Hubble Space Telescope (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. 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 shuttle Atlantis in 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 astronauts of the space shuttle Endeavour in Dec., 1993, who 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 Mar., 2002, astronauts from the space shuttle Columbia 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.

Most sophisticated optical observatory ever placed into orbit around Earth. Because it is above Earth's obscuring atmosphere, it can obtain images much brighter, clearer, and more detailed than ground-based telescopes can. Named for Edwin Hubble, it was built under NASA supervision and deployed on a 1990 space-shuttle mission. The reflector telescope's mirror optics gather light from celestial objects and direct it to an array of cameras and spectrographs (see spectroscopy). A defect in the primary mirror initially caused it to produce fuzzy images; in 1993 another shuttle mission corrected this and other problems. Subsequent missions to the HST have been for maintenance, repairs, and instrument upgrades.

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