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.

Silent Skies for Radio Astronomers

With the ever-increasing expansion of radio-frequency mobile telecommunication chatter via cellular / mobile phones and other wireless devices, will our skies be “silent” enough for radio astronomy?


By: Ringo Bones


The International Astronomical Union and the International Dark Skies Association had made significant progress recently in stamping out the scourge of urban light pollution during the celebration of the UN sponsored 2009 International Year of Astronomy. Especially when the Galloway Forest Park in Scotland was established as a protected dark sky area for stargazers and amateur astronomers. Unfortunately, nothing has been done for radio astronomers when it comes to the “trivial” problem of the increasing radio-frequency traffic that denied a chatter-free silent sky condition for astronomers who explore the cosmos in the radio portion of the electromagnetic spectrum. Especially in radio frequencies of interest used in exploring our universe like galactic structure and evolution to signs of “extraterrestrial technology”.

As far back as the 1970s, astronomer Carl Sagan raised concerns over the US Department of Defense’s heavily encrypted DARPA Net “Hotline” operating so close a frequency to the hydroxyl radical radio frequency. In 1995, the Strasbourg-based European Science Foundation issued a warning that the rapid expansion of the mobile communications / mobile phone / cellular phone industry’s excessive “radio-frequency pollution” – a.k.a. RF pollution - was a serious threat to radio astronomers worldwide. Back then, Dr. James Cohen of Britain’s Jodrell Bank observatory decried the ongoing deployment of numerous low-Earth-orbit telecommunications satellites used to serve the mobile / cellular phone industry.

Around that time, Dr. James Cohen stated that even if the sideband emissions from satellite / mobile / cellular telephones were small, they would still devastate radio telescopes equipped with large dishes which are so sensitive they can detect extremely weak and distant RF signals on the sub-nanovolt level. Like urban light pollution plaguing astronomers who work in the optical portion of the electromagnetic spectrum, Dr. Cohen likened the problem to a professional photographer having a light shining into his or her lens every time he or she tried to snap a picture. Dr. Peter Napier of the US National Radio Astronomy Observatory concurred with Dr. Cohen, saying that the problems of excessive RF pollution in the radio spectrum of our skies were severe and getting worse as the years go by. Dr. Napier characterized common telecommunications engineering practices as “inadequate” to prevent severe disruption to radio astronomy.

Due to a lack of a legally binding international treaty designed to protect the world’s radio astronomers against excessive RF frequencies reaching into their astronomical instruments or radio frequency pollution. The International Telecommunications Union (ITU) had assigned a frequency of 1410MHz – previously the sole domain of the US DoD during the height of the Cold War to send heavily encrypted data streams – available for civilian use for satellite / mobile / cellular phone systems. Unfortunately, this radio frequency band is dangerously close to the 1412MHz signature of the hydroxyl radical – a hydrogen / oxygen molecular fragment widely distributed in space and is used by radio astronomers to map our universe.

In the time since the European Science Foundation issued its warning against excessive radio frequency traffic ruining radio astronomy, “celestial traffic” via telecommunications satellites around the Earth had increased tremendously. These now support an ever-growing market of dedicated ISDN modem / broadband modem lines / wi-fi / and mobile / cellular phones – not to mention an “experimental” system intended to prevent auto collisions. Not only do these satellites contribute to the RF frequency pollution that spoils radio astronomy - their highly reflective Teflon-coated antennae can also be a source of light pollution to astronomers working in the optical spectrum. These satellites – like the various Iridium satellites and the 24-satellite Global Positioning System in geosynchronous orbits - are probably the only “stars” visible in urban areas plagued by sodium-vapor street-lamp light pollution.

The radio frequency pollution problem shows no sign of abating. Mike Cousins, who runs the Stanford Research Institute’s radio-telescope research program, told the San Francisco Examiner that the problem is “constant, sometimes severe”. There is now a cellular phone tower just over the hill from Stanford’s 150-foot dish, Cousins told the San Francisco Examiner’s science writer Keay Davidson. “There is nothing we can do about it” Cousins says. The Stanford dish is sensitive enough to pick up radar reflections from ships in the Western Pacific and Citizen’s Band radio transmissions from as far away as Florida. Even Seth Shostak of the SETI Institute in Mountain View, California, characterized the radio frequency pollution problem in radio astronomy as “science versus heavy-duty commerce”. Scientists at Seth Shostak’s institute search for radio frequency signals from extraterrestrial civilizations.

Will the radio frequency pollution problem that plagues radio astronomers like urban light pollution problems plaguing astronomers using optical telescopes ever be solved? Most radio telescopes, like their optical counterparts, stand in once-remote and once-uninhabited locations that are now surrounded by highly urbanized civilization with their inherently light and RF polluting lifestyle. Even the International Dark Skies Association had failed to solve the increasing light pollution problem around Mount Wilson Observatory in Pasadena, California.

Recently, radio astronomers have developed a few techniques for separating the extraterrestrial radio signals of interest from the more mundane RF pollution noise. One is by computer correlation of signals received by two or more dishes spaced hundreds of miles apart. Using this technique, radio astronomers can filter out the transmissions of cellular phone “yakking yuppies” and concentrate on the spectrum of interest. The better – if not the best technique – for radio astronomers to beat radio frequency pollution is to build a radio astronomy dish as far away from the earthbound RF noise as possible – like on the dark side of the Moon. Sadly, this project won’t be getting any US Congressional funding anytime soon after the Bush-Cheney Consortium ran their “Global War on Terror in a malfeasant manner that left the US Government mired in a 12 trillion dollar debt burden. Some gifted scientist now smoking weed in Amsterdam could have built a faster-than-light capable spacecraft that carries a crew of 150 with that kind of money.