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Jared Sherman wrote on Thu Sep 30 2004 - 20:30:06 EDT:

>only in one direction, >

>Alexandre, this is probably a writer's error in the manual.

This is not the error the the manual, but my incomplete
or non-precise citation of the manual.
My own understanding of this recommendation agrees with
your explanation: I have to rotate in one chosen direction
immediately before reading:

> It doesn't matter which way you go
> --only that it be
> consistent. And of course, that should be the same
> direction that you go
> in when zeroing for index error as well.

In Russian manuals, the error component explained in your
message has a special name, literally in Russian "dead movement"
of the worm. In the web translation of the SNO-T manual it
is translated as "backlash between sextant worm and frame".
They always care to include this "dead movement" of each
particular sextant into its certificate. They guarantee that it
is to be most 6" (some sextant manuals say 12" depending
of the year of production)
but measure it and include to the
certificate for each sextant.
For example, in my sextant certificate it is 4".
They also have a special check for this quantity that they recommend
to do periodically together with the other standard checks.
(The procedure is evident: measure the index error twice,
one time for each direction of rotation,
and then take the difference).

Alex.

   

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