Max Planck Institute for Astronomy - Annual Report 2005

106 IV. Instrumental Development
IV.11 Pacs – Far-Infrared Camera and Spectrometer
for the hErschEL Space Telescope
Within a consortium of 15 European institutes, MPIA is
participating – with a share of 15 percent – in the development
of the Pacs instrument, a camera and spectrometer
for wavelengths of 60 to 210 µm.
The most important contributions from Heidelberg are
the focal plane chopper, the characterization of the large
Ge:Ga-spectrometer-cameras and their – 270 °C cold
readout electronics, the investigation and avoidance of
radiation damages to these components, and the calibration
of the instrument before and during flight.
Already in June 2005, MPIA was able to deliver the
focal plane chopper – as one of the consortium's first contributions
to the Pacs flight model – to MPE Garching,
which is responsible for the overall instrument. The
chopper manufactured by C. ZEiss is a derivative of a
prototype designed and tested at MPIA. It is of excellent
quality and surpasses many of the technical requirements.
In the summer of 2005, construction of the flight
spare unit of the chopper was started, which promises an
even better performance.
During the characterizations of the 16 3 25 Pixel-Ge:
Ga-cameras for the flight model unexpectedly numerous
Fig. IV.11.1: Seven detector rows with 16 pixels each of a camera
for the Pacs spectrometer unit. The light funnels in front of
the Ge:Ga pixels are visible. The two cameras each have 25 of
such modules. At MPIA, these detector modules are characterized
in detail with respect to their sensitivity, noise, dark current
etc. at temperatures below – 270 °C.
Fig. IV.11.2: PhD student Jutta Stegmaier prepares a test of detector
modules (as in Fig. IV.11.1) to be conducted in the liquid
helium cryostat of MPIA.
failures occurred. MPIA participated in a series of experiments
lasting several months to explain the malfunctions.
For this we had to move the extensive test facilities of
MPIA to the manufacturer of the cameras. Finally, two
main causes for the high failure rates were determined:
1. destruction of electronic amplifier and readout stages
by electrostatic discharges and 2. bad bonding of the thin
steel connecting leads; steel is used because of its low
thermal conductivity. To avoid the electrostatic problems
the handling of the components had to be regulated
very carefully at the institutes and firms involved (air
humidity, groundings, protection circuits …). With the
new procedures the success rate of the detector/readout
electronic tests rose steeply. So by the end of the year
most of the camera modules were fully characterized and
could be delivered to the manufacturer astEq for the
assembly of the cameras. Because of the unexpectedly
time-consuming tests with the Ge:Ga-spectrometer-cameras,
the radiation tests with gamma-ray sources to
simulate an optimal operation at the Lagrangian Point L2
were delayed.
After the work for the actual instrument is drawing to
a close at MPIA, the tests and development of the ground
observatory are reaching the critical stage. Scientists
from Heidelberg were involved in the cold tests of the
Pacs qualification model and in the integrated module
tests of all three hErschEL instruments. For this, test
procedures were designed and an extended data analysis
was carried out. Naturally, the focus of the contributions
from Heidelberg was on detectors, chopper, and calibration.
Because the development of subsystems and their
mutual interfaces from the 15 institutes of the Pacs
consortium are still not quite perfected, all these tests
took noticeably more time than originally planned.
Moreover, MPIA is contributing to the design of the
scientific observing templates for future users of the
hErschEL Observatory and has taken on a leading role
in the development of the interactive data analysis for
Pacs.