Aperture Synthesis: Making Radio Telescopes “See”?

Developed by Sir Martin Ryle back in 1952, can aperture synthesis be used to make radio telescopes finally “see” celestial objects when before it can only “hear” them?


By: Ringo Bones


When the Rayleigh Criterion is taken into account, radio telescopes seem to “hear” rather than actually “see” the celestial objects of interest they are aimed at due to the much longer wavelengths of the radio spectrum in comparison to the optical / visible light part of the electromagnetic spectrum. This is the reason why RADAR or radio wave-based imagery of the universe has always been inferior in resolution terms in comparison to optical astronomy. The most obvious solution is to make ever-larger radio telescope dish antennas so that their resolution capabilities would equal that of optical telescopes, but that presents its own problems. Even the 1,000-foot diameter RADAR dish in Arecibo, Puerto Rico - in imaging terms - still cannot match the detail resolution of the human eye, and making ever bigger dishes presents its own problems.

Since radio astronomy began, astronomers had been constructing ever-larger dish antennas in an attempt to detect radio emissions from objects millions of light years away from Earth where the optical portion of the electromagnetic spectrum – i.e. visible light – cannot be picked up by optical telescopes. Radio astronomers knew, however, that to “see” into more distant objects would require the construction of antennas several miles wide. And it had taken awhile for radio astronomy to rival that of its optical sibling in resolution terms.

Ever since Albert A. Michelson used his interferometer in optical astronomy to bypass the inherent Rayleigh Criterion limitations of reflecting telescope with finite-sized mirrors, radio astronomers have adopted this technique in radio astronomy. The simplest kind of radio interferometer is based on the Michelson interferometer – which consists of two small reflectors. Radio waves arriving at an angle to the baseline of the reflectors reach each other slightly before the other, producing a multi-lobed antenna pattern similar to the fringe pattern in optical interferometers. The farther apart the reflectors, the narrower the width of the lobes – or beams – of the antenna’s radiation pattern, thus greatly increasing the radio telescope’s resolution.

A very example of this type of radio telescope is the California Institute of Technology twin-element interferometer, which consists of two 90-foot steel-mesh parabolic antennas. The antennas are mounted on special vehicles that move along special railroad tracks that can be separated by up to 1,600 feet in either east-west or north-south direction. With a beam-width or acceptance angle as narrow as 0.03 degrees, the telescope is used to record the 960-MHz radio noise in our Milky Way galaxy.

Another type of radio interferometer consists two linear arrays at right angles to each other that electronically compares the fan-shaped beams of the two arrays that result in a single pencil beam being produced that can accurately pinpoint radio sources. Developed in the early 1950s by the Australian radio astronomers W. N. Christiansen and B. Y. Mills. Each array may consist of many dipole antennas (Mills cross), parabolic antennas (Christiansen cross), or a parabolic cylinder.

The Mills cross near Sydney Australia was completed in 1952 and has legs 1,500 feet in length. Later upgraded to a version in 1957 with 3,500-foot legs, and a 1.6-kilometer version. The cross antenna at Stanford University consists of 16 steerable 10-foot parabolic reflectors in a row 375 feet long bisected by a similar row at right angles to it. This antenna is roughly equivalent to a single paraboloidal antenna 375 feet in diameter. Other cross radio telescopes include the Soviet Union’s 0.62-mile (1-kilometer) cross consisting of two parabolic cylinders, while a similar instrument is also found in the University of Bologna in Italy.

Radio interferometer resolution further improved when British astronomer Sir Martin Ryle developed a more effective method called aperture synthesis. He discovered that if he periodically varied the distances between a number of small radio telescopes, they would yield the resolving power of a single mammoth-sized radio telescope. Aperture synthesis was developed over two decades starting in 1952. This technique revolutionized radio astronomy by allowing radio astronomers to achieve an accuracy and resolution rivaling that of optical science. Together with Antony Hewish, Ryle won the Nobel Prize in physics in 1974. The first ever given to astronomers since it began in 1901. In fact, the long time-exposure of Ryle’s telescopes allow “viewing” with a definition so sharp that in the words of the Nobel selection committee: “it corresponds to an observer on Earth being able to see details of a postage stamp on the Moon.”

Years later, aperture synthesis even influenced the development of the US Navy’s Synthetic Aperture RADAR technology that allows their ship and plane based radar systems to achieve the same capabilities to that of the Distant Early Warning (DEW) Line base in Thule, Greenland. Many pundits claim that Synthetic Aperture RADAR is the main technology that made Operation Desert Storm a success in 1991.