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Early Techniques in Radio Astronomy
ADVANCES IN IMAGING AND ELECTRON PHYSICS, VOL. 91
Early Techniques in Radio Astronomy A. HEWISH Cavendish Laboratory, Cambridge, United Kingdom
Radio astronomy in the U.K. essentially began in late February 1942, when anti-aircraft radars operating at meter wavelengths were jammed by intense radio noise emitted from the sun. It was J. S. Hey, working with the Army Operational Research Group, who identified the solar source of the noise signals and who later initiated systematic observations in the immediate postwar years. Following up Jansky’s discovery of radio emission from the Milky Way in the U.S.A. in 1937, Hey also located radiation from a source in the constellation of Cygnus which ultimately led to the identification of the first radio galaxy. Over the years, detailed studies of radio galaxies have revolutionized our picture of the Universe, and this paper outlines the development of techniques in radio astronomy by which these advances were achieved. Astronomical radio signals are generated by a number of different mechanisms and usually have the character of electrical noise (or “hiss”)the equivalent of white light in conventional astronomy. Frequently the signals are much weaker than the front-end noise from the receiver itself, even when the best low-noise amplifiers are used, and long integration times are necessary for their detection. Small fluctuations of receiver gain are an immediate problem, and this was solved by a variety of switching techniques, for example alternating between the signal and a constant noise source, so that an ac output is generated. Suitable filtering then removes the effects of slow variations of receiver gain, allowing long integration time to be achieved. A more fundamental problem concerns the necessity for exceptionally large receiving antennas, both to obtain a detectable signal and to achieve high angular resolution‘. The latter depends upon the ratio LID, where L is the wavelength and D the aperture of the antenna. In the earliest work, large values of D were obtained by connecting two antennas separated by a distance D to a single receiver. The earth’s rotation causes the resultant signal to vary periodically in response to the changing phase of the two components, and this device is a radio analogue of the Michelson interferometer in optical astronomy. An equivalent system with a single antenna mounted on a cliff and utilizing Lloyd’s mirror interference by reflection from the sea was employed by Pawsey and his co-workers in Australia. It was the 285
Copyright 0 1995 by Academic Press, Inc. All righls of reproduction in any form reserved. ISBN 0-12-014733-5
286
A. HEWISH
enhanced angular resolution of a simple east-west interferometer used by Graham-Smith that enabled the Cygnus source to be identified with a distant galaxy. An important early development was Ryle’s phase-switching interferometer, in which the outputs from two separated antennas were combined alternately in phase and in anti-phase. The modulated signal corresponds only to signals which are correlated at both antennas, so that sources of angular size much larger than L/D are rejected. This is an advantage when detecting sources against the sky background, and local man-made radio interference is also greatly reduced. The first large-scale sky surveys were carried out by Ryle at Cambridge in the early 1950s, using phase-switching interferometers and antennas of considerable collecting area economically constructed with reflecting surfaces composed of steel wires stretched between a framework of parabolic pylons (Fig. 1). A more conventional approach to achieving large collecting areas was pioneered by Love11 at Jodrell Bank who constructed a fully steerable parabolic reflector 250 ft. in diameter which became operational in 1957. A similar reflector of diameter 100 m was completed in Germany in 1970. The latter incorporated the principle of structural homology, whereby flexure
FIGURE1.
EARLY TECHNIQUES IN RADIO ASTRONOMY
287
under gravity deforms the surface so that it remains parabolic, but of varying focal distance. This provides a more accurate surface, so that the radio telescope can be operated at much shorter wavelengths. The largest reflector yet built is the 1,OOO ft. spherical bowl at Arecibo in Puerto Rico. This fixed bowl was excavated from the limestone hills, and scanning over a limited angular range is achieved by movement of the receiving antenna at the focus. Another means of constructing partially steerable reflectors of large area was that of Kraus, in the U.S.A., who used a tiltable flat surface to direct radiation horizontally into a second focusing reflector having a focal point at ground level. Massive radio telescopes adopting similar ideas were later constructed in France and Russia. By the late 1950s it was evident that the high angular resolution needed to study radio galaxies in detail could not be obtained by conventional methods. Apertures of at least one mile were required. One solution was the unfilled aperture technique adopted in the Mills Cross telescope built in Australia in 1966. Two narrow cylindrical parabolic reflectors, each one mile long and arranged as an east-west, north-south cross, were connected to a correlating receiver, giving an output corresponding to the product of the voltage polar diagrams of the separate antennas. Combining the intersecting fan beams in this way provides a pencil beam of high angular resolution. The rapid development of electronic computers in the 1960s was exploited by Ryle in a still more radical approach known as aperture synthesis, for which he was awarded the Nobel Prize in 1974 (Fig. 2). Steerable antennas of modest size are arranged as correlation interferometers, producing outputs corresponding to components in the two-dimensional Fourier transform of the intensity distribution across the field of view. Inverse transformation of the stored data from interferometer pairs on different baselines, usually employing the Earth’s rotation to vary their position angle in relation to the sky, ultimately yields an image of angular resolution L/D, where D is the maximum baseline employed. Knowledge of the phase of the signals is necessary for Fourier inversion of the image, so all the receivers in use at one time require a common local oscillator. The flexibility of aperture synthesis methods, and power of computer techniques for image analysis, and image enhancement where sampling of the Fourier transform by the interferometer pairs is incomplete, produced a revolution in high-resolution radio astronomy. The 5-km telescope designed by Ryle and completed in 1972 incorporated eight parabolic reflectors, four of which could be moved along rails to provide variable baselines (Fig. 3). This arrangement provided 16 Fourier components simultaneously, and the high-resolution images obtained gave the first clear evidence that the
288
A. HEWISH
FIGURE2.
