error
I do not understand. Here is my typical recent lunar:

Dec. 1, GMT 15:32 Lat 40d27.2N, Long 86d55.8W Sun-Moon IC=0.
Temperature 33F Pressure 30.1 Reducing with Frank's calculator,
last column is the error.

15:43:46    127d12.2     0.8
15:46:15    127d10.8     0.6
15:47:40    127d09.8     0.3
15:49:45    127d09.1     0.7
15:53:28    127d07.2     0.7

The average error is 0.6, and maximal
deviation from this average is 0.3

This 0.6 overshot occurs in all my
measurements of distances larger than 90.
But what is this?
Repeated measurements of the index error
give zero. Well, never more than 0.2.
All my attempts to measure collimation error
also give zero.

So this is either some kind of
or some problems with the arc excentricity.

Alex.

```
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