Introduction

1. Therise of radio astronomy in the Netherlands: a peculiar history Astrid ElbersLeiden Observatory, Leiden University, The Netherlands The 21-cm hydrogen line At a clandestine meeting of the NederlandseAstronomenclub (NAC) (Dutch Astronomers Club) on April 15th 1944 at Leiden Observatory, van de Hulst could confirm that a spectral line of hydrogen existed at 21 cm. Only a few days after van de Hulst’s talk at the NAC, Oort asked dr. C.J. Bakker of Philips factories to construct a receiver for a radio telescope with a mirror of 10 to 20 metres. The mirror could be built by the mechanical workshop of the Leiden Observatory. Bakker replied this certainly would be possible after the war. At this moment, cooperation with the industry was a fact. Shortly after the war, Oort applied to the Department of Science and Education for funding for his telescope. But since the cost was much too high, he didn’t get what he asked for. Kootwijk (Netherlands Organisation for the Advancement of Pure Research). It was a lucky coincidence that the director of ZWO, J.H. Bannier, a physicist and friend of Oort’s, had a great interest in astronomy. For the radio astronomical project, a foundation was erected in 1949, namely the StichtingRadiostraling van Zon en Melkweg (SRZM) (National Foundation for Radio Astronomy or NFRA). The board of the SRZM consisted not only of several astronomers, but also of members of the PTT (which was interested in the radio emission of the sun, because this affects the ionosphere and influences radio propagation on the Earth), Philips, and the KNMI (Royal Netherlands Meteorological Institute). Dwingeloo and Westerbork In the years of the Kootwijk observations, Oort kept pursuing his plans for a much bigger and more sensitive telescope.The successful observations in Kootwijk quickly caused ZWO to approve the plans for a new radio telescope, with a 25-m dish. They provided financial support for its design and construction. The telescope was built in Dwingeloo and inaugurated in April 1956. For a full year, it would be the largest telescope in the world. observations than the Kootwijk one – several During the ‘Dwingeloo period’, the Netherlands had recovered from the war and a period of extensive economic growth began. The universities benefited from this. In the late 1950’s, there was for instance a considerable increase in the budget of the ZWO. However, despite the economic growth till 1975, it has sometimes been suggested that Dutch natural scientific research was not very innovative. It adopted and elaborated issues that had already started elsewhere, but did not come up with truly new things. A big exception in that picture was Dutch astronomy. With radio astronomy, the Netherlands did not simply follow what was going on elsewhere, but took the lead. In the same way as Oort had been pursuing his plans for Dwingeloo during the Kootwijk period, he again started planning as early as in 1957 the building of a new telescope. Its creation was a long and complex process, which started as a Dutch/Belgian venture. After the withdrawal of the Belgians, the Dutch had to go on on their own. The project resulted in the Westerbork Synthesis Radio Telescope in 1970, an interferometer consisting of a linear array of 14 antennas. Today, this telescope is still in use. Conclusions By 1970, radio astronomy was firmly established as a (sub)discipline in the Netherlands. The reasons for this are multiple and need to be further explored. Certainly, the role of the Leiden astronomer Jan Hendrik Oort was crucial in this respect. Besides being a great scientist, he excelled in efficient planning and lobby work. The contacts Oort established with Bok were fruitful for the relations between the Netherlands and the US. And the fact that Oort was a personal friend of Bannier, the president of ZWO, was also an advantage. Above all, Bannier was a physicist with a great interest in astronomy. Oort managed to recruit important allies in industry and government. The question is why all these parties were willing to cooperate in this project and what they had to gain from it. Last but not least, we need to consider the more structural circumstances that made it possible that the initiatives of Oort turned out to be successful. Introduction The postwar take-off of Dutch radio astronomy had a special character in several respects. In Australia, England and the USA, radio-astronomical research was initiated in 1945 by radio engineers and physicists who had