A German-American-British X-ray and ultraviolet astronomy satellite, named for Wilhelm Röntgen (1845–1923) who discovered X-rays; it operated for almost nine years. The first six months of the mission were dedicated to an all-sky survey in X-rays (0.1–2.5 keV) and ultraviolet (62–206 eV) using an imaging telescope with a sensitivity about 1,000 times greater than that achievable with the instruments on Uhuru. During the subsequent pointed phase of its mission, ROSAT made deep observations of a wide variety of objects. Its operation ended in Feb. 12, 1999.The Roentgensatellit (ROSAT) was a joint German, US and British X-ray astrophysics project. ROSAT carried a German-built imaging X-ray Telescope (XRT) with three focal plane instruments: two German Position Sensitive Proportional Counters (PSPC) and the US-supplied High Resolution Imager (HRI). The X-ray mirror assembly was a grazing incidence four-fold nested Wolter I telescope with an 84-cm diameter aperture and 240-cm focal length. The angular resolution was <5 arcsec at half energy width. The XRT assembly was sensitive to X-rays between 0.1 to 2 keV.
In addition, a British-supplied extreme ultraviolet (XUV) telescope, the Wide Field Camera (WFC), was coaligned with the XRT and covers the wave band between and 6 angstroms (0.042 to 0.21 keV).
ROSAT's unique strengths were high spatial resolution, low-background, soft X-ray imaging for the study of the structure of low surface brightness features, and for low-resolution spectroscopy.
The ROSAT spacecraft was a three-axis stablized satellite which can be used for pointed observations, for slewing between targets, and for performing scanning observations on great circles perpendicular to the plane of the ecliptic. ROSAT was capable of fast slews (180 deg. in ~15 min.) which makes it possible to observe two targets on opposite hemispheres during each orbit. The pointing accuracy was 1 arcminute with stability <5 arcsec per sec and jitter radius of ~10 arcsec. Two CCD star sensors were used for optical position sensing of guide stars and attitude determination of the spacecraft. The post facto attitude determination accuracy was 6 arcsec.
The ROSAT mission was divided into two phases:
(1) After a two-month on-orbit calibration and verification period, an all-sky survey was performed for six months using the PSPC in the focus of XRT, and in two XUV bands using the WFC. The survey was carried out in the scan mode.
(2) The second phase consists of the remainder of the mission and was devoted to pointed observations of selected astrophysical sources. In ROSAT's pointed phase, observing time was allocated to Guest Investigators from all three participating countries through peer review of submitted proposals. ROSAT had a design life of 18 months, but was expected to operate beyond its nominal lifetime.
Re-entry into the Earth's atmosphere
In 1990, the satellite was put in an orbit at an altitude of 580 km and inclination of 53° . Since that time, due to atmospheric drag, the satellite has slowly lost height. As of September 2011, the satellite continues to orbit approximately 270 km above the Earth, and is expected to re-enter the Earth's atmosphere in October 2011.
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