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Dome of the 26-inch "Great Equatorial" telescope at dusk, U.S. Naval Observatory, Washington, DC
The dome of the 26-inch "Great Equatorial" refractor at dusk
The largest telescope located on the Observatory grounds in Washington, D.C. is the historic 26-inch (66-cm) "Great Equatorial" refractor, originally built by the firm of Alvan Clark & Sons of Camridge, Massachusetts. Acquired in 1873 at a cost of just over $46,000, it was the world's largest refracting telescope until 1883, when it was surpassed by another Clark instrument, the 30-inch made for the Imperial Russian Observatory at Pulkovo near St. Petersburg. Used by Professor Asaph Hall in 1877 to discover Phobos and Deimos, the two moons of Mars, this telescope is now used for determining the orbital motions and masses of double stars using a special camera known as a speckle interferometer.  On November 12, 2023, the Great Equatorial celebrated the sesquicentennial of its "first light", making it the oldest continuously operated research telescope in the world.
 
The Observatory's largest optical telescope is located at the Flagstaff Station (NOFS) in Arizona. It is the 1.55-meter Kaj Aa. Strand astrometric reflector, used to obtain parallax distances of faint objects and to measure the brightness and colors of stars with Charge-Coupled Device (CCD) technology. In 1978, photographic plates taken with this telescope led to the discovery of Charon, the largest moon circling the dwarf-planet Pluto. A 1.3-meter reflector, optimized for use with a wide-field infra-red mosaic CCD camera, was commissioned in 1996. The 1-meter Ritchey-Chretien reflector, originally installed at the Washington site in 1934, was the first telescope to be re-located to Flagstaff in 1955, where it continues to make photometric and spectroscopic observations.   The smaller, but very prolific Ron Stone 0.2-meter automated transit telescope is also maintained at NOFS, providing rapid astrometric data on solar system objects.
 
USNO is currently working on the construction of a 1.8-meter reflector at NOFS, which will make complimentary observations with an identical telescope at Siding Spring Observatory in Australia.  It is expected to begin operation in 2022.  We also operate the 1-meter USNO Deep South Telescope at the Cerro Tololo Inter-American Observatory (CTIO) in Chile.
 
Other telescopes that have been used at the Observatory include transit circles, once used for astrometry and time-scale determination, Photographic Zenith Tubes, which recorded polar motion and nutation in Earth's rotational axis, and a 12-inch Alvan Clark/George Saegmüller refractor.
 
USNO maintains a radio telescope at Kokee Park, Kauai, Hawai'i that is used in conjunction with other radio telescopes at various locations around the world to determine astronomical time and the orientation of the Earth in space.

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