eROSITA system functionality and operation

eROSITA (extended ROentgen Survey with an Imaging Telescope Array) is the primary scientific payload on board the SRG (Spektrum Rontgen Gamma) mission, scheduled for launch in March 2019. Its destination is the Lagrangian point L2, and its mission is to make a full survey of the X-ray sky. eROSITA is a complex instrument composed of seven identical co-aligned X-ray telescopes with a focal length of 1600mm and an aperture of 7x350mm. Each telescope is equipped with an independent CCD camera. The cold redundant ITC manages all seven cameras as well as the thermal control of the telescope and the interface to the spacecraft. The complexity of this system resides in the fact that the hardware and software interfaces to the spacecraft are minimal. In this respect the ITC has a key role in the management of the instrument, as well as in the science data management. This proved to be an optimal solution in terms of instrument flexibility, but also presented some challenges. The paper presents an overview on the eROSITA system functionality, its interfaces and operational modes together with an outline of the main challenges, which were faced during integration and verification of the complete system. The function of the instrument was verified in the frame of an extensive end-to-end test in vacuum under representative thermal control conditions to verify that the instrument functions under worst case conditions. Currently tests are performed in combination with the spacecraft to verify communication between eROSITA and the spacecraft and the data transfer through the Russian radio complex (TM/TC system). This includes all operational and survival modes. The paper summarizes the main results of the functional tests on instrument level and on spacecraft level.