During the first ten years after the second world war the development of radio astronomy occurred simultaneously in many places. Other large radio telescopes were designed and built during approximately the same period in which the Dwingeloo telescope was constructed.
The next step in the development of Dutch radio astronomy is very intimately related to the developments in other countries during the late forties and the fifties. Of course, it was clear to everyone that poor angular resolution, the problem inherent to observing at long radio wavelengths, could ultimately not be overcome by constructing larger single radio telescopes.
The solution to that problem could only be found in combinations of two or more telescopes at some distance from each other working as an interferometer. Many of the developments in Australia and England especially were concentrated on interferometric telescopes even in the very early years of radio astronomy.
Already in 1946 Ryle and Vonberg constructed a radio-equivalent of the Michelson interferometer near Cambridge, England. It consisted of two dipole arrays, tuned to a wavelength of 1.7 metres; the distance between the two arrays could be varied over a range of 10 to 140 wavelengths.
The resolution of that very early interferometer was comparable to that achieved by the Dwingeloo telescope which was operating at a wavelength eight times as short. The very first records of interferometric observations of the sun were made very early in 1946 by McCready, Pawsey and Payne-Scott (1947), who used one of the Australian coastal radar installations to combine the direct image of the sun and its image reflected by the sea.
Using this method the locations of active regions on the sun's surface could just about be determined. This sea interferometer was used by Bolton and Stanley (1948) to determine the size of Cygnus A, one of the strongest radiosources in the sky, to be smaller than 8 arcminutes. They could also determine its position with an accuracy of that order.
Using a two-element interferometer in Cambridge at a wavelength of 3.5 metres, Martin Ryle and and Graham Smith (1948) discovered Cassiopeia A, a radio source even brighter than Cygnus A (or Cygnus 1) which had been observed before. Two years later Ryle, Smith and Elsmore (1950) published a catalogue of 50 radio sources observed with the same instrument.
Accurate determination of the positions of these sources was important in order to identify some of them with objects seen optically. Without optical identification calibration of the radio-positions to the level of the internal accuracy of the observations was extremely difficult. Mills (1952), in Australia, achieved position accuracies of order 1 arcminute for the first time in 1952.
At Jodrell Bank near Manchester, Jennison and Das Gupta (1953), using an intensity interferometer, showed that the radio source Cygnus A was not a simple structure, but consisted of more than one major component. After further measurements with three phase-sensitive interferometers simultaneously Jennison (1958) concluded that Cygnus A had to be a double source.
Around the time that the Dwingeloo telescope was commissioned, the middle 1950s, it became obvious that many more discrete radio sources be identified with known astronomical objects so that statistical work could be done. As a result survey-instruments for high-resolution observations were constructed.
At Cambridge Ryle and Hewish (1955) constructed a large telescope consisting of four antennas at the corners of a rectangle of 580 by 49 metres (East-West and North-South, respectively). Contrary to most of the earlier instruments this telescope provided not only resolution in the East-West direction but also in the North-South direction. Initially it operated at 81.5 MHz (3.7 m wavelength).
Mills and co-workers (1958) built a telescope in Fleurs (near Sydney) consisting of two 457 m long arrays of antennas in the form of a cross. The North-South array produced a beam elongated in the E-W direction but narrow in the N-S direction, while the East-West array produced an elongated beam orthogonal to that of the North-South array. When the two arms were combined, the Mills cross had a `pencil-beam' of 49 arcminutes in both directions at 85.5 MHZ frequency (3.5 m wavelength).
The sky survey produced by this instrument, published by Mills, Slee and Hill (1958,1960,1961) contained a list of 2200 sources. Comparison of part of this source list with the results of the Cambridge interferometer in an overlapping part of the sky showed poor agreement, which was found to be due to source confusion in the lower resolution Cambridge observations.
This comparison led to the doubling of the operating frequency of the Cambridge interferometer to 159 MHz (1.9 m wavelength). This reduced the area of the beam by a factor four. The new list of 471 sources produced with the modified Cambridge interferometer (Edge et al. 1959) was the famous 3C catalogue.
During the same period Christiansen and Warburton (1955) produced a two-dimensional map of the quiet sun at a wavelength of 21 cm, using a combination of an East-West and a North-South array of parabolic reflectors. In fact, this could be called the first application of earth rotation aperture synthesis, although Ryle described the principle more explicitly in 1962.
In the second half of the 1950s, this was the international background against which Jan Oort and his co-workers started thinking about the next instrument to be built for Dutch astronomy. As we will see in the paragraphs below the further developments in Cambridge and Australia, and later also in Italy and the Soviet Union influenced the design of what was to become the Westerbork Synthesis Radio Telescope.