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Frank,

On Sun, 5 Dec 2004, Frank Reed wrote:

> Sextant users are advised to eliminate "side error".
> This the little
> left-right gap that shows up between the direct and reflected
> image when the  sextant
> is set to zero. Side error is eliminated when the index mirror

And also the horizon mirror.
With my sextant, I find it much harder to adjust the horizon
mirror perpendicularity than that of the index mirror.
(Because the screw on the horizon mirror is over-sensitive)
In practice, I always have a very small side error due to
the horizon mirror, just because I am unable to adjust
the horizon mirror precisely.

Chauvenet has a formula:
err=-2l^2 sin(1')tan (A/4),
where err is the error measured in minutes,
l is the inclination of the index mirror in minutes, and A is
the measured angle.

But this formula assumes that the horizon mirror is perfectly
adjusted
according to the usual procedure and its inclination is EQUAL
to that of the index mirror.

(Which means that the images of two stars would exactly coincide
when the reading is zero).

Alex.

   

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