eROSITA in-orbit calibration strategy and plan: from the ground to the science

eROSITA is a German X-ray astronomical observatory on-board Spectrum-Roentgen-Gamma (SRG) with an expected launch in 2014 and operation at libration point L2. It consists of 7 identical mirror modules with 7 identical CCD cameras as focal plane instrumentation. It will observe the X-ray sky in the 0.25 - 10 keV range (4 yr all-sky survey, then pointed observations). Another instrument aboard SRG, ART-XC, will extend the spectral range to higher energies. We describe how the on-ground calibration is planned for the PANTER and PUMA X-ray test facilities, with respect to maximum scientific exploitation, in combination with an in-orbit calibration. This will be performed within one module and between the 7 eROSITA modules. If XMM-Newton will still be operational, a dedicated cross-calibration campaign is envisaged. The selection of the celestial targets and the procedure of the measurements with the internal Fe-55 calibration source are outlined.