The camera housing provides a vacuum environment for the CCDs. This enclosure must be kept continuously under vacuum during all CCD operations to prevent frost buildup on the cold CCDs. An external pump is used to maintain a low pressure inside the CCD camera during ground testing operations. A schematic drawing of the CUBIC CCD camera system is shown in Figure 8.
The CCD camera includes a getter intended to provide passive pumping once disconnected from the external vacuum pump. We have found that the camera is either outgassing or leaking at a rate of about 5 T/week. This means that the pressure at launch may be as high as 30 T. A 1/2 psi relief valve in the camera body will vent any excess gas during launch, so that the internal pressure should be less than 26 T on orbit before the door is opened. We have verified in laboratory tests that the filters will not break if the door is opened with this pressure inside the camera. We have also verified that the relief valve opens at about 20 Torr, and that the filter does not break under protoflight acoustic levels in a 100 Torr environment. We therefore expect that the filter will survive both launch and door opening.
Each CCD is mechanically supported by a niobium block (
pure)
bonded to its back.
Nb was chosen in order to minimize the differential thermal expansion
between the CD package and the mounting block (the Nb and ceramic package
both have CTE of about 
/C). The Nb block is screwed
down to an Al plate which is clamped to the top of the TEC. A thin
piece of indium foil between the niobium mounting block and Al cold
plate ensures good thermal contact between them. A platinum
RTD mounted between the Nb mounting block and the CCD package monitors
the temperature of each CCD, while AD590s monitor the temperature of the
backside of the TEC and the camera housing.
