The cooling concept of the X-ray telescope eROSITA

eROSITA (extended ROentgen Survey with an Imaging Telescope Array) is the core instrument of the Russian SRG satellite which will be launched in 2011 into an orbit of 600 kin height and 30 degrees inclination. It is being developed by the Max-Planck-Institute fur extraterrestrische Physik (MPE) in Garching, Germany. It comprises seven nested Wolter-I grazing incidence telescopes, each equipped with its own CCD camera. The seven eROSITA CCD cameras require a stable operating temperature of about (-80 +/- 0,5)degrees C. Therefore the thermal control system is vitally important. The cooling system consists of passive thermal control components only: two radiators, variable conductance heat pipes (VCHP) and two special thermal storage units. By reason of the low-earth-orbit and the special scan geometry it is impossible for one radiator to look into the cold space at all times. The cameras and the radiators are connected by variable conductance heat pipes which can be cut off when a radiator gets too warm. A novel "latent cold storage unit" guarantees an absolute constant temperature without any further control mechanism.