# NavList:

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

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Perpendicularity again. was: Adjusting Central Mirror
From: Alexandre Eremenko
Date: 2004 Oct 11, 18:04 -0500

```Dear Bruce,
I am very glad to the opportunity to repeat my question again
(The question was addressed to everyone who EVER checked
perpendicularity of the index mirror,
but surprisingly nobody replied when I asked it
for the first time about a month ago).
This has nothing to do with SNO-T sextants or adjusting.

The question was: how do you properly CHECK perpendicularity
of the index mirror to the sextant frame.

this check on his/her sextant
(and nobody replied, which is surprising).

There are several versions of this check.
The simplest one and most common one says:
look into your index mirror so that you see a part of the arc
directly
and another part of the same arc reflected in the mirror.
If everything is OK, you see the arc as a straight continuation
of its reflected image.

But whether this is so or not, DEPENDS on the angle of your
sight
with respect to the plane of the sextant frame!
I just cannot believe that nobody has noticed this.

In other words: you can move your eye (or the sextant) so that
the point, where the direct and reflected images of the arc meet,
moves along the right vertical edge of the index mirror.
You cannot get a match of these arcs at every point of this
vertical edge. Either they match at the lower point of the edge of the
mirror,
or in the upper point of this edge  or somewhere in the middle.
But never everywhere.

This does not depend on the particular sextant.
Every sextant with sufficiently large mirrors will
behave like this (and this can be shown mathematically).

So WHERE exactly should you place your eye with respect to the frame
for the proper test?

No manual says this precisely.

Alex.

On Mon, 11 Oct 2004, Bruce Stark wrote:

> I don't know if I remember this right, but it seems Alex mentioned a problem
> in setting the index mirror of his SNO-T perpendicular to the frame of the
> instrument.
>
> Also, as best as I can recall, Joel mentioned some time ago that the
> reflecting surface of these front-silvered index mirrors is not centered on the pivot,
> and so can't be checked for perpendicularity in the usual way, by aligning
> the reflected image of the sextant's arc with a direct image of it. As I take
> it, that's the reason for the little L shaped vanes in the sextant box.
>
> Anyway, I had no trouble setting up the index mirror on my SNO-T using the
> vanes, and just now got to wondering why it didn't work for Alex. I'm thinking
> he may not have set both vanes exactly the same distance from the center pivot.
> So I've put the SNO-T on the table and tried it again. To be sure we're
> talking about the same thing, here is how it's set:
>
> Index set at 35 degrees. One vane covering 0 degrees on the arc, the other
> covering 120 degrees. Vanes are set with the long, flat side facing toward the
> central mirror. The corner edge of each vane is precisely on the inner edge of
> flat part of the arc.
>
> Move either of the vanes slightly toward or away from the central mirror and
> you get the effect Alex described.
>
> I don't trust my memory of what Alex and Joel had to say and hope they, or
> other List members, will correct me. There may be more to this than I suppose.
>
> Bruce
>

```
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