Radio telescopes of large resolving power.

It was not until 1958 that It could be shown with some certainty that most of the radio sources were powerful extragalactic objects, but the possi­ bilities were so exciting even in 1952 that my colleagues and I set about the task of designing instruments capable of extending the observation to weaker and weaker sources, and of exploring their internal structure. Early observations of radio sources were severely limited both by the poor angular resolution and by the limited sensitivity. It was usually impossible to obtain any information about the structure of a source, and adjacent sources could often not be properly separated, whilst attempts to relate the radio sources to optically visible objects were often prevented by the poor positional accuracy. The use of interferometers allowed better posi­ tions to be obtained, and sometimes made it possible to derive simple models for the source structure. Few of the sources were found to have an angular size greater than 2-3’ arc. The problem of making detailed maps of such sources arises simply from the fact that the wavelengths used are some million times greater than optical wavelengths — so that even to obtain a radio picture with the same resolution as that of the unaided human eye (~1’ arc) a tele­ scope having a diameter of about 1 km operating at a wavelength of 50 cm would be needed. At the same time the instrument would be effective only if the surface accuracy was good enough to make a proper image, corresponding to errors of ≤ λ/20 or a few cm; the engineering problems of building such an instrument are clearly enormous. An entirely different approach to the problem is to employ small aerial elements which are moved to occupy successively the whole of a much larger aperture plane. The develop­ ment and use of “aperture synthesis” systems has occupied much of our team in Cambridge over the past 20 years. The principle of the method is ex­ tremely simple. In all methods used to obtain a large resolving power, that is to distinguish the wavefront from a particular direction and ignore those from adjacent directions, we arrange to combine the field measur­ ed over as large an area as possible of the wavefront. In a paraboloid we do this by providing a suitably shaped reflecting surface, so that the fields incident on different parts of the sampled wavefront are combined at the focus (Fig. 1a); the voltage pro­ duced in the receiving dipole repre­ sents the sum of these fields. We can achieve the same result if we use an array of dipoles connected together through equal lengths of cable (Fig. 1b). Suppose now that only a small part of the wavefront is sampled, but that different parts are sampled in turn (Fig. 1c); could we combine these signals to produce the same effect? Since in general, we do not know the phase of the incident field at dif­ ferent times this would not normally be possible but if we continue to measure one of the samples while we measure the others we can use the signal from this one as a phase refer­ ence to correct the values measured in other parts of the wavefront. In this way, by using two small aerial ele­ ments, we can again add the fields over the wavefront — the area of which is now determined by the range of relative positions taken by the two aerial elements. It might be thought that this method would be extremely slow, for if we are Energy and Physics