The CCD camera requires cooling and vacuum enclosure. For ground operation the CCD dewar is sealed by an electrically actuated, spring-loaded pop-top door. To allow convenient ground verification and calibration a 100 Curie Fe source is installed in the inside of the door, visible to the CCDs when the door is closed. This source provides a high enough count rate to monitor the camera performance in short ground tests.
The door will be opened by command from the ground following attainment of operational orbit and completion of initial instrument checkout. Once opened in flight, it will not (and cannot) be closed again. The door is actuated by a thermal wax motor.
The power to the Wax Motor Heater is redundantly protected against accidental opening. A pair of pressure sensors, one in the CCD camera and the other on the analog electronics board, measure the pressure on both sides of the door. The signals from these sensors are monitored by the ICP, which controls the Wax Motor Heater power through a relay. The ICP will not operate this relay unless the pressure sensors read the same value, in order to prevent a catastrophic opening of the door with imbalanced pressures across it. In addition, the heater power is wired through the normally closed contacts of the TEC power relay and the normally open contacts of the heater power relay in series to ensure that the TEC and heater power cannot both be on simultaneously. This will prevent the possibility of an excess power drain by the CUBIC heater. Two heaters on the wax motor provide redundancy in the event of a heater failure. These are selected by a relay actuated by ground command through one of our Pulse Relay command channels.