The calibration of eROSITA on SRG

The power of eROSITA (extended ROentgen Survey with an Imaging Telescope Array), the core instrument on the Russian–German Spectrum–Roentgen–Gamma (SRG) mission, is high sensitivity at high spectral, spatial, and temporal resolution over a large field of view. This is achieved by combining 7 coaligned X–ray telescope modules, each composed of 54 paraboloid / hyperboloid mirror shells in a Wolter–I geometry, to focus the X–rays through optical blocking filters onto PNCCDs with a total of 1 million pixels, providing a spectral resolution of 60 – 160 eV FWHM over an energy range 0.2 – 8 keV and a 1 degree field of view at a time resolution of 50 ms. In order to make full scientific use of the unique capabilities of eROSITA, all the individual components have to be calibrated. The initial ground calibration was done at the PANTER and PUMA facilities of MPE as well as at BESSY, and was supplemented by an in–orbit calibration. We report on the extensive and often challenging calibration activities performed on ground and in space.