June 19, 1997

Some facts (and a little history) about Arecibo

· Beginnings: Arecibo Observatory was built in 1963 by the U.S. Air Force under the initiative of Professor William Gordon in the Department of Electrical Engineering and his colleagues at Cornell. It was primarily intended for radar studies of the Earth's ionosphere, but it was realized that the telescope would be a very significant new instrument for the then relatively new fields of radio and radar astronomy. In addition to its astronomical observations, it is still used for atmospheric and ionospheric studies. It has been managed by Cornell since its construction, first for the Air Force and, after 1970, for the National Science Foundation.

· First upgrade: The original 1,000-foot-diameter fixed spherical reflector had a wire mesh surface that limited its operation to radio frequencies below 600 MHz (50 cm wavelength). Shortly after the observatory was made one of the NSF's National Astronomy and Ionosphere Centers in the early 1970s, this surface was replaced by 38,788 very accurately shaped aluminum panels, which allowed the telescope to operate at much higher frequencies, with the highest about 3 GHz (10 cm wavelength). At the same time, a high-powered transmitter (420 kilowatts) was installed for planetary radar studies. This upgrade was financed by the NSF and NASA.

· Second upgrade: Begun in 1992 and completed in 1997, this upgrade replaced the line feeds with a Gregorian reflector system as the main method of focusing radio waves reflected from the 1,000-foot dish. The Gregorian reflector system will allow the telescope to operate over the full frequency range allowed by the accuracy of the 38,788 panels of the primary reflector, up to 10 GHz. Also included in this upgrade are a 50-foot-high, steel wire mesh ground screen around the perimeter of the 1,000-foot dish, which shields the telescope's receiving system from radio noise radiated from the surrounding ground, and a new 1 megawatt transmitter for planetary studies. This upgrade also was funded by the NSF and NASA, with a contribution from Cornell.

· The dish and screen: The reflector dish is 1,000 feet in diameter (305 meters) with a depth of 161 feet (51 meters) , as big as 26 football fields, covering 18 acres. Its surface is made of 38,788 reflective aluminum panels, each 3 feet by 6 feet. The ground screen is 50 feet high surrounding the perimeter of the primary antenna, the reflector dish. This screen has an area of about 16,000 square meters, the size of five football fields. The screen reduces radio noise emitted by the ground that gets into the receiver systems.

· The new mirrors: The Gregorian reflectors are suspended 450 feet (137 meters) above the primary reflector dish. The suspended mirrors, and associated sensitive receiver systems and new radar transmitter, are housed in a six-story, 90-ton, 86-foot diameter enclosure. One reflector is 72 feet in diameter, the other, 26 feet. The whole structure is attached to trolleys that move along the 304-foot-long curved feed arm suspended above the dish.

· Frequencies and range: The system operates at frequencies between 50 MHz and 10,000 MHz (10 GHz), with wavelengths between 6 meters and 3 centimeters. Its range is as close as 4 miles (6 kilometers) above the Earth to several billion light years away, at the edge of the known universe. It is sensitive enough to eavesdrop on a cellular phone conversation at the distance of Venus.

· Radar: The 1997 upgrade includes a doubling of the power of the transmitter, to 1 million watts from about 420,000 watts, used for radar studies of the solar system. The new transmitter combined with the telescope forms the world's most powerful radar system. This can result in images of remarkable resolution: about one-half mile (1 km) for the surface of Venus, down to 50 feet (15 meters) for asteroids and comets. That is sensitive enough to detect a steel golf ball at the distance of the moon.