The thermal layout is shown in Fig. 2. The cooling path from the CCDs travels through the CCD42-90 package's precision shims to the Invar mounting plate. Copper straps connect the Invar plate to a set of four cold copper bars. These copper bars are connected to two IRLabs ND-14 dewars mounted on opposite sides of the central dewar. The copper straps were trimmed once after the initial cool down and the CCDs vary in temperature from -130°C to -115°C across the focal plane. A more uniform distribution could have been obtained with further iterations of trimming. The temperature of the CCDs can be stabilized using a set of heater resistors mounted on the back side of the Invar plate. The heaters are all wired in a single zone.
The total LN2 consumption of the Megacam is 40 W or 20 liter/day. The ND-14 dewars are filled from the bottom and thus contain a tube that extends inside the tank to prevent the liquid from running back out. We have chosen to make this tube extend 60% the height of the tank. This means we can only fill the tank 60% full, but are not concerned with the dumping of large quantities of LN2 when the telescope tips over. The hold time in this configuration is roughly 36 hours. The thermal load is dominated by radiation coupling from the dewar window to the CCDs. Several features minimize other sources of thermal loading. We have gold-plated the Invar plate, and copper straps and bars. The flex circuits connecting to the CCDs have two ground layers to provide electrical isolation between the AC and DC signals. Instead of a solid plane, these layers are laid out in a serpentine path to minimize the thermal conductivity. Titanium was chosen for the flexures because of its high stiffness and low thermal conductivity. The flexures contribute only 5 W of the 40 W load. Finally, we put radiation shields between the flex circuits and the Invar cold plate.
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