The UVCS thermal control system employs passive and active means to maintain proper instrument temperature distributions during all phases of the SOHO mission. Passive control hardware such as thermal radiators, multilayer insulation (MLI) and thermal films and finishes are employed on the instrument exterior to maintain proper instrument external thermal exchange with the neighboring SOHO hardware and environment. The aft thermal radiators exchange the high heat dissipations from the sunlight trap and XDL detector electronics with the exterior environment. The telescope assembly MLI maintains interior heat energy and allows a more uniform temperature distribution while its outer indium-tin-oxide coating maintains blanket conductivity and eliminates a source of static charge buildup. Films and finishes such as silverized teflon and gold paint provide low-absorbance, reflective barriers where instrument hardware such as the door interior and roll ring front surface are directly exposed to solar heat energy.
Active control measures include the use of nonoperational and operational heater banks. Nonoperational heaters, which maintain a capacity of 89.7 Watts and are powered (upon SOHO-initiated command) by an independent 27-volt power bus, maintain instrument hardware within nonoperational survivable temperature limits during the pre-operational phases of the SOHO mission. Instrument operational heaters are duty-cycle modulated to achieve various desired power levels and are used in an open-loop heater control system. These heaters maintain a capacity of 41.2 Watts which is based on minimum instrument operation during beginning-of-life cold-case conditions.
The TSU hardware, with the exception of the front support assembly (FSA), is individually-controlled apart from the SOHO payload module thermal system. The REU and the FSA are collectively-controlled with the interfacing SOHO payload module hardware.