History of the WSRT

Radio astronomy began in America in the early 1930's, with the discovery by Karl Jansky of radio emission from the centre of the Milky Way. Although this led to some theoretical speculations, the only practical follow up was by the American radio engineer Grote Reber, who made the first sky maps as an amateur in the 1940's. His published results reached Holland during the war, and stimulated Prof. Jan Oort to think seriously about how this new technique could be put to use to tackle astronomical problems.
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After the war, the first radio telescope in the Netherlands was built in Kootwijk, making use of a German radar reflector. With this instrument neutral hydrogen (HI) from the Milky was detected, the first maps of HI in the Galaxy were made, and the basis was established for the 25 m Dwingeloo Telescope.
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At the same time, the radio technique was being explored and refined elsewhere, notably in England and Australia, and some of these developments would prove crucial to the design of the next Dutch telescope: Westerbork
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The origins of the Westerbork Synthesis Radio Telescope (WSRT) go back to the Benelux Cross Antenna Project, which began shortly after the completion of the Dwingeloo 25 m telescope in 1956.
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Suffice to say that the Benelux Cross was probