Traditionally, the effect of gravity has been minimized by designing the movable structure to be as stiff as possible in order to reduce the deflections resulting from gravity. A more effective technique, based on the principle of homology, allows the structure to deform under the force of gravity, but the cross section and weight of each member of the movable structure are chosen to cause the gravitational forces to deform the reflecting structure into a new paraboloid with a slightly different focal point. It is then necessary only to move the feed or secondary reflector to maintain optimum performance. Homologous designs have become possible only since the development of computer-aided structural analysis.
Some radio telescopes, particularly those designed for operation at very short wavelengths, are placed in protective radomes that can nearly eliminate the effect of both wind loading and temperature differences throughout the structure. Special materials that exhibit very low absorption and reflection of radio waves have been developed for such structures, but the cost of enclosing a large antenna in a suitable temperature-controlled radome may be almost as much as the cost of the movable antenna itself.
The cost of constructing a very-large-aperture antenna can be greatly reduced by fixing the structure to the ground and moving either the feed or the secondary reflector to steer the beam in the sky. For parabolic reflecting surfaces, the beam can be steered in this way over only a limited range of angle without introducing aberration and a loss of power gain.
Radio telescopes are used to measure broad-bandwidth continuum radiation as well as spectroscopic features due to atomic and molecular lines found in the radio spectrum of astronomical objects. In early radio telescopes, spectroscopic observations were made by tuning a receiver across a sufficiently large frequency range to cover the various frequencies of interest. This procedure, however, was extremely time-consuming and greatly restricted observations. Modern radio telescopes observe simultaneously at a large number of frequencies by dividing the signals up into as many as several thousand separate frequency channels that may range over a total bandwidth of tens to hundreds of megahertz.
The most straightforward type of radio spectrometer employs a large number of filters, each tuned to a separate frequency and followed by a separate detector to produce a multichannel, or multifrequency, receiver. Alternatively, a single broad-bandwidth signal may be converted into digital form and analyzed by the mathematical process of autocorrelation and Fourier transformation (see below). In order to detect faint signals, the receiver output is often averaged over periods of up to several hours to reduce the effect of noise generated in the receiver.
Radio interferometry and aperture synthesis
Angular resolution, or ability of a radio telescope to distinguish fine detail in the sky, depends on the wavelength of observations divided by the size of the instrument. Yet even the largest antennas, when used at their shortest operating wavelength, have an angular resolution of only about one arc minute, which is comparable to that of the unaided human eye at optical wavelengths. Because radio telescopes operate at much longer wavelengths than do optical telescopes, it was thought for many years that the resolution of radio telescopes must be much poorer than that of optical instruments. In actuality, this is not the case, for several reasons.
First, although the theoretical resolution of an optical telescope may be as good as a few hundredths of a second of arc, distortions of the incoming light signal by the Earth's atmosphere, known as seeing, diffuse the image, so that even at a good mountain site under good observing conditions the best angular resolution is only a little better than one arc second. At the much longer radio wavelengths, the distortions introduced by the atmosphere are less important, and so the theoretical angular resolution of a radio telescope can in practice be achieved. Because radio signals are easier than light signals to distribute over large distances without distortion, it is possible to build radio telescopes of essentially unlimited dimensions. In fact, the history of radio astronomy has been one of solving engineering problems to construct radio telescopes of continually increasing angular resolution.
The high angular resolution of radio telescopes is achieved by using the principles of interferometry to synthesize a very large effective aperture from a number of small elements. In a simple two-element radio interferometer, the signals from an unresolved, or "point," source alternately arrive in phase and out of phase as the Earth rotates and causes a change in the difference in path from the radio source to the two elements of the interferometer. This produces interference fringes in a manner similar to that in an optical interferometer. If the radio source has finite angular size, then the difference in path length to the elements of the interferometer varies across the source. The measured interference fringes from each interferometer pair thus depend on the detailed nature of the radio "brightness" distribution in the sky.
During the late 1940s Australian radio astronomers realized that each interferometer measurement is one "Fourier component" of the brightness distribution of the radio source. Further developments during the 1950s by Martin Ryle and his colleagues in Cambridge, Eng., involved the use of movable-element interferometers and the rotation of the Earth to sample a sufficient number of Fourier components with which to synthesize the effect of a large aperture and thereby reconstruct high-resolution images of the radio sky. The laborious computational task of doing Fourier transforms to obtain images from the interferometer data is accomplished with high-speed computers and the fast Fourier transform (FFT), a mathematical technique that entails the application of a group of algorithms specially suited for computing discrete Fourier transforms
In recognition of their contributions to the development of the Fourier synthesis technique, more commonly known as aperture synthesis, Ryle and Antony Hewish were awarded the 1974 Nobel Prize for Physics. During the 1960s the Swedish physicist Jan Hogbom developed a technique called "clean," which can be used to remove spurious responses from a celestial radio image caused by the use of discrete, rather than continuous, spacings in deriving the radio image. Further developments, based on a technique introduced in the early 1950s by the British scientists Roger Jennison and Francis Graham Smith, led to the concept of self-calibration, which is used to remove errors in a radio image due to uncertainties in the response of individual antennas as well as small errors introduced by the propagation of radio signals through the terrestrial atmosphere.
The combination of up to millions of data points to form a single image, together with the lengthy calculations required to clean and self-calibrate, is a formidable computational task. For more complex images, such calculations are made practical only by using large, high-speed computers.
Very long baseline interferometry
In conventional interferometers and arrays, coaxial-cable, waveguide, or even fibre-optic links are used to distribute a common local oscillator reference signal to each antenna and also to return the received signal from an individual antenna to a central laboratory where it is correlated with the signals from other antennas. In cases in which antennas are spaced more than a few tens of kilometres apart, however, it becomes prohibitively expensive to employ real physical links to distribute the signals. Very high frequency (VHF) or ultrahigh frequency (UHF) radio links can be used, but the need for a large number of repeater stations makes this impractical for spacings greater than a few hundred kilometres.
Interferometer systems of essentially unlimited element separation can be formed by using the technique of very long baseline interferometry, or VLBI. In a VLBI system the signals received at each element are recorded by broad-bandwidth videotape recorders located at the element. The recorded tapes are then transported to a common location where they are replayed and the signals combined to form interference fringes. The successful operation of a VLBI system requires that the tape recordings be synchronized within a few millionths of a second and that the local oscillator reference signal be stable to better than one part in a trillion. For the most precise work, hydrogen maser frequency standards are used to give a timing accuracy of only a few billionths of a second and a frequency stability of one part in a quadrillion.
For many VLBI applications, modified consumer-type videocassette recorders (VCRs) provide adequate performance, and the low cost and widespread availability of these devices have allowed as many as 18 radio telescopes throughout the world to be used simultaneously to obtain high-resolution images. The most sensitive observations, however, require the use of special recorders that are able to record up to several hundred megabits of data per second.
Radar techniques
Techniques analogous to those used in military and civilian radar applications are employed with radio telescopes to study the relatively nearby objects in the solar system. By measuring the spectrum and the time of flight of signals reflected from planetary surfaces, it is possible to examine topographical features, deduce rates of rotation, and determine with great accuracy the distance to the planets. Nonetheless, radio signals reflected from the planets are weak, and high-power radar transmitters are needed in order to obtain measurable signal detections. The time it takes for a radar signal to travel to Venus and back, even at the closest approach of the planet to the Earth, is about five minutes. For Saturn, it is more than two hours.
Major applications of radio telescopes
Radio telescopes permit astronomers to study many kinds of extraterrestrial radio sources. These astronomical objects emit radio waves by one of several processes, including (1) thermal radiation from solid bodies such as the planets, (2) thermal, or bremsstrahlung, radiation from hot gas in the interstellar medium, (3) synchrotron radiation from relativistic electrons in weak magnetic fields, (4) line radiation from atomic or molecular transitions that occur in the interstellar medium or in the gaseous envelopes around stars, and (5) pulsed radiation resulting from the rapid rotation of neutron stars surrounded by an intense magnetic field and energetic electrons.
Radio telescopes enabled investigators to discover intense radio emissions from Jupiter and have been used to measure the temperature of all the planets. Astronomers have relied on radar observations to map the large-scale features on the surface of Venus, which is completely obscured from visual scrutiny by the heavy cloud cover that permanently enshrouds the planet. In addition, radar studies have shown that Venus is rotating in the retrograde, or reverse, direction from that of the other planets. Radar measurements also have revealed the rotation of Mercury, which was previously thought to keep the same side toward the Sun. Accurate measurements of the travel time of radar signals reflected from Venus near superior conjunction have indicated that radio waves passing close to the Sun slow down owing to gravity and have thereby provided a new independent test of Albert Einstein's general theory of relativity.
Broadband continuum emission throughout the radio-frequency spectrum is observed from a variety of stars (especially binary, X-ray, and other active stars), from supernova remnants, and from magnetic fields and relativistic electrons in the interstellar medium. The discovery of pulsars (from pulsating radio stars) in 1967 revealed the existence of rapidly rotating neutron stars throughout the Milky Way Galaxy and led to the first observation of the effect of gravitational radiation.
Utilizing radio telescopes equipped with sensitive spectrometers, researchers have discovered more than 50 separate molecules, including familiar chemical compounds like water vapour, formaldehyde, ammonia, methanol, ethanol, and carbon dioxide. The important spectral line of atomic hydrogen at 1421.405 MHz (21 centimetres) is used to determine the motions of hydrogen clouds in the Milky Way Galaxy and other spiral systems. This is done by measuring the change in the wavelength of the observed lines arising from Doppler shift. It has been established from such measurements that the rotational velocities of the hydrogen clouds vary with distance from the galactic centre. The mass of a spiral galaxy can, in turn, be estimated using this velocity data.
Radio telescopes have discovered powerful radio galaxies and quasars beyond the Milky Way system. These cosmic objects have intense clouds of radio emission that extend hundreds of thousands of light-years away from a central energy source located in an active galactic nucleus (see photograph), or quasar. VLBI observations made with worldwide networks of radio telescopes have revealed apparent faster-than-light motion in many quasars. (For more specific information about quasars and other extragalactic radio sources, see Cosmos: Components of the universe: Quasars and related objects and galaxy: Extragalactic radio and X-ray sources: Quasars.)
Measurements made by Arno Penzias and Robert W. Wilson with an experimental communications antenna at Bell Telephone Laboratories detected the existence of cosmic background radiation at a temperature of 3 K. This radiation, which comes from all parts of the sky, is thought to be the remaining radiation from the hot big bang, the primeval explosion from which the universe presumably originated some 15 billion years ago.
Important radio telescopes
The first really large fully steerable radio telescope was completed in 1957 at Jodrell Bank, Eng. This 76-metre instrument is still used for a number of research programs (Table 2). The world's largest fully steerable radio telescope is the 100-metre-diameter antenna operated by the Max Planck Institute for Radio Astronomy at Effelsberg, near Bonn, Ger. (see photograph). It is used in a number of different wavelength bands as short as one centimetre for atomic and molecular spectoscopy and for other galactic and extragalactic studies. Because of its large collecting area and full sky coverage, the Effelsberg radio telescope is frequently used for worldwide VLBI observations.
The Commonwealth Scientific and Industrial Research Organization (CSIRO) in Australia maintains near Parkes, N.S.W., a 64-metre radio telescope that is the largest of its kind in the Southern Hemisphere. The world's largest radome-enclosed radio telescope is the 36-metre Haystack antenna operated by the Northeast Radio Observatory Corporation under agreement with the Massachusetts Institute of Technology (MIT).
A 91-metre fixed-azimuth radio telescope with limited elevation motion was operated by the National Radio Astronomy Observatory in Green Bank, W.Va., U.S., until its unexpected collapse in late 1988. A 43-metre equatorially mounted radio instrument, however, remains in operation in Green Bank; this telescope is used primarily for molecular spectroscopy at wavelengths as short as one centimetre. Green Bank is located in the national Radio Quiet Zone, which offers unique protection for radio telescopes from sources of man-made interference.
