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The Hubble Space Telescope High Speed Photometer (HSP) thermal control system uses a software control system instead of mechanical thermostats to control heaters. The most unreliable part of a conventional thermal control system, the thermostat, is eliminated in this design. In addition, the software control design provides great operational flexibility impossible to obtain with thermostats.The heaters can be controlled by a "software thermostat" with its in-flight adjustable set points. The control system can also be operated in a variety of other modes, namely, a constant power mode, a power profile mode, and a direct commanding mode. The system can provide a given amount of energy into the heaters over a wide range of input bus voltages because bus voltage is sensed by the control software. Heater power is switched by the same relays that are needed in a thermostat system for power control.The system has proven to be adaptable t.o the changing needs of the Hubble Space Telescope mission. Several modes of operation have been used in flight. The results and experience to date are described along with additional ,and so far, unexercised capabilities of the system.A similar system was designed and built into the Diffuse X-Ray Spectrometer instrument that was launched in January 1993. Experience from that mission is also described.