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On 2016-04-02 22:33, I wrote:
> The T2 circle has 10 minute graduations.

Correction: the circle is graduated every *20* minutes. The two halves
of the split image in the reading microscope move in opposite
directions, so the graduations coincide every 10 minutes. The micrometer
has a 10 minute range.


> In the reading microscope,
> opposite sides of the circle are visible simultaneously, one above the
> other in a split image. The lower half is inverted and moves right when
> the upper half moves left.

Correction: the *lower* half is upright. That's where you read the angle.

The reason for superimposing images from opposite sides of the circle is
eccentricity error. In general, the geometric center of the graduated
circle is not exactly on the instrument axis of rotation. Therefore,
even if perfectly graduated, the instrument will read high or low
depending on the direction it's pointed. (Significant eccentricity error
used to be common on sextants, though I think modern manufacturing
methods have practically eliminated that.)

The solution is to read the circle at two points 180° apart. For
instance, with an old fashioned theodolite the observer would set the
telescope on the target, then step around to the side of the instrument
to read the angle with a microscope. At the same time his assistant took
a reading on the opposite side. The mean reading was free of
eccentricity error.

Heinrich Wild devised an optical system to superimpose both images in
the observer's view, plus a micrometer to measure their offset. Moreover
(except the T4), the microscope eyepiece is on the telescope. The
observer need only shift his head a few millimeters to read the circles.
No walking around the instrument, no eccentricity error.

   

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