astronomical observatory in Hawaii, U.S., that has become one of the most important in the world owing to its outstanding observational conditions. The Mauna Kea Observatory is operated by the University of Hawaii and lie at an elevation of 4,205 metres (13,796 feet) atop the peak of Mauna Kea, a dormant volcano on north-central Hawaii island.

The observatory was founded in 1964 at the urging of the influential American astronomer Gerard Kuiper, and a 2.2-metre (88-inch) reflector used for planetary studies went into service there in 1970. Mauna Kea subsequently became the site of the world's most important collection of telescopes designed for observations in the infrared range. Three large reflectors, the 3.8-metre (150-inch) United Kingdom Infrared Telescope, the 3.6-metre (142-inch) Canada-France-Hawaii Telescope, and the 3-metre (118-inch) NASA Infrared Telescope Facility, went into service there in 1979. In addition, a 15-metre British-Dutch submillimetre- and millimetre-wavelength telescope, the James Clerk Maxwell Telescope, was completed in the late 1980s, and a similar 10.4-metre millimetre wave telescope, the Caltech Submillimeter Observatory, owned by the California Institute of Technology (Caltech), was completed early in the '90s. Another radio astronomy facility, the Submillimeter Array, a group of eight 6-metre (20-foot) diameter antennas owned by the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics of Taiwan, was added in 2003. The Keck telescope, a 10-metre multimirror telescope operated jointly by Caltech and the University of California, was completed at Mauna Kea in 1992; it is the largest reflector in the world and is used for both optical and infrared observations. Another Keck telescope went into operation on Mauna Kea in 1996. Two other large optical telescopes, the Japanese 8.2-metre (27-foot) Subaru and the multinational 8-metre (26-foot) Gemini North, began observations in 1999.

Mauna Kea is the site of many major telescopes because its viewing conditions are the finest of any Earth-based observatory. The site lies at an elevation almost twice that of any other major observatory and above 40 percent of the Earth's atmosphere; there is thus less intervening atmosphere to obscure the light from distant stellar objects. A high proportion of nights at Mauna Kea are clear, calm, and cloudless owing to local weather peculiarities and the fact that the mountaintop lies above cloud cover most of the time. The high elevation and extremely dry, clear air make the site ideal for observing astronomical objects that emit radiation at far-infrared wavelengths, which are easily blocked by atmospheric water vapour.