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Dear Robert,
The only book I know that contains a COMPLETE
theory of sextant is Chauvenet. (I am sure that there
are others, but this is really complete, very concise,
and written in good English:-)

In particular, on p. 115 (vol. 2) the following formula
is derived:
Err=-2 L^2 sin1" tan(h/4),
where Err is the resulting error, in seconds
L is the deviation of your mirror from
perpendicularity (in seconds) and h the angle you measure.

In more modern notation this becomes:
Err=-0.00057 L^2 tan(h/4)
where both Err and L are in minutes now.

For example, if L=5'
Then Err=0.008 or 05", which is completely negligible.

Now, according to SNO manual, you can adjust this mirror
(using special factory-supplied visors in place of dominoes)

That's why visual adjustment (without dominoes) is OK too.
Two years ago there was a large discussion on the list about
how exactly to do this adjustment, and the general conclusion
was that dominoes (or visors) are only recommended for sextants
with front-silvered index mirros.
For back silvered mirrors, the usual eye method is OK.

Alex

On Mon, 17 Apr 2006, Robert Eno wrote:

> I recall asking this question before so if I did, I apologize for repeating 
myself. It seems to me that I never received a definitive answer.
>
> What is the practical consequence of the index mirror not being perfectly 
perpendicular to the frame of the sextant? I have dozens of navigation texts, 
some of them dating back quite a few years and while all of them direct the 
navigator to check for it, none of them reveal the consequences of not having 
the index mirror perpendicular to the frame of the sextant.
>
> Furthermore, I know of a few techniques for adjusting the index mirror for 
perpendicularity, including the eyeball method and placing objects (such as a 
pair of dominos) of exactly the same height at opposite ends of the sextant 
limb but it seems to me that these methods are rather coarse and not nearly 
as precise as those for reckoning index and side errors.
>
> Any comments on this most fundamental topic are welcome.
>
> Robert
>

   

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