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The British-German gravitational wave detector GEO 600 has concluded a long observational period called Astrowatch, which lasted from November 2007 to July 2009. Together with the LIGO-H2 detector, GEO 600 was kept observing, while other detectors of the world-wide network of laser-interferometers like LIGO(L1 and H1) and Virgo were upgraded. A fraction of the time during the astrowatch period was set apart for noise investigations and experiments preparing future upgrades. Even with these investigations GEO 600 reached an observation time of 86.0% of the overall time, such that a total of 522 days worth of data were collected. The average sensitivity was roughly a factor of 2 lower than that of the LIGO-H2 detector for frequencies above 500 Hz. In July 2009 GEO 600 has started an upgrade program called GEO-HF. Within this program we aim at improving the sensitivity by a number of sequential upgrades, like tuned signal recycling, DC readout, output mode-cleaning, injection of squeezed vacuum states and the increase of circulating light power. Tuned signal recycling and DC readout have already been implemented and can be operated robustly, due to a new technique associated with the automatic alignment system.
H. Grote (Tue,) studied this question.