2.2.3.2 ACIS Thermal Configurations and Modeling

The ACIS Detector Assembly is designed to operate in a 10,000 by 140,000 kilometer elliptical orbit. Under normal operation, the focal plane will operate at -120 C. However, there are other thermal configurations for which the detector will be required to operate on-orbit.

On-Orbit in Shuttle

While the AXAF-I is in the cargo bay and on-orbit, the normal orientation is +Z nadir which orients the radiators toward the earth at an altitude of approximately 130 nautical miles. In this orientation, the focal plane and detector housing will operate at around room temperature. In contingency modes, it is possible that +Z can be oriented anti-nadir. Full sun on the radiators must be limited to less than 30 minutes, followed by a 12 hour cool-down, to prevent the focal plane from overheating.

AXAF Transfer Orbit

While the AXAF-I is being transferred to its final elliptical orbit of 10,000 by 140,000 kilometers, the radiators view of the full sun must be restricted. The limitation on hot temperature extremes are that the radiators shall be exposed to full sun no more that 30 minutes with a 12 hour cool-down. Operations at survival temperatures are also possible depending on sun orientation and survival heaters will turn on.

On-Orbit Normal Operation

During normal operations of the AXAF, the sun will never shine on the radiators. The only potential heating which can occur on the radiators is at perigee when the radiators can get full albedo and earth IR radiation when they are in the nadir direction. The focal plane can heat up during these heating pulses which occur once an orbit. However, perigee heating occurs very quickly and the focal plane temperature will come back under control within 8 hours or by the time an altitude of 60,000 kilometers has been obtained. In order to maintain control of the focal plane 0.43 Watts of heater power is required on the focal plane and 1.0 Watt on the detector housing. Figure 2.9 shows the hot case analysis results with full sun on the sun shade. Cold case conditions simply results in a higher duty cycle of the detector housing and focal plane heaters.

On-Orbit Eclipse

During eclipses the ACIS will be turned off and all heater control will be lost. For extended eclipses the focal plane and detector housing can be expected to reach their survival temperature limits. Survival heaters supplied by the telescope will maintain the detector housing above -76 C which will keep the focal plane above -136 C. There are also survival heaters on the PSMC and Support Structure (to maintain survival limits for the DEA and DPA).

On-Orbit Bake-out

The last mode of operation which can be expected on-orbit is the contamination bake-out mode. Heaters mounted on the focal plane and on the detector housing are used to heat the focal plane to +30 C and the camera body to +25 C. The required steady state power on the camera body is 37 Watts and 16 Watts on the focal plane.