eROSITA on SRG. A X-ray all-sky survey mission

eROSITA (extended ROentgen Survey with an Imaging Telescope Array) is the core instrument on the Russian Spektrum-Roentgen-Gamma (SRG) mission which is scheduled for launch in late 2012. eROSITA is fully approved and funded by the German Space Agency DLR and the Max-Planck-Society. The design driving science is the detection of 50 - 100 thousands Clusters of Galaxies up to redshift z 1.3 in order to study the large scale structure in the Universe and test cosmological models, especially Dark Energy. This will be accomplished by an all-sky survey lasting for four years plus a phase of pointed observations. At the time of writing the instrument development is currently in phase C/D.