dumbell structure of many radio galaxies was caused by beams of energy emitted from active nuclei in the central regions. Some years later came the Very Large Array in the U.S.A., consisting of 27 reflectors arranged in a “Y” configuration, each arm being 13 miles long.
EARLY TECHNIQUES IN RADIO ASTRONOMY
289
FIGURE3.
This provides more than 300 Fourier components at the same time. The largest baselines so far employed in aperture synthesis are those of the MERLIN array operated by Jodrell Bank, which extend to nearly 200 kilometers. Direct cable connections between the antennas are impractical at this separation, so radio links with calibrated phase-paths are used. The highest angular resolutions in radio astronomy have been obtained by interferometers spanning intercontinental baselines using the VLBI technique. At this separation, radio links to maintain the phase of interferometer signals cannot be used. Independent local oscillators of high stability provide phase-coherent signals at each antenna which are recorded and correlated later to provide interferometric data. Fourier inversion to generate the image is no longer straightforward, as the true phase of the components has been lost, but image-analysis techniques using relative phases in the phase closure method supplies a partial solution to this difficulty. VLBI methods were initiated by Hanbury Brown and Palmer at Jodrell Bank in the late 1950s and early 1960s. It is noteworthy that radio telescopes operating at wavelengths typically lo5 times longer than optical wavelengths now provide images of substantially higher resolution than those available from the largest ground-based optical telescopes. This is due to atmospheric turbulence, which degrades the performance of ground-based telescopes. An outstanding challenge for the future is the imaging of very small thermal variations in the microwave background radiation, which will provide clues about the origin of galaxies
290
A. HEWISH
in the early universe. This requires the detection of changes of a few parts in a million of the 2.7 kelvin background temperature on angular scales exceeding several arcminutes and poses new technical problems. It seems likely that aperture synthesis arrays of small overall size, but operating over very wide bandwidths at a variety of frequencies, will provide the best solution.
Early Techniques in Radio Astronomy A. HEWISH Cavendish Laboratory, Cambridge, United Kingdom
Radio astronomy in the U.K. essentially began in late February 1942, when anti-aircraft radars operating at meter wavelengths were jammed by intense radio noise emitted from the sun. It was J. S. Hey, working with the Army Operational Research Group, who identified the solar source of the noise signals and who later initiated systematic observations in the immediate postwar years. Following up Jansky’s discovery of radio emission from the Milky Way in the U.S.A. in 1937, Hey also located radiation from a source in the constellation of Cygnus which ultimately led to the identification of the first radio galaxy. Over the years, detailed studies of radio galaxies have revolutionized our picture of the Universe, and this paper outlines the development of techniques in radio astronomy by which these advances were achieved. Astronomical radio signals are generated by a number of different mechanisms and usually have the character of electrical noise (or “hiss”)the equivalent of white light in conventional astronomy. Frequently the signals are much weaker than the front-end noise from the receiver itself, even when the best low-noise amplifiers are used, and long integration times are necessary for their detection. Small fluctuations of receiver gain are an immediate problem, and this was solved by a variety of switching techniques, for example alternating between the signal and a constant noise source, so that an ac output is generated. Suitable filtering then removes the effects of slow variations of receiver gain, allowing long integration time to be achieved. A more fundamental problem concerns the necessity for exceptionally large receiving antennas, both to obtain a detectable signal and to achieve high angular resolution‘. The latter depends upon the ratio LID, where L is the wavelength and D the aperture of the antenna. In the earliest work, large values of D were obtained by connecting two antennas separated by a distance D to a single receiver. The earth’s rotation causes the resultant signal to vary periodically in response to the changing phase of the two components, and this device is a radio analogue of the Michelson interferometer in optical astronomy. An equivalent system with a single antenna mounted on a cliff and utilizing Lloyd’s mirror interference by reflection from the sea was employed by Pawsey and his co-workers in Australia. It was the 285
Copyright 0 1995 by Academic Press, Inc. All righls of reproduction in any form reserved. ISBN 0-12-014733-5
286
A. HEWISH
enhanced angular resolution of a simple east-west interferometer used by Graham-Smith that enabled the Cygnus source to be identified with a distant galaxy. An important early development was Ryle’s phase-switching interferometer, in which the outputs from two separated antennas were combined alternately in phase and in anti-phase. The modulated signal corresponds only to signals which are correlated at both antennas, so that sources of angular size much larger than L/D are rejected. This is an advantage when detecting sources against the sky background, and local man-made radio interference is also greatly reduced. The first large-scale sky surveys were carried out by Ryle at Cambridge in the early 1950s, using phase-switching interferometers and antennas of considerable collecting area economically constructed with reflecting surfaces composed of steel wires stretched between a framework of parabolic pylons (Fig. 1). A more conventional approach to achieving large collecting areas was pioneered by Love11 at Jodrell Bank who constructed a fully steerable parabolic reflector 250 ft. in diameter which became operational in 1957. A similar reflector of diameter 100 m was completed in Germany in 1970. The latter incorporated the principle of structural homology, whereby flexure
FIGURE1.
EARLY TECHNIQUES IN RADIO ASTRONOMY
287
under gravity deforms the surface so that it remains parabolic, but of varying focal distance. This provides a more accurate surface, so that the radio telescope can be operated at much shorter wavelengths. The largest reflector yet built is the 1,OOO ft. spherical bowl at Arecibo in Puerto Rico. This fixed bowl was excavated from the limestone hills, and scanning over a limited angular range is achieved by movement of the receiving antenna at the focus. Another means of constructing partially steerable reflectors of large area was that of Kraus, in the U.S.A., who used a tiltable flat surface to direct radiation horizontally into a second focusing reflector having a focal point at ground level. Massive radio telescopes adopting similar ideas were later constructed in France and Russia. By the late 1950s it was evident that the high angular resolution needed to study radio galaxies in detail could not be obtained by conventional methods. Apertures of at least one mile were required. One solution was the unfilled aperture technique adopted in the Mills Cross telescope built in Australia in 1966. Two narrow cylindrical parabolic reflectors, each one mile long and arranged as an east-west, north-south cross, were connected to a correlating receiver, giving an output corresponding to the product of the voltage polar diagrams of the separate antennas. Combining the intersecting fan beams in this way provides a pencil beam of high angular resolution. The rapid development of electronic computers in the 1960s was exploited by Ryle in a still more radical approach known as aperture synthesis, for which he was awarded the Nobel Prize in 1974 (Fig. 2). Steerable antennas of modest size are arranged as correlation interferometers, producing outputs corresponding to components in the two-dimensional Fourier transform of the intensity distribution across the field of view. Inverse transformation of the stored data from interferometer pairs on different baselines, usually employing the Earth’s rotation to vary their position angle in relation to the sky, ultimately yields an image of angular resolution L/D, where D is the maximum baseline employed. Knowledge of the phase of the signals is necessary for Fourier inversion of the image, so all the receivers in use at one time require a common local oscillator. The flexibility of aperture synthesis methods, and power of computer techniques for image analysis, and image enhancement where sampling of the Fourier transform by the interferometer pairs is incomplete, produced a revolution in high-resolution radio astronomy. The 5-km telescope designed by Ryle and completed in 1972 incorporated eight parabolic reflectors, four of which could be moved along rails to provide variable baselines (Fig. 3). This arrangement provided 16 Fourier components simultaneously, and the high-resolution images obtained gave the first clear evidence that the
288
A. HEWISH
FIGURE2.
dumbell structure of many radio galaxies was caused by beams of energy emitted from active nuclei in the central regions. Some years later came the Very Large Array in the U.S.A., consisting of 27 reflectors arranged in a “Y” configuration, each arm being 13 miles long.
EARLY TECHNIQUES IN RADIO ASTRONOMY
289
FIGURE3.
This provides more than 300 Fourier components at the same time. The largest baselines so far employed in aperture synthesis are those of the MERLIN array operated by Jodrell Bank, which extend to nearly 200 kilometers. Direct cable connections between the antennas are impractical at this separation, so radio links with calibrated phase-paths are used. The highest angular resolutions in radio astronomy have been obtained by interferometers spanning intercontinental baselines using the VLBI technique. At this separation, radio links to maintain the phase of interferometer signals cannot be used. Independent local oscillators of high stability provide phase-coherent signals at each antenna which are recorded and correlated later to provide interferometric data. Fourier inversion to generate the image is no longer straightforward, as the true phase of the components has been lost, but image-analysis techniques using relative phases in the phase closure method supplies a partial solution to this difficulty. VLBI methods were initiated by Hanbury Brown and Palmer at Jodrell Bank in the late 1950s and early 1960s. It is noteworthy that radio telescopes operating at wavelengths typically lo5 times longer than optical wavelengths now provide images of substantially higher resolution than those available from the largest ground-based optical telescopes. This is due to atmospheric turbulence, which degrades the performance of ground-based telescopes. An outstanding challenge for the future is the imaging of very small thermal variations in the microwave background radiation, which will provide clues about the origin of galaxies
290
A. HEWISH
in the early universe. This requires the detection of changes of a few parts in a million of the 2.7 kelvin background temperature on angular scales exceeding several arcminutes and poses new technical problems. It seems likely that aperture synthesis arrays of small overall size, but operating over very wide bandwidths at a variety of frequencies, will provide the best solution.