Astronomy

Aperture Synthesis In Radio Astronomy

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In 1946, a researcher, J.C Hey, found a highly localized source, Cygnus. He suggested it could be a star-like object. It soon turned out to be correct. An Australia verified it. He used a series of radio telescopes over a wide area connected to a central point. A radio star was discovered. Many more such stars were soon found. An expert in radar detectors, Martin Ryle at Cambridge (UK), assisted by Francis Graham Smith, used another type of interferometer, called aperture synthesis.

The astronomers were encouraged by being able to resolve Sunspots of small diameter. In 1964 Martin Ryle, ( who later won the Nobel Prize for physics made history by making a one-mile telescope; using aperture synthesis. It uses small telescope dishes to produce the angular resolution of a much larger dish. In this method, one or more small dishes were fixed, while some other dishes moved over the surface, resulting in a large baseline and a better resolution.

An optical telescope can distinguish between objects 2.6 cm apart or about 1/2000th of the diameter of the Moon. A radio telescope has to be 25 km across to have the same resolution. As it is not practical to have such a big telescope, a technique called aperture synthesis is used. It creates a virtual big antenna, by synthesizing the inputs received by several smaller antennas spread around a central instrument. As the Earth rotates, the smaller antennas cover more and more of the surface and the signals from them are synthesized by a “correlator”, which is a digital computer and gives the output that would have been observed by a single big-sized telescope.

Ryle also constructed a 5-km telescope to study individual new-born stars in the Milky Way. His radio telescope detected quasars, quasi-stellar radio sources that are among the most distant and so the youngest objects in the Universe. By 1951, radio sources outside our galaxy were found. Before 1960, many extragalactic radio objects were discovered.

Two Basic Trends

Two basic trends have marked the progress of radio astronomy: Interferometry and detection in shorter wavelengths. Interferometry means signals from multiple antennas are electronically combined to obtain the resolution possible with a single very large array.

Several key technologies have made these trends possible: low-noise radio receivers; highly stable and accurate time references; advanced digital electronics and massive storage systems that can take hundreds of megabits of data per second.

One of the limitations of radio observation is that only a small number of wavelengths could be used as experienced in the early days, when radio telescopes studied radiation from tens of meters to centimeters. Engineers soon acquired the ability to detect the phase and amplitude of the waves at these frequencies in a technique called interferometry. It was used in England and Australia. In England, Martin Ryle introduced aperture synthesis and gave a new dimension to interferometry. In this technique, the aperture of a receiver can be synthetically or electronically expanded, so that the signals can be recombined at the correct phase.

How does one increase the resolution? One can increase the diameter of the dish antenna. But there is a limit. Alternatively more than one antenna is used; signals arriving in phase at two antennas separately placed reinforce each other, while those which are out of phase will cancel. If the two antennas are kept farther apart from each other, the angular resolution (the angle subtended by the smallest “visible” detail) improves. The distance between the antennas is called the baseline and each new baseline adds to the resolution.

Computers have emerged in a big way in processing the signals from numerous antennas. For example, a Very Large Baseline Array can have ten radio telescopes around the Earth that can operate in unison. In May 2008, a world wide radar telescope network with a combined diameter of 11,000 km went live. It links the Arecibo Observatory in Puerto Rico with other radio telescopes in North and South Americas, Europe and Africa.

Russia has completed a gigantic set of three radio telescopes located 2000 to 4000 km away from each other in a sort of a triangle, all controlled by one central computer. The area encompassed by the telescopes is 12 million km.

Signals from all antennas are brought to a central point in what is known as Very Large Array (VLA) and combined in real time. When they cannot be operated in real time, the signals are sampled. The output of each antenna is processed and multiplied with that of every other antenna is processed and multiplied with that of every other antenna. One can have an idea of the complexity recently achieved in this sort of processing by looking at the work of the computers involved: they process as many as 750 billions floating point operations per second.

A remarkable innovation has boosted the power of the telescope. Special electronic chips, made of indium phosphide, have increased the power by three times. It works at the radio wavelength of 3 mm and 12 mm as against 3 cm or more in the beginning. The electronics developed for the detection of millimeter waves from space has revealed new celestial objects, such as radio galaxies and quasars. Radio galaxies are extremely powerful and are among the most remote phenomena that are receding from the Earth at nearly the speed of light. Both radio galaxies and quasars, it is believed, harbour massive black holes that account for their enormous energy. In the last one decade, much work has gone into reception and analysis of signals above 70 GHz (wavelengths shorter than about 4 millimetres).