acquired extensive experience with radio receivers during World War Two. In the Netherlands, such experience was lacking. Moreover, the war had brought Dutch science to a grinding halt. In spite of these unfavourable conditions, the Netherlands would rapidly emerge as a centre for radio astronomy. My aim is to account for this unlikely success. The initiative did not come from engineers and physicists, but from leading individuals at classical astronomical research, especially Jan Hendrik Oort at Leiden. Figure 5. The Dwingeloo telescope With this telecope – which could make much more detailed observations than the Kootwijk one – several projects were undertaken. Among them are the galactic and extragalactic research of hydrogen (for example of the Andromeda Nebula) Lacking money, instruments, and expertise, a large part of the success of Dutch radio astronomy was due to his recruiting of several allies both in astronomical and physical communities as well as in industry and government circles. Figure 1. Professor Jan Hendrik Oort of Leiden Observatory The Dutch PTT (Post, Telephone and Telegraph) however, gave Oort access to a German radar reflector at its tele- communication station in Kootwijk and Philips provided a receiver for 21-cm wavelength in 1948. Oort also managed to obtain a funding from the ZWO Archives and Literature Oort archive, Leiden University Casimir, H.B.G., Voorzieningen ten behoeve van de research binnen de faculteiten der wis- en natuurkunde der Nederlandse universiteiten, ’s-Gravenhage, 1959. Raimond, E., Historical notes: Four decades of Dutch radio astronomy, twenty-five years of Westerbork telescope, in: Raimond, E. and Genee, R. eds., The Westerbork Observatory; Continuing Adventures in Radio Astronomy, (Astrophysics and Space Science Library, 207), Dordrecht, Boston and London, 1996, pp.11-51. Spoelstra, T.A.Th. ed., Een zilveren spiegel. 25 jaar Radiosterrenwacht Dwingeloo. 17 april 1956 – 17 april 1981, Dwingeloo, 1981. Van Berkel, K., Kwaliteit en zuinigheid in het wetenschappelijk onderwijs, in: Schuyt, K. and Taverne, E. eds., 1950: Welvaart in zwart-wit (Nederlandse cultuur in Europese context. Deel 4) Den Haag, 2000, pp. 331 – 354. Van Woerden, H. and Strom, R.G., The Beginnings of Radio Astronomy in the Netherlands, in: Journal of Astronomical History and Heritage, 9,1 (2006), pp. 3-20. Figure 3. The Kootwijk telescope Communication in wartime Just before World War Two, the American electronic engineer Grote Reber from Wheaton built a radio telescope in his garden with which he made the first maps of radio emission of the Galaxy. He published his findings in various journals, such as the Astrophysical Journal. During the war, Europe’s access to international publications was restricted. Thanks to Oort’s complaints about this to Bart Bok, a Dutch astronomer who immigrated to the US in 1929 to work at Harvard College Observatory in Cambridge, Bok made sure astronomical literature from the US reached Holland. These contacts between Oort and Bok were crucial in establishing the relations between the Netherlands and the US during and after the war. At a meeting of the American Astronomical Society in September 1940, the Council appointed a Committee for the Distribution of Astronomical Literature (CDAL) with Bart Bok as president. Acknowledgments We thank Frans Van Lunteren, David Baneke and Martin Weiss for their useful remarks. Further, we thank Colin Purrington of Swarthmore College for placing a very useful postertemplate at our disposal. Figure 4. In 1954, the observations in Kootwijk led to the publication of the first large-scale map of the distribution of neutral hydrogen in the Galaxy. In 1958, the data were combined with similar ones of the Southern part of the Galaxy observed by Australian astronomers. Together, they formed the Leiden-Sydney map of the Milky Way. Figure 6. The Westerbork telescope Through this channels, Oort got familiar with the publications of Reber and understood that it was important to find radiation at one specific frequency. As a result, a spectral line that could be observed by a radio telescope had to be found. Oort asked astronomy student Henk van de Hulst to investigate whether such a line existed. For further information Please contact Drs. Astrid Elbers: Leiden Observatory, P.O. Box 9513, 2300 RA Leiden ,The Netherlands. Phone: +31 (0)71 527 8412 elbers@strw.leidenuniv.nl You can find an on line version of the poster at: http://www.strw.leidenuniv.nl/~elbers/ Figure 2. Bart Bok