The largest single radio telescope in the world is the 305-metre fixed spherical reflector operated by Cornell University near Arecibo, P.R. The 305-metre antenna has an enormous collecting area, but the beam can be moved through only a limited angle of about 20 from the zenith. It is used for planetary radar astronomy as well as for studying pulsars and other galactic and extragalactic phenomena. The Russian RATAN-600 telescope (RATAN stands for Radio Telescope of the Academy of Sciences), located near Zelenchukskaya in the Caucasus Mountains, has 895 reflecting panels, each 7.4 metres high, arranged in a ring 576 metres in diameter. Using long parabolic cylinders or dipole elements, researchers in Australia, France, India, Italy, and Ukraine have also built antennas with very large collecting areas.
Several smaller, more precise radio telescopes for observing at millimetre wavelength have been installed high atop mountains, where clear skies and high altitudes minimize absorption from the terrestrial atmosphere. A 45-metre radio dish near Nobeyama Plateau, Japan, is used for observations at wavelengths as short as a few millimetres.
The French-German Institut de Radio Astronomie Millimetrique (IRAM) in Grenoble, Fr., operates a 30-metre antenna at an altitude of 2,850 metres on Veleta Peak in the Spanish Sierra Nevada for observations at wavelengths as short as one millimetre. Several radio telescopes that operate at submillimetre wavelengths are located on La Silla Hill in Chile at an elevation of 2,350 metres and near the summit of Mauna Kea, Hawaii, U.S., at elevations of 4,050 and 4,092 metres.
The largest of these, the James Clerk Maxwell Telescope, has a diameter of 15 metres. Millimetre interferometers and arrays are operated at the Owens Valley Radio Observatory of the California Institute of Technology, the Hat Creek Observatory Laboratories of the University of California at Berkeley, the IRAM Plateau de Bure facility in France, and the Nobeyama Observatory.
The Very Large Array
The world's most powerful radio telescope is the Very Large Array (VLA) located on the Plains of San Agustin near Socorro, N.M., U.S. The VLA consists of 27 parabolic antennas, each measuring 25 metres in diameter. The total collecting area is equivalent to a single 130-metre antenna. Each element of the VLA can be moved by a transporter along a Y-shaped railroad track; it is possible to change the length of the arms between 600 metres and 21 kilometres to vary the resolution of the system (see photograph). Each antenna is equipped with receivers that operate in six different bands from wavelengths of approximately one centimetre to one metre. When used at the shorter wavelength in the largest antenna configuration, the angular resolution of the VLA is several tenths of one arc second. The VLA is operated by the U.S. National Radio Astronomy Observatory as a national facility and is used by more than 500 astronomers each year for a wide variety of research programs devoted to the study of the solar system, Milky Way Galaxy, and extragalactic systems.
There are a number of other large radio telescope arrays around the world. The Netherlands Foundation for Radio Astronomy operates the Westerbork Synthesis Radio Telescope in continental Europe (see Table 2). The Commonwealth Scientific and Industrial Research Organization maintains the six-element Australia Telescope at Culgoora, N.S.W., for studies of the southern skies. A number of smaller arrays are operated by the Mullard Radio Astronomy Observatory near Cambridge, Eng., including the pioneering One-Mile Radio Telescope and a simple yet very powerful array of Yagi antennas operating at 151 MHz.
The Multi-Element Radio-Linked Interferometer Network (MERLIN), operated by the Nuffield Radio Astronomy Laboratories at Jodrell Bank, employs microwave radio links to connect seven antennas separated by up to 200 kilometres in the southern part of England. It is used primarily to study compact radio sources associated with quasars and active galactic nuclei.
The Very Long Baseline Array (VLBA), which will consist of ten 25-metre dishes spread across the United States from the Virgin Islands to Hawaii upon completion in the early 1990s, is expected to yield radio images of quasars and other compact radio sources of unprecedented angular resolution and quality. Other radio telescopes being constructed in such countries as Italy and Australia will be dedicated to VLBI research programs. When used together with the VLBA and other radio telescopes throughout the world, the effective resolution of the system will be comparable to that of a single antenna whose diameter is roughly equivalent to the Earth's. Future space-based radio antennas are expected to increase the resolution still further to produce images of cosmic radio sources in even finer detail.
- Encyclopedia Britannica
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