WIRCam

WIRCam is the newest wide-field imaging facility at CFHT and represents one of the largest astronomical mosaic of infrared detectors ever built.

WIRCam contains 4 2048 x 2048 pixel HAWAII2-RG detectors as shown in Fig.1, and covers a 20 arcminute x 20 arcminute field-of-view with a sampling of 0.3 arcsecond per pixel. To properly sample the 0.4 arcsecond infrared seeing often offered by the CFHT at Mauna Kea, WIRCam will use its image stabilization unit to micro-step the image with 0.15 arcsecond sampling. The image stabilization signal will be obtained by repeatedly reading out a small region of the detectors centered on a bright star, while the exposure continues for the rest of the pixels.

SDSU controller has been adapted to execute the image readout task. The major functions of controller are to provide precise bias and clocking signals that needed by arrays. Each video board is capable of 8 channels readout and we used totally 16 video cards to provide 32 output channels of each H-2RG array. It will provide a 1.5s CDS image to minimize the overhead of WIRCam observations.

SDSU controller has been adapted to execute the image readout task. The major functions of controller are to provide precise bias and clocking signals that needed by arrays. Each video board is capable of 8 channels readout and we used totally 16 video cards to provide 32 output channels of each H-2RG array. It will provide a 1.5s CDS image to minimize the overhead of WIRCam observations.

The optics of infrared camera is very different from visible instrumentation. Heat of lenses should be reduced to eliminate its own emission. The optical components are assembled in two cans and housed inside the WIRCam cryostat. The barrel which holds lenses at 85K is shown in Fig.5.

The mechanical structure of WIRCam and its cryogenic systems are shown in Fig.7 and Fig.8, which were designed by the Laboratoire d'Astrophysique de Grenoble. The structure was optimized to have extremely low flexures, and the large filter wheels to have reliable fast movements in the cold environment (15s maximum time for filter changes).

The commission run of WIRCam started from March 2005. Fig 9. shows a test image of M17. Some fine tunes of the system are in progress to provide best condition for the coming science operations . We are so glad to see the success of WIRCam. Lots of precious experiences have been learned by our engineer staff and hopefully can be brought into future projects of ASIAA.

Fig.1 Four HAWAII-2RG infrared detectors are assembled into one module to form the mosaic and improve resolution. Pixel size is 18 μm and field of view is 0.3 arcsecond. Active pixel number is 2040x2040 with 4 bordered reference pixels.


Fig.2 The sky is clear and pure very often on the top of Mauna Kea. The stabilization of the atmosphere is the most important fact that excellent seeing could be provided for 300 days per year.


Fig.3. The dome of CFHT. The prime focus mirror CFHT is 3.6m and it occupied a very good site on the summit due to its old history.


Fig.4 SDSU controlling system. Eight video cards shown in the figure are used to offer 64 channels readout. Another controller with the same configuration is used to handle the other two arrays’ readout and is synchronized with controller no.1 by hardware handshaking.


Fig.5 The principle of optics is displayed in Fig.6. In real engineer world, the stress of lenses induced by thermal contraction and its large size made the design and mounting very challenging.


Fig.6 Preliminary optical design diagram of WIRCam.


Fig.7 The mounting of mosaic module. Flexible cable were used to conduct I/O signals..


Fig.8 WIRCam in the prime focus cage.


Fig.9 M17 image produced by WIRCam.