Status of the CCD camera for the eROSITA space telescope

The approved German X-ray telescope eROSITA (extended ROentgen Survey with an Imaging Telescope Array) is the core instrument on the Russian Spektrum-Roentgen-Gamma (SRG) mission. After satellite launch to Lagrangian point L2 in near future, eROSITA will perform a survey of the entire X-ray sky. In the soft band (0.5 keV - 2 keV), it will be about 30 times more sensitive than ROSAT, while in the hard band (2 keV - 8 keV) it will provide the first complete imaging survey of the sky. The design driving science is the detection of 100,000 clusters of galaxies up to redshift z ~ 1.3 in order to study the large scale structure in the Universe and test cosmological models including Dark Energy. Detection of single X-ray photons with information about their energy, arrival angle and time is accomplished by an array of seven identical and independent PNCCD cameras. Each camera is assigned to a dedicated mirror system of Wolter-I type. The key component of the camera is a 5 cm • 3 cm large, back-illuminated, 450 μm thick and fully depleted frame store PNCCD chip. It is a further development of the sensor type which is in operation aboard the XMM-Newton satellite since 1999. Development and production of the CCDs for the eROSITA project were performed in the semiconductor laboratory of the Max-Planck-Institutes for Physics and Extraterrestrial Physics, the MPI Halbleiterlabor. By means of a unique so-called 'cold-chuck probe station', we have characterized the performance of each PNCCD sensor on chip-level. Various tests were carried out for a detailed characterization of the CCD and its custom-made analog readout ASIC. This includes in particular the evaluation of the optimum detector operating conditions in terms of operating sequence, supply voltages and operating temperature in order to achieve optimum performance. In the course of the eROSITA camera development, an engineering model of the eROSITA flight detector was assembled and is used for tests since 2010. Based on these results and on the extensive tests with lab model detectors, the design of the front-end electronics has meanwhile been finalized for the flight cameras. Furthermore, the specifications for the other supply and control electronics were precisely concluded on the basis of the experimental tests.