Max Planck Institute for Astronomy - Annual Report 2007

Fig. IV.3.2.: The design of Panic. Above: Light enters from top
into the cryostat, three mirrors fold the light path to a compact
design. The diameter of the cryostat is about 100 cm. All optical
elements are mounted to an optical bench (red) to minimize
flexure. Below: view on the optical bench showing the folding
mirrors, the lens optics and the filter wheels (yellow).
The New Control System for the 3.5 m Telescope
The 3.5 m telescope had a control system that dated from
the 1970s and its contemporary hardware had reached the
limits of its useful life; repairs to the outdated technology
had become extremely difficult as replacement parts
were no longer available – not even equivalent types. In
order to prevent breakdowns of purely technical nature,
the system was completely replaced.
The new system controls the telescope’s drives in hour
and variation, the rotation of the Cassegrain flange, and
the focus drive. The telescope computer system no longer
consists, as before, of only one computer where the bus
extended across the entire telescope building in order to
communicate with the individual drives to be controlled,
but of a total of five VME computers which are assigned
to the drives. They are networked with the central computer
across a standalone Ethernet. Commercially available
digital, highly integrated, intelligent modular components
control and regulate the individual drives. The old,
IV.3 Instruments for Calar Alto 111
mostly analog hardware was thus replaced with modern,
highly integrated, and intelligent digital electronics.
Because there are very high precision demands on
such a control system, the required control loops are
complex. Thus, for example, the most complicated and
important drive, the hour drive, consists of three control
loops which are interconnected with each other. These
structures, which were established by Zeiss in experiments,
were retained in simplified form in the design in
order to save on development time.
After two years of operation, it is clear that the new
control is excellent. The computer system has been running
for months without a hitch and the installed modular
components have had no breakdowns so far.
In addition to reliability, an important criterion for
controlling a telescope is obviously the precision achieved.
Here too there has been improvement: With the original
control from the desired position, the rms-deviation
was 0.15 arcseconds; with the new control it is now only
0.1 arcseconds.
Josef Fried, Karl Zimmermann, Rainer Wolf,
in cooperation with Calar Alto’s engineers