Precision dimensional structures, for example optical structures such as space telescopes, require accurate temperature control in order to minimize distortion resulting from uneven thermal expansion. Such distortion might cause optical aberrations such as defocusing. Very precise optical structures can require temperature stability beyond the capability of modem heater circuits to control or even to measure.
Previously, such precision optical structures have been constructed from a material having a low rate of thermal expansion such as Invar or graphite so as to minimize the effects of temperature differences over the structure. However, such materials are heavy and expensive. Moisture absorption in graphite outgasses during orbiting in space, presenting the danger of contamination. In addition, significant temperature differences can result in some amount of distortion even with low thermal expansion material.
A proposal in the past to avoid distortion of structures in warm-biased environments, from which heat flows out, was to use a material having high thermal conductivity, such as aluminum, and to control the temperature of the structure by dividing the surface of the structure into a large number of contiguous zones, affixing a heater element onto each zone of the structure, sensing the temperature in each zone of the structure, and controlling a proportional controller to control the respective heater elements based on the sensed temperatures in the corresponding zones. Such a system and method, however, have shortcomings. Placement of the heater elements directly on the zones of the structure limits the influence which each heater element can have on adjacent zones and on the overall structure. This can be of particular concern if a temperature sensor/heater element combination should fail, making it necessary for adjacent heater elements to maintain the heat in the zone of the failed components. In addition, very accurate temperature sensors are required for the proportional heater controllers. Errors might arise due to errors in temperature measurement, errors in the proportional temperature controllers, or errors due to the locations of the temperature sensors in that the temperature in the location on a zone at which the temperature is sensed may not be the same as the average temperature of the zone.