Untitled - Laboratoire d'Astrophysique de l'Observatoire de Grenoble

have taken responsibilities: A. Chelli (EII board, Coordinator of JRA4), G. Duvert (PI of a JRA4 workpackage),
P. Kern (OPTICON board), C. Perrier (EII science council), G. Zins (project manager of a JRA4
work-package). The JMMC and EII activities are developed in a separated chapter in this report.
11.3 Cameras and detectors
11.3.1 Towards wide-field cameras: WIRCam
WIRCam (Wide-field InfraRed Camera) is the second instrument of the Wide Field Imaging Plan of the Canada
France Hawaii Telescope (CFHT), providing a 20.5 × 20.5 arcminute field of view in the infrared ([0.9 µm; 2.4
µm]), and completes in the infrared the MegaCam instrument operational on the CFHT since January 2003.
The LAOG, under the lead of the local project manager E. Stadler, was responsible of the cryovessel design and
manufacturing, including the filter wheels design and control/command. The LAOG was also responsible of the
cryogenic system design, manufacturing and testing and of the temperature regulation (optics and detectors)
as well. The optics was developed by the University of Montreal and the detectors control was performed by
the CFHT.
A challenging thermal design. This innovative instrument is based on four Hawaii 2RG detectors arrays
developed by Rockwell in a close buttable package. This camera is designed to be placed on the prime focus
of the 3.6 m CFHT telescope to take benefit of the simpler opto/mechanical design for a wide-field camera.
It uses a Gifford Mac-Mahon closed-cycle cryo-cooler to avoid strenuous daily re-fillings on the telescope due
to poor accessibility. An optimal thermo-mechanical design has been defined to meet the stringent stability
requirements with minimal thermal losses. To provide excess cooling power was not possible due to weight
constraint on the camera of 250 kg. As the cryo-cooler must be easily dismounted for maintenance operation,
the cooling power is transmitted by a system of two cones fitted together, one male and one female, made of
OFHC copper and having exactly the same shape. The thermal link between the two cones is enhanced by using
ultra-high vacuum grease. A tunable load between the two cones is applied by a system of titanium springs.
All the cold structure is attached to the warm part of the cryostat by ten G12 composite twin blades.
Thermal-mechanical modelling. In the past decade, new computing tools have been offered to the system
designers in terms of thermal and mechanical modeling. In addition to an overwhelming increase of computer
capabilities, these tools are now mature enough to drive the design of complex astronomical instruments, in
particular if these instruments have to be cooled. This allows to better understand the cryogenic performances,
which is a huge advantage in a new design approach, and to waste time during the instrument integration.
A complete thermal-mechanical model of the camera using Finite-Element Analysis (FEA) under the I-deas
software was carried out. The capabilities of the I-deas thermal module (TMG) was demonstrated for our
particular application 5 : studies included conduction, radiation and free-convection management, variations of
the cooling power and thermal characteristics of the materials as a function of the temperature, and studies in
permanent regime and transient analysis (Figure 11.10).
The WIRCam cryovessel was successfully installed on the CFHT telescope by December 2004 with a team
of LAOG people and has obtained its first light in March 2005 after the optics and detector integration (Figure
11.9). A temperature regulation of the detector at the 0.002 K level was obtained on the telescope at the
nominal detector temperature of 81 K.
11.3.2 Research and development activities in photon counting superconducting
detectors
Superconducting Tunnel Junctions (STJ) Superconducting Tunnel Junctions have been developed as
photon counting detectors for a wide range of applications since they are energy resolving photon counters and
5 P. Feautrier; E. Stadler; P. Puget; 2004 ; Interest of thermal and mechanical modeling for cooled astronomical instruments:
the example of WIRCam, SPIE Proc., 5497, 149-160
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