According to model calculations a heat load of 65 W had to be accommodated by the cooling system, which is mounted below the CCD detector head. As no options for external compressors or cooling lines through the telescope cable wrap existed, a self contained LN2-based system was developed (see Figures 5 and 6). It utilizes the maximum cooling power of liquid nitrogen through the combination of an integrated 40 liter LN2 tank, combined with the main top heat exchanger operated in continuous flow mode by the cryostat cooling controller. The top heat exchanger is coupled by means of electrically isolated silver foils to cooling clamps which link directly to the bottom cooling fingers of the CCD mosaic table in the CCD detector head. This way the weight of the cooling system (about 120 kg) could be decoupled from the flexure of the mosaic table. This also minimized vibration and allowed us to overcome position-angle dependence of conventional bath cryostat systems at Cassegrain focus. As the cooling clamps can be opened and closed through the vacuum vessel, the cooling connections are easily separable before the detector head is mounted or dismounted.
During laboratory operation, very good margins have been obtained to reach the necessary CCD operating temperature of -120 °C and a hold time of about 40 hrs. For the cryogenic and vacuum system control, the cooling system has integrated (heated) sensors for level sensing at top and bottom, a heat-able charcoal sorption pump, and two fully redundant sets of temperature and vacuum sensors. Further details about the cooling system can be found in Lizon, et al. .
100 temperature sensors with optional permanent logging were integrated into the cryostat at intermediate levels. These were implemented as diagnostics of the new construction of both the detector head and the cooling system (considering all integrated electronics). They will prove very useful for checking the cooling clamp interface after reintegrating the detector head and cooling system in Chile.
Pt100 boards with standardized glue-free mechanical mounting interface have been developed for universal use. They interface by means of an interconnection kit in different lengths to four multiplexers within the head. Heat dissipation and wiring are minimized.
The Pt100 boards are connected to PULPO2 , responsible for reading, logging and alarm signaling of up to 125 temperature and vacuum sensors. Furthermore PULPO2 operates four independent PID heater control loops with freely programmable control sensor assignments. In full CCD mosaic operation a homogeneity of 2 K (pp) was achieved across the focal plane.
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