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.

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.