The high-energy Universe is capricious, doing the most interesting things at the most unexpected times. Because of this, astronomers would like to look everywhere all the time, if possible. All sky observatories like the free-flying Fermi Gamma Ray Space Telescope and the Monitor of All-Sky X-ray Image (MAXI) on the International Space Station build up pictures of stunning occurrences and massive structures all over the sky. In the 1990’s, the ROSAT X-ray observatory scanned the sky for six months from pole to pole, providing an all-sky map of the low-energy (soft) X-ray band that remains unriveled to this day. But this is about to change. A upcoming X-ray observatory, Spectrum-Roentgen-Gamma (shown in the illustration above), is expected to be launched this June. Spectrum-Roentgen-Gamma is a Russian-German X-ray observatory, with contributions from NASA. The primary instrument on SRG is the «extended ROentgen Survey with an Imaging Telescope Array«, better known as eROSITA. eROSITA is the upper instrument on SRG in the illustration, just above SRG’s ART-XC instrument. One goal of eROSITA is to perform an unprecedented, 4-year long survey of the entire X-ray sky, with about 20 times the sensitivity of the ROSAT All-Sky Survey over a 5-times wider X-ray energy band. In addition to extending the ROSAT survey, the e-ROSITA survey (and companion observations by the ART-XC instrument) will provide, for the first time ever, images of the entire X-ray Universe in the hard energy band above 3 kilo-electron volts. One of the primary goals of SRG is to detect tens of thousands of distant galaxy clusters, to determine the large scale structure in the Universe and test our understanding of the mysterious dark energy which drives the Universe apart.