Six subsystems make up the AXAF-I spacecraft. The most basic is the structure and mechanical subsystem, which consists of the structure of the spacecraft and provides for the support and alignment of the X-ray telescope, as well as attachment points for the orbital transfer stage. One of the more obvious elements in this subsystem is the sunshade door (see Fig. 1 of The X-ray Telescope) which opens to permit X-radiation to enter the X-ray telescope and optical light to enter the aspect camera, and which protects the X-ray mirror from direct sunlight.
A thermal control subsystem is also part of the spacecraft. This subsystem consists of radiators, thermistors, switches, heaters, and insulating material to control the temperatures of critical components of AXAF-I. The operation of the active part of this system depends on the provision of adequate electrical power: during eclipses and power emergencies some heaters may be turned off to conserve power for the most critical components, which may be safeguarded by backup thermostats. It is particularly important that the temperatures near the X-ray mirror be well controlled: extensive use is made of thermal coatings, multilayer insulation, and heaters. Information about the temperature at many points is contained in engineering telemetry, passed to the ground as part of the 32 kbps data stream.
The electrical power subsystem is responsible for generating the primary electrical power from the solar arrays, storing it in three banks of batteries, and distributing a regulated power supply to the Observatory. The solar arrays generate more than 2 kW throughout the nominal 5-year lifetime of the mission, and about the same power can be supplied from the three battery banks (which are kept charged by the arrays). If less than three battery banks are functioning then various electrical loads are shed during eclipses (by command of the on-board computer or backup devices) to ensure the survival of the Observatory.
Spacecraft ranging, commanding of observatory systems, data acquisition and storage, data telemetry generation and transmission, command telemetry acquisition and processing, on-board processing of data from spacecraft sensors, and the spacecraft clock are provided by the communication, control, and data management subsystem. This subsystem is central to the operation of the Observatory: for the present, note only that the telemetry streams that are generated are stored on a solid-state recorder on board, and are transmitted to the ground during scheduled contacts with the NASA Deep Space Network about once every 8 hours. These same contacts also up-load the commands needed to operate the Observatory for 72 hours.
Control of the pointing of the Observatory and the collection of data that allow the aspect to be reconstructed from the telemetry are performed by the pointing control and aspect determination subsystem. This system is also responsible for taking the Observatory into safe modes during emergencies. The most obvious component of this subsystem is the aspect camera, whose stray light shield can be seen near the entrance to the X-ray mirrors (Fig. 1 of The X-ray Telescope), and which is used as the primary aspect reference of the Observatory, although on short timescales the pointing is monitored through the outputs from the gyroscopes. Other information on the orientation of the Observatory is provided by fine and coarse sun sensors and an earth sensor. A subset of the six reaction wheels are used to control the angular motion of the Observatory.
The final component of the spacecraft is the orbital transfer stage (`propulsion module' on Fig. 2 of The X-ray Telescope) which raises the orbit after AXAF-I has been deployed from the Shuttle.
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