WIRCam (Wide-field InfraRed Camera, see Fig. 1) is the second instrument of the "Wide Field Imaging Plan" of the Canada France Hawaii Telescope, providing a 20.5^20.5 arc minute field of view in the infrared
(0.9-2.4 ^m). This will complete the infrared aspect of the MegaCam instrument which has been operational on the CFHT since January 2003.
This innovative instrument is based on four Hawaii 2RG detectors arrays developed by Rockwell in a close buttable package. WIRCam uses the special guiding capability of these arrays. Details of this instrument are described in Puget, et al. . This camera is placed on the prime focus of the 3.6 m CFHT telescope in order to benefit from the simple opto-mechanical design of a wide-field camera. The camera uses a Gifford Mac-Mahon closed-cycle cryo-cooler to avoid strenuous daily refillings on the telescope due to poor accessibility because of the camera location. Moreover, it is necessary to define the thermo-mechanical design to meet the stringent stability requirements of the optical design with minimal thermal losses. This allowed the correct choice of cryo-cooler and to predict the cryogenic performances of the camera: permanent regime, temperature regulation needs, cooling performance, cryostat warming design, cryogenic margin, etc. The provision of excess cooling power was not possible due to weight constraint on the camera of 250 kg from the focusing unit features of the telescope.
The focusing unit, attached to the prime focus upper end, is used to focus the instrument on the telescope. Thermal simulations are also used to assist in the mechanical design and demonstrate the achievement of the required low temperature of the detectors during the design phase.
As the cryo-cooler must be easily dismounted for maintenance operations, cooling power is transmitted by a system of two cones fitted together, one male and one female, made of OFHC copper with exactly the same shape. The thermal link between the two cones is enhanced by using ultra-high vacuum grease manufactured by Pfeiffer vacuum. A tunable load between the two cones is applied by a system of titanium springs. They provide the required load while minimizing thermal leaks between ambient and colder temperatures. The cold structure is attached to the warm part of the cryostat by ten G12 composite twin blades. This design minimizes thermal leaks while achieving the required stiffness.
Figure 2 shows the WIRCam cryovessel during the cryovessel integration at the CFHT on the Mauna Kea summit in December 2004.
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