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Thermal System Design

It is planned to launch the AXAF-I into a highly elliptical 10,000 by 100,000 kilometer orbit from the space shuttle in late 1999. With this high orbit, the CCDs will have a higher radiation exposure than low earth orbit. Therefore, it is desirable to improve the radiation tolerance of the CCDs by increased radiation shielding and lowering the CCD operating temperatures which is made possible by this higher orbit. The current design goal is to operate the CCDs at -120 degrees C. This is lower than any other currently operating spaceborne CCD camera and presents a significant design challenge. The cooling approach calls for a two stage passive radiator which radiates to deep space.

The Thermal Control System for the ACIS consists of two passive radiators, thermal isolators and the thermal control surfaces. Cut away views of the radiators can be seen in Figure 4. The two stage approach cools the camera housing to -70 degrees C with the first stage (warm) radiator and the focal plane to -120 degrees C with the second stage (cold) radiator. The warm radiator is used to reject the heat leaks from the SIM (0 degrees C) and the telescope (10 degrees C) to deep space. The 2.5 sq-foot radiator uses a high emittance Martin Black finish to reject 13 watts to space at a temperature of -86 degrees C.

The cold radiator rejects the conducted and radiated heat from the camera housing and the thermal dissipation of the CCDs. The total area of this radiator is about 2.0 sq-foot and also has a Martin Black finish. This radiator rejects 3 Watts to space at a temperature of -140 degrees C. Heaters will be used on the focal plane to regulate its temperature at -120 degrees C.

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