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Department of Physics

The Cavendish Laboratory

Prof. F.G. Smith

Radio Astronomy in Cambridge grew from J.A. Ratcliffe's determination to re-create at the Cavendish Laboratory the ionospheric research that had been dissolved during the war. Among other members of the Telecommunications Research Establishment, which carried out wartime radar research, he brought Ryle from the team, and encouraged him to branch out into ionospheric physics by working on the recently discovered radio emission from the sun. In 1946 I helped in one of the early experiments, when Ryle and Vonberg were measuring the angular diameter of the radio emission above a very large sunspot. This was done using a "Michelson" interferometer, named after the original optical version. The apparatus was on the old rifle-range off Grange Road (behind Ryle's house); when the baseline of the interferometer had to be extended we carried one of the two aerial systems over the fence into the University rugger ground.

Although the early research was concerned with the sun, it soon appeared that the techniques could be used for observing and locating the other celestial radio sources already discovered by J.S. Hey and J. Bolton. An interferometer was again used, to distinguish the discrete sources from the background of Galactic radiation. I well remember seeing the first recording of the known source in Cygnus, and the astonishment with which we greeted the following part of the record which showed a new and previously unknown source. I knew practically nothing of conventional astronomy, or even of spherical trigonometry, and it was a whole day before I could tentatively say to Ryle and Ratcliffe that "I thought the new source was in a constellation called Cassiopeia". This was in 1948. Since then the accuracy of position-finding has improved fairly consistently by a factor of two every three years or so, so that the positions of quasars are now quoted to a fraction of a second of arc.

Solar work continued in Cambridge for several years, and notably included the first extensions of the Michelson interferometer towards the technique of aperture synthesis. The excitement of the "cosmic radio sources" was, however, pointing the way ahead to the major work on the objects now known as radio galaxies and quasars. The positions I found for the two major sources, Cyg A and Cass A, led to identifications by Baade and Minkowski at Mount Palomar. Ryle began the design of the series of interferometer radio telescopes which have produced the Cambridge surveys of radio sources. The importance of these in cosmology and astrophysics hardly needs emphasising.

An outstanding impression of the early days is of the co-operative effort of a small team with limited resources but enormous enthusiasm. As at Jodrell Bank, much of the early equipment was adapted from wartime surplus, even including some German radar dishes (one of these was eventually flown by helicopter from the rifle range to the present Mullard Observatory at Lord's Bridge). All the work was done by our own hands, especially the digging, fencing and wiring that was needed each time our interferometer activities were extended. It was a very healthy business, as well as a very exciting time. But we could hardly have predicted the scale of radio astronomical research that our efforts have now gone into.