Among the smaller projects mentioned in the introduction, the Spektrum-Roentgen-Gamma (SRG) project is of special relevance. It addresses the nature of the mysterious dark energy, which is one of the most exciting questions facing astronomy and physics today. It may be the vacuum energy providing the cosmological constant in Einstein's theory of General Relativity, or it may be a time-varying energy field. The solution could require a fundamental revolution in physics. The discovery of Dark Energy has come from three complementary techniques: observations of distant supernovae, the microwave background, and clusters of galaxies.
In 2005, a new concept was defined for a revived Spektrum-Roentgen-Gamma (SRG) mission . A medium size satellite will be launched by the Russian space agency Roskosmos in 2011 into a 600 km low-Earth orbit, which provides a significantly lower particle background than those of Chandra and XMM-Newton, and will allow detailed studies of low-surface-brightness diffuse objects. The payload includes eROSITA (extended Roentgen survey with an imaging telescope array) provided by a German consortium led by MPE , the wide field X-ray monitor Lobster provided by a consortium led by Leicester University, UK, and a hard X-ray telescope (ART) provided by IKI, Russia. The eROSITA X-ray telescope consists of seven mirror modules (Wolter-I optics) each having a CCD-detector in the focus. By using frame store pn-CCDs with a field of view of about 1 square degree, newly developed by our dedicated semiconductor laboratory  and based on the technology of the successful pn-CCD detector aboard XMM-Newton, eROSITA achieves a grasp (solid angle area product) which is 3-4 times larger than that of XMM-Newton. Recently the addition of a calorimeter instrument has been discussed.
The mission will conduct the first all-sky survey with an imaging telescope in the medium energy band up to 12keV, with an unprecedented spectral and angular resolution and a factor of 30 and 100 more sensitive than the previous ROSAT and HEAO-1 surveys in their respective energy bands. This survey will detect several Million new X-ray sources, most importantly about one hundred thousand clusters of galaxies which can be used to investigate the nature of dark matter and dark energy, and several hundred thousand obscured supermassive black holes. With Lobster the first all-sky imaging X-ray time variability survey will be performed. In addition to the all-sky surveys it is foreseen to do pointed observations of selected sources. The new SRG mission would thus be a highly significant scientific and technological step beyond Chandra and XMM-Newton and would provide important and timely inputs for the next generation of giant X-ray observatories like XEUS or Con-X.
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