(HST), the first large optical orbiting observatory . Built from 1978 to 1990 at a cost of $1.5 billion, the HST (named for astronomer E. P. Hubble ) was expected to provide the clearest view yet obtained of the universe from a position some 350 mi (560 km) above the earth. Using a Ritchey-Chrétien design that affords wider and flatter fields of view than traditional Cassegrain systems, the telescope has a 7.9-ft (2.4-m) primary mirror that can observe 24 hours a day (but usually observes less than 20% of the time) in a sky that is always clear and always has perfect seeing . Among the instruments are two high-resolution cameras and two spectrographs. The HST was launched from shuttlein 1990. Initial tests taken after its launch showed that the primary mirror was astigmatic, and it was discovered that the mirror had been mistakenly ground to the wrong figure. The telescope was repaired by space shuttle astronauts in Dec., 1993; they replaced critical instruments and added corrective optics while in orbit. Subsequent servicing missions in 1997 and 1999 added capabilities to HST, which observes the universe at ultraviolet, near-ultraviolet, visible, and near-infrared wavelengths. In 2002 astronauts made repairs and improvements designed to enable the observatory to function for another decade, but in 2004 the power supply for the ultraviolet spectrograph failed. A final shuttle servicing mission in 2009 made additional repairs, replacements, and enhancements, including replacing the gyroscopes and the batteries and installing a new wide-field camera and a new ultraviolet spectrograph.

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