The X-ray telescope eROSITA is the core instrument besides the Russian ART-XC on the Russian Spektrum-Roentgen- Gamma satellite which will be launched in 2012 to an orbit around the L2 point of the Earth-Sun-system. During both survey and pointing phase the solar panels and the antenna constrain the possible mission scenario. The scan axis is supposed to point constantly towards the earth in the survey phase. In combination with the orbit, the points of largest exposure - the scan poles - then would be areas of a few hundred deg² instead of small singularities. The background as a permanent interference factor is limiting the performance as well as transient disruptions like solar flares. Constraints on the instrument's side are amongst others vignetting, effectivity and aligning of the different components. The mission objectives and related performance imply very stringent requirements. Extremely challenging mechanical requirements in terms of mirror accuracy, alignment and dimensional stability have to be ensured by design and realized during manufacturing and integration. Although the Wolter telescope design is quite similar to those of XMM, the manufacturing of the mirrors is even more challenging due to the more unfavorable geometry of the mirrors. Mirrors, CCD-cameras and camera electronics all have their own, partly narrow working temperature ranges. Therefore accurate thermal control has to be implemented to ensure that the telescopes are performing within specification. Objectives of this work are to find the optimum mission scenario as well as certain operating parameters, taking into account all environmental boundary conditions.