and its corresponding time is recorded on the same
time scale as the observations resulting from the
mechanical contacts. Assuming uniform motion,
the true transit times corresponding to the observa
tions can be computed by linear interpolation.
Therefore the calibration of the micrometer scale
with respect to the reference points is necessary.
Finally, from the differences between the observed
and the computed contact times at each transit, the
personal error and the standard deviation can be
computed.
The equipment used for the time measurement is
shown in Fig. 4. The observations as well as the
pulses of the photocell are recorded by an Omega
timerecorder. Besides recording all the contact times
individually, the mean value of these contact times
is computed simultaneously by a computer-clock
("chronoputer"), synchronised with the Omega
time recorder. These mean times, however, have
been used only for check.
The computer-clock, mentioned above, was devel
oped by the Central Electronic Service of the Delft
University of Technology. It is able to compute
the mean value of an arbitrary number of times
(between 1 and 99). If required, a number of times
at the start (between 0 and 9) can be excluded from
the computation.
For the investigation of the motor-micrometer the
DKM 3A was set up on an observation pillar at a
distance of about 40 meter from the artificial star.
All the mechanical errors of the artificial star can,
at this distance, be neglected. The observations are
carried out with five different velocities (14.42,
11.91, 9.55, 7.17 and 4.78"/sec), measured in
arbitrary sequence. The measurement of each
velocity is repeated three times in both horizontal
directions ("meridian transit"). Consequently, one
observation programme consists of 30 transits,
which can be measured, including the calibration
measurements, in less than one hour.
4 Variance of the time
The variance of the time of a star's transit with the
vertical wire is expressed by [4]
(al -f sec2 <5 sec2 <7^ (1)
N v
where
N number of contacts
v magnifying power of the telescope, for the
DKM 3A u 45
<5 declination of the star
q parallactic angle (in our case 0° or 180°)
The parameters a0 and b0 depending mainly on the
observation method applied, can be determined
empirically. Some numerical values obtained in The
Netherlands from Laplace point measurements
(Black method) and from observations with an
artificial star [2], [3], are given in table 1.
Fig. 4. Instruments for the time measurement
1. remote control for the artificial star; 2. signal
converter; 3. Omega time recorder; 4. computer
clock ("chronoputer"); 5. Voumard frequency
standard.
ngt 73
51
