Arecibo Observatory, radio-astronomy facility located near Arecibo, Puerto Rico, that includes the world's largest single-dish radio telescope, part of the National Astronomy and Ionosphere Center. Conceived by, designed by, and built under the supervision of William E. Gordon, it was completed in 1963 and is now operated by SRI International and its partners under contract with the U.S. National Science Foundation. The principal instrument, the William E. Gordon Telescope, is a fixed antenna of spherical section, 1,000 ft (305 m) in diameter, that is built into a natural limestone bowl. Although the antenna is too large and heavy to be moved, it can be pointed as much as 20° from the zenith by moving the line feeds to the antenna's focus. As a result of the resurfacing of the antenna, which was completed in 1974, observations are possible up to a frequency of 4,000 MHz. A 100-ft (30-m) satellite antenna can be used in conjunction with the large antenna for interferometer observations. In addition there is a wide range of instrumentation for measuring ionospheric conditions. Principal research programs include studies of radio emissions from many types of objects, especially the cores of supernovas called pulsars; radar studies of comets and asteroids; and ionospheric studies.