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Frank,
My description of the arc error measuring device
was not actually posted; this was just a reply
to Brooke Clarke who asked off the list.
Here it is.

---------- Forwarded message ----------
Date: Mon, 23 May 2005 15:45:19 -0500 (EST)
From: Alexandre Eremenko 
To: Brooke Clarke 
Subject: Re: Visit to Freiberg

On Mon, 23 May 2005, Brooke Clarke wrote:

> Do you have photos of the test equipment or could you make sketches of it?

Unfortunately, the idea to bring my camera,
and to ask their permission to make photos
did not occur to me.

But I can describe the device from my memory.
(Sorry, I never remember "device" or "devise" and on
my German computer there is no English spell checker:-)
(I can certainly make a sketch but don't know whether
I will find a scanner here in Germany, and I am not sure
that I know how to scan:-) So I apologize for the low-tech pictures
below:-)

The device essentially consists of a horizontal platform
(made of "pig iron" or something like this, very sturdy and heavy)
with three tubes (scopes) and a rotating mirror.
All tubes are parallel and are about 2 ft long and 2.5 inches thick.
(I suppose these are Kepler telescopes). The configuration
is approximately shown in the picture. M is the device's own
rotating mirror.

                                  mirror M   ---------tube 2
                     sextant index mirror
 tube 3-------------  sextant horizon mirror ---------tube 1

Tubes 1 and 2 work as reversed telescopes. The sources of light
are placed on the right of the picture, in the foci of tubes 1 and 2.
Their object lenses create parallel beams going right to left.
The beam from tube 2 is reflected by the mirror M (rotating and
attached to the device's own arc) and then by the index mirror of the
sextant, and then follows the normal way in the sextant,
ending in the tube 3. The sextant telescope is removed,
and the observer looks through the tube 3 (left to right).
He puts the two lights into coincidence by rotating the mirror M.

The general idea should be clear: the device controls the angle of the
mirror M. The arc and the arm controlling this mirror was hidden inside
the device and the observer read the angle through a microscope.

The light ray from the mirror M comes at the known, controlled angle
to the sextant index mirror, and then measured with the sextant
in the normal way.

Then the results are compared.

In practice, the procedure looks like this.
The sextant is clamped rigidly on the platform.
Then it is assured that the plane of the sextant
arc is strictly parallel to the plane of the devise.
Then the axis of rotation of the index mirror is made
to coincide exactly with certain prescribed point on the device.
(This was the longest part of the procedure).
Then the perpendicularity of sextant mirrors to the
plane of the device is adjusted.
Then the index error is set to be zero by adjusting the index mirror.
Then the backlash was regulated.
And then the angles of 10, 20, 30, ... degrees exactly were first
set on the sextant, then the images of the point sources were
aligned and the true angles were read through the device microscope.

Unfortunately, I could not talk to the master who was doing this
and ask him questions. He did not speak English at all, and the
translating secretary was not familiar with special
English technical
terminology
(This is not to say that I am so much familiar with it:-)

In a modern construction I would use laser beams instead of the telescopes
1 and 2, but all equipment I saw seemed to be 40-50 years old at least.
(Possibly 70 years old).

The hardest part in making this sort of device seems to be
mechanical. Graduation of an arc to 1" precision,
and various backlashes and excentricities.

Alex.

   

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