Since the 1970s, radio astronomy has turned to millimeter and sub millimetre regions, in contrast to the earlier trend of using metre and centimeter wavelengths. The advantage of using shorter wavelengths is that they would get higher resolution of the cosmic phenomena. It would also be possible to detect certain molecules in outer space.

The signals from molecules in space are considerably weakened. The frequencies are no longer the same when the waves left their source but are slowed down when they are received on Earth. In order to catch the signals, innovative technologies have come up: ultra-low noise mixing and amplification techniques. For example, carbon monoxide is detected at 97 GHz. In other words, such signals are just whispers from space. And the source involves more chemistry than physics. Since the millimeter and submillimetre radio frequencies are too high to be processed as they are, they are mixed with the frequency of a local oscillator to obtain a longer frequency, though recent advances in amplification directly amplify the signals from space.

Millimeter waves have revealed over a 140 molecules in interstellar space, contrary to the earlier notion. Atomic hydrogen (a basic building block of the Universe) is detected at 12.1 cm. Hydrogen absorbs as well as emits radio waves. Radio telescopes can detect measure and analyse radio waves that are stretched by the expansion of the Universe. Carbon monoxide, the second most abundant element after molecular hydrogen in the galaxy, emits at 2.6 millimetres. In the interstellar space, atomic gases at 100 to 10000 atoms per cubic centimeter, become molecules.

The discovery of molecules has opened a new fascinating window on the early Universe. Based on astrochemistry, it is found that in the first two or three billion years of the Universe, the cycle of birth and death of massive stars had already run its course.

An interferometer operating at millimeter waves is also envisaged. One proposal is to have 40 dishes in such an array. Another proposal envisages interferometers in orbit in space.

Dishes to Dipoles

Astronomical observations in low frequencies have recently become possible. The most important reason for it is a technological breakthrough in 1991. Computer algorithms were developed to overcome the effects of ionosphere interference to radio waves.

Until recently, radio astronomers used frequencies above 1 GHz, as otherwise the ionosphere—the Earth’s radio mirror—would distort the lower frequencies. It was found that if the length of the radio wave is close to the diameter of the dish or longer, the dish cannot record it at all. But, arrays of wire antennas (multiple dipoles) can be designed to catch any wavelength with a right arrangement and super computers could convert the waves into meaningful images. It has been found that dipoles are more efficient than dishes somewhere in between 200 MHz and 100 MHz.

The gain in efficiency, however, needs software to replace massive signal-gathering dishes. For example, a very Large Array, a Y-shaped assemblage of 27 dish antennas, is in New Mexico. It operates in the low frequency of 74 MHz. In other words, the waves can be detected in any direction or in all directions at once as long as computers are available to process the data.

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Low-frequency Array (LOFAR) came up in Germany and the Netherlands. It has 25000 antennas over 350 km in diameter. It can filter out unwanted radio and TV signals. Other long wavelength arrays are in the United States and Australia.

A wide field array is under construction at Mileura in Western Australia. It will have 500 antenna tiles spread over 1.5 km. unlike other arrays that aim at particular targets the Mileura array will “eavesdrop” on accidental leakage of radio waves from other civilizations around 1000 nearest stars! The array is different from most of the radio SETI projects that were once famous for searching extra terrestrial intelligence, as they operated at frequencies higher than 1 GHz. Mileura is located far from TV-infested areas. The array will operate at the same radio wavelength at which FM and TV usually operate.

A dipole array has an 180º view. It can detect transient phenomena. It can for example, pick up radio counterparts to gamma rays, and tell experts how such radio bursts are distorted.

Theory states that ultraviolet rays from the first stars would blow off hydrogen atoms, resulting in ions that do not radiate at certain wavelengths. If this theory is right, then radio telescopes should find the so-called “silent zones” which do not put any radio waves in say 100-150 MHz frequency bands.

Although radio astronomy was discovered at low frequencies (near 20 MHz corresponding to a wavelength of 15 m), which is well below the current FM band, astronomers soon used higher frequencies to overcome the distortions caused by the ionosphere. The trend is now towards lower frequencies, but with complex computers and software. There are plans to deploy 13,000 antennas and integrate their recordings.

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