In the years immediately after the end of World War II it was obvious that using existing equipment and the help of industry was the only possible road towards getting radio observations.
Thanks to the Dutch PTT and Philips a 7.5-metre German radar reflector at the telecommunication station in Kootwijk and a receiver for 21-cm wavelength were made available. After some modification of the receiver by C.A. (Lex) Muller, he and Oort could observe the interstellar emission line of hydrogen at 21 cm wavelength in 1951 and confirm its first detection by Ewen and Purcell (1951) of a few months bef ore.
At the 8th General Assembly of the International Astronomical Union in Rome in 1952 the results of the observations of the first year were presented and enthusiastically received. After some further improvement of the receiver a full year (July 1952 to June 1953) of systematic observations in Kootwijk led to the publication by Van de Hulst, Muller and Oort (1954) of the first large-scale map of the neutral hydrogen distribution in the galaxy. This was generally considered a real break-through in galactic research. Combination of the Kootwijk map with a similar one of the Southern part of the galaxy observed by Frank Kerr and his Australian collaborators led to an even more complete picture, the Leiden/Sydney survey by Oort, Kerr and Westerhout in 1958.
Simultaneously with the activities leading to the first observations in Kootwijk, Oort kept pursuing his plans for a much bigger, more sensitive radio telescope with a better resolution than that of the Kootwijk dish.
As a result of his tentative investigations into the possibilities of achieving his goals, a first, preliminary design for a large telescope was made by Werkspoor in 1946. It soon became clear that the plans were outgrowing the capabilities of the Leiden University Observatory. Also there was interest in radio-observations of the Sun from the Utrecht observatory, where Minnaert was director.
In 1948 Oort and Minnaert took the initiative to set up an inter-university organisation to serve the Dutch national interests in radio astronomy. As a result, the Netherlands Foundation for Radio Astronomy (NFRA), with a board consisting of astronomers and prominent people from industry, was formally founded in 1949 under the auspices of the Netherlands Organisation for the advancement of Pure Research (ZWO).
The NFRA became the base for the engineering efforts in electronics with C.A. Muller as leader from late 1950 onwards. In November 1951 Werkspoor was given the contract to make a final design for an alt-azimuthally mounted radio-telescope with a mirror of 25 metres diameter. The design was completed in February 1954. Since computers were too expensive and too slow to transform equatorial to azimuthal coordinates, a complex mechanical-electrical coordinate transformer had been designed.
While the telescope was being designed a survey was made of several sites suitable to locate the new telescope. By the end of 1953 the site in Dwingeloo at the edge of the State Forest and overlooking a protected piece of moor in Southerly direction was chosen as the most suitable one. Radio interference from motor vehicles and other sources could be kept at a minimum at this location.
Actual construction started in the summer of 1954 under supervision of B.G. Hooghoudt. In November 1955 the partly completed Dwingeloo telescope was used for the first time to observe the occultation of the Crab-nebula by the moon at 75 cm wavelength (Westerhout and Seeger 1957). In April 1956 the telescope was inaugurated by Her Majesty Queen Juliana.
Regular observations started in September 1956 with Westerhout's survey (1958)of the continuous radiation from the galaxy at a frequency of 1390 MHz (21.5 cm wavelength), which was soon followed by 21-cm line observations of the Galactic Centre region (Oort and Rougoor 1959, Rougoor and Oort 1959) and the first observations of hydrogen in the Andromeda nebula (Van de Hulst, Raimond and Van Woerden 1957). For many years the Dwingeloo telescope was to be the bread-and-butter instrument of the Dutch radio astronomers.
The sensitive all-sky survey of the 21-cm line radiation by Hartmann (1994) and the discovery of the galaxy Dwingeloo-1 by Kraan-Korteweg et al. (1994) show that, after 38 years, the instrument is still capable of producing significant scientific results today.
The Dwingeloo telescope, the largest radio telescope in the world for a short time, was a big step forward towards solving problems of galactic structure, motions in the interstellar matter in our galaxy and in nearby galaxies as well as of structure of galactic magnetic fields. However, it was not going to solve the very fundamental problems of the structure of the universe in which Jan Oort and many othe rs were interested.
For that purpose a radio-telescope with significantly higher angular resolution was required. The size of such a telescope would have to be of the order of kilometers rather than tens of metres.