|Annu. Rev. Astron. Astrophys. 1980. 18:
Copyright © 1980 by . All rights reserved
1.1. Techniques for Measuring Source Structure
Because of their limited resolution, single dishes are useful only for studying the largest extragalactic sources at short wavelengths. Until recently, owing to receiver instabilities, dynamic ranges greater than 10 to 1 were difficult to achieve. With new observing techniques incorporating multiple beams (Emerson et al. 1979) and cleaning methods (Reich et al. 1978) much larger dynamic ranges have been obtained with the 100-m Effelsberg telescope in Germany.
Several tools have been developed to achieve the higher resolutions needed for studying the structure of extragalactic radio sources. In the technique of interplanetary scintillations, measurements of the degree and time scale of scintillation (flickering) of a source and their variation with distance from the Sun are used to provide useful information on small-scale structure (< 1"; Hewish et al. 1964, Cohen et al. 1967, Little & Hewish 1968). The scintillation method has the advantage of providing relatively high resolution at low frequencies. However, all but the crudest inferences about structure depend on the model assumed for the solar wind. Moreover, only sources within a few tens of degrees from the Sun are observed to scintillate and can be studied extensively.
Another way to achieve high resolution is to observe a source as it is occulted by the moon. The variation of flux density then gives its one-dimensional brightness distribution convolved with the diffraction pattern of the moon (Getmansev & Ginzburg 1950, Scheuer 1962, von Hoerner 1964). In contrast to other techniques, the resolution that can be obtained with lunar occultations is not strongly frequency dependent and is mainly governed by signal to noise. A resolution of a few tenths of an arcsecond is readily achievable. There are however three disadvantages of this method. First, it can only be applied to sources that lie on the Moon's path. Second, it is difficult to reconstruct the two-dimensional structure without observing several occultations covering a large range of orientations. Third, dynamic ranges of more than 10 to 1 cannot easily be obtained. Despite these limitations the occultation technique has scored two big successes. The occultation of 3C273 (Hazard et al. 1963) was one of the most crucial observations leading to the discovery of quasars. Also, it was by means of occultation measurements that the relation between the angular sizes and flux densities of radio sources was discovered (Swarup 1975, Kapahi 1975).
By far the most important tool for investigating the structure of radio sources has been interferometry (see Fomalont & Wright 1974 for an excellent account). This uses two or more telescope elements extending over a baseline B to give an angular resolution of / B radians. A Michelson two-element interferometer was first applied to astronomy in 1946 by Ryle & Vonberg (1948). Since then the method has undergone many sophisticated developments along two broad fronts.
First, the technique of earth rotational aperture synthesis (Ryle & Hewish 1960) made it possible to produce two-dimensional maps of source structures by Fourier transforming the complex fringe amplitudes using digital computers. The power of aperture synthesis was subsequently increased by the development of restoration methods such as "clean" which removed the distortion in maps due to incomplete sampling of the aperture (Högbom 1974, Schwarz 1978). Several arrays of telescopes linked by cables were built primarily to map the brightness and polarization (Conway & Kronberg 1969, Weiler 1973) distributions of radio sources. Particularly important contributions in delineating radio-source structure on the scale of seconds to minutes of arc have been made by the "One Mile" and "5 km" telescopes at Cambridge, England, the Westerbork telescope in the Netherlands, and NRAO interferometer system at Green Bank. West Virginia. The VLA (Very Large Array) at present being built by the NRAO can attain higher resolution than any of these. Even in its unfinished state the VLA is now routinely producing maps with resolution of better than 1" and sensitivity better than 1 mJy. (2)
This resolution is not high enough to probe the range of angular scales within individual sources, which sometimes exceeds 106. However, from the beginning of interferometry a second distinct path evolved in which detailed mapping was subordinated to a quest for higher and higher resolution. At Jodrell Bank, England, radio links were used to connect the telescope elements (Brown et al. 1955, Elgaroy et al. 1962). In this way, by 1966 baselines of more than 100 km and resolutions of ~ 1/20" were achieved, at the cost of sacrificing fringe phase information (Palmer et al. 1967). Next, accurate atomic clocks and video tape recorders, which became available during the sixties, were used to develop linkless interferometers. The first examples were completed almost simultaneously in Canada (Broten et al. 1967) and the US (Bare et al. 1967). Very long baseline interferometry (VLBI) with angular resolutions of a fraction of a milliarcsecond could then be carried out. Since the absolute phase of the interferometer fringes cannot yet be generally measured using VLBI, unambiguous Fourier reconstruction of maps is impossible. However, considerable information about fine-scale structure in sources can be extracted by means of the many sophisticated model-fitting schemes now available and from these models a pseudo-map is often produced. The so-called "closure phases," which can be measured with interferometer arrays (Jennison 1958, Readhead & Wilkinson 1978), are particularly valuable in constructing VLBI pseudo-maps.
Aperture-synthesis and VLBI arrays now in operation are listed by Fomalont (1979) and Moffet (1979) respectively.
2 1 Jansky = 10-26 W m-2 Hz-1 = 103 mJy. Back.