The Universal Theodolite DKM 3A with self-recording
motor-micrometer
G. J. H'jsti
1 Introduction
The precision of time measurement attainable in
star observations depends by far on the equipment
used (stopwatch, chronometer, chronograph) and
on the method applied (eye and ear, tapkey, self-
recording micrometer). In the following article
attention will be paid to first order geodetic-astro
nomical observations, generally carried out using
a self-recording micrometer and a chronograph.
The accuracy of time recording with digital chrono
graphs, including the clock correction from radio
time signals, is today quite satisfactory. It is well
known that with a little effort an accuracy of 0.v001
can be obtained. However, generally an accuracy
of 0.s01 suffices. Consequently, significant improve
ments can only be expected from better methods of
observation. The methods of observation should
not only be improved for the sake of obtaining higher
accuracy, but also to make the observations in a
more convenient way. This may be done in different
manners
- improving the visual methods
- applying photographic methods
- applying photo-electric methods
As an example of improving the visual methods the
tracking method of Ramsayer [7], [8] with moving
telescope should be mentioned. A more simple and
less expensive method, however, is to provide the
self-recording micrometer of Repsold with motor-
driving. After all the idea of a motor-micrometer is
not new. Already in the year 1901 the astronomer
Struve [13] obtained good results with a self-
recording micrometer driven by a timepiece. At
the present time a number of first order instruments
(Danjon astrolabe, Askania transit instrument AP
70) are equipped with electric motor-micrometers
[4], At the same time it should be noted that the
instruments mostly used by geodesists, the universal
theodolites Wild T4 and Kern DKM 3A, are still
provided with a manually operated micrometer.
Tracking stars, and especially slow moving stars,
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using a manually operated micrometer, is not con
sidered as an easy observation method. More
over, the personal error of the observer is not always
negligible. Several investigations have shown that
the observer keeps the movable wire systematically
behind the star. Lorck [5] reports differences up to
0."4, while at the Delft University of Technology
still greater values are found, namely 0."5, and
with less experienced observers even 0."8.
In addition, it is not inconceivable that the observer
influences the line of sight when touching the micro
meter during the measurement.
2 DKM 3A with motor-micrometer
The universal theodolite Kern DKM 3A is mainly
designed for first order geodetic-astronomical
observations. Therefore it is provided with a broken
telescope, precision striding level and a self-record
ing micrometer [6]. This instrument has been in
vestigated at the Delft University of Technology,
Sub-department of Geodesy, in the following way.
To the self-recording micrometer, normally operated
by hand, a micromotor is attached. The complete
equipment is shown in Fig. 1. One of the tracking
knobs of the self-recording micrometer is removed,
and to the axis of this knob the micromoter is
connected by means of a plastic tube (friction
coupling). The micromotor, fixed on a mounting
plate, can simply be dismantled. This attachment
fits any DKM 3A.
It should be pointed out that the micromotor to be
used must have a good speed control. A D.C. motor
with voltage regulation is less suitable for this pur
pose, because of the time delay in the regulation.
Very good results are obtained with a step motor,
which has the advantage of a perfect speed control
realised electronically. The discontinuity of the
rotation is no objection when the steps are small.
In our case a motor of 48 steps with a transmission
10:1 is used, which means that the movable wire is
jumping with intervals of 0."5.
SUMMARY
The accuracy of tracking stars has been investigated with the aid of a motor driven
self-recording micrometer attached to the universal theodolite DKM 3A. This article
describes the equipment and analyses the results of the observations using an artificial
star. The most important conclusion is that the observer's personal error can practically
be neglected.
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