Untitled - Laboratoire d'Astrophysique de l'Observatoire de Grenoble
Untitled - Laboratoire d'Astrophysique de l'Observatoire de Grenoble 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, 149-160 126
Figure 11.9: Left View of the WIRCam cryovessel during its installation on the CFHT (March 2005). Right: raw data (no flat-fielding) from an engineering test J band image of the M17 nebula obtained in 30 s with a seeing of 0.4 arcsec. The quadrants show the 4 detector array and the gap in between. The excellent cosmetic quality can be judged from the zoom below. Bottom: zoom of the previous image, raw data without flat-fielding, showing the amazing cosmetic quality of the last Rockwell infrared detectors. can be used from the infrared (2 µm) to X-ray wavelengths with good quantum efficiency (60%) 6 . The aim of our work was to investigate the interest of this kind of detectors for ground based low-light astronomical applications and compare them to Superconducting Single Photon Detectors (SSPD) (see next paragraph). Since the conventional detector arrays based on semi-conductor devices have recently progressed toward photon counting detectors in the visible 7 , some niches have to be found for this type of detectors where conventional devices cannot compete. The astronomical applications that could be investigated are wave-front sensing detectors for adaptive optic systems, fringe sensors and focal plane instruments for interferometry in the near infrared. This work has been carried out in a collaboration between three laboratories from Grenoble: the LAOG (responsible of the activity under the lead of P. Feautrier), the CRTBT(under the lead of A. Benoit), and the CEA-Grenoble/DRFMC/LCP (under the lead of Jean-Claude Villégier) in charge of the device fabrication (Figure 11.11). At the end of the last LAOG PhD thesis based on this subject, we decided to put our efforts on some slightly different superconducting detectors, the SSPD (see next paragraph), which present important advantages compared to STJ: 6 G. Brammertz, A. Peacock, P. Verhoeve, D.D.E. Martin, and R. Venn, Optical photon detection in Al superconducting tunnel junctions, Nucl. Inst. and Meth. A, 520, 508 (2004) 7 C. D. Mackay, R. N. Tubbs, R. Bell, D. J. Burt, P. Jerram, and I. Moody, Sub-electron read-out noise at MHz pixel rates, Proc. SPIE, 4306, 289 (2001) 127
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Figure 11.9: Left View of the WIRCam cryovessel during its installation on the CFHT (March 2005). Right:<br />
raw data (no flat-fielding) from an engineering test J band image of the M17 nebula obtained in 30 s with a seeing<br />
of 0.4 arcsec. The quadrants show the 4 <strong>de</strong>tector array and the gap in between. The excellent cosmetic quality<br />
can be judged from the zoom below. Bottom: zoom of the previous image, raw data without flat-fielding,<br />
showing the amazing cosmetic quality of the last Rockwell infrared <strong>de</strong>tectors.<br />
can be used from the infrared (2 µm) to X-ray wavelengths with good quantum efficiency (60%) 6 . The aim<br />
of our work was to investigate the interest of this kind of <strong>de</strong>tectors for ground based low-light astronomical<br />
applications and compare them to Superconducting Single Photon Detectors (SSPD) (see next paragraph).<br />
Since the conventional <strong>de</strong>tector arrays based on semi-conductor <strong>de</strong>vices have recently progressed toward photon<br />
counting <strong>de</strong>tectors in the visible 7 , some niches have to be found for this type of <strong>de</strong>tectors where conventional<br />
<strong>de</strong>vices cannot compete. The astronomical applications that could be investigated are wave-front sensing <strong>de</strong>tectors<br />
for adaptive optic systems, fringe sensors and focal plane instruments for interferometry in the near<br />
infrared. This work has been carried out in a collaboration between three laboratories from <strong>Grenoble</strong>: the<br />
LAOG (responsible of the activity un<strong>de</strong>r the lead of P. Feautrier), the CRTBT(un<strong>de</strong>r the lead of A. Benoit),<br />
and the CEA-<strong>Grenoble</strong>/DRFMC/LCP (un<strong>de</strong>r the lead of Jean-Clau<strong>de</strong> Villégier) in charge of the <strong>de</strong>vice fabrication<br />
(Figure 11.11).<br />
At the end of the last LAOG PhD thesis based on this subject, we <strong>de</strong>ci<strong>de</strong>d to put our efforts on some slightly<br />
different superconducting <strong>de</strong>tectors, the SSPD (see next paragraph), which present important advantages compared<br />
to STJ:<br />
6 G. Brammertz, A. Peacock, P. Verhoeve, D.D.E. Martin, and R. Venn, Optical photon <strong>de</strong>tection in Al superconducting tunnel<br />
junctions, Nucl. Inst. and Meth. A, 520, 508 (2004)<br />
7 C. D. Mackay, R. N. Tubbs, R. Bell, D. J. Burt, P. Jerram, and I. Moody, Sub-electron read-out noise at MHz pixel rates, Proc.<br />
SPIE, 4306, 289 (2001)<br />
127
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