The largest telescope located on the Observatory grounds in Washington, D.C. is the historic 26-inch (66-cm) refractor. Acquired in 1873, 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 Pulkowa. Used in 1877 to discover the two moons of Mars, this telescope is now used chiefly for determining the orbital motions and masses of double stars using a special camera known as a speckle interferometer, and for planetary satellite observations.
The Observatory's largest optical telescope is located at the Flagstaff Station (NOFS) in Arizona. It is the 1.55-meter astrometric reflector, used to obtain distances of faint objects and to measure the brightness and colors of stars both photographically and with Charge-Coupled Device (CCD) technology. In 1978, photographic plates taken with this telescope led to the discovery of a moon circling the planet Pluto. A 1-meter reflector, also located in Arizona, makes photometric and spectroscopic observations. Together with the 26-inch telescope's program of double star work, the observations made by the 1.55-meter and the 1-meter provide data not only satisfying the needs of the Navy, but also essential for all of astrophysics and cosmology. The smaller, but very prolific Ron Stone 0.2-meter automated transit telescope is also maintained at NOFS.
USNO's newest telescope is a 1.3-meter instrument optimized for use with a wide-field mosaic CCD camera. It is also located at the Flagstaff Station.
Other telescopes that have been used by the Observatory include transit circles, once used exclusively for astrometry, Photographic Zenith Tubes, and a 12-inch refractor.
Radio telescopes are used at various locations to determine astronomical time and the orientation of the Earth in space.
Our newest instrument, under development at the Lowell Observatory's Anderson Mesa Station near Flagstaff, is the Navy Prototype Optical Interferometer (NPOI), which will provide unprecedented ground-based astrometry data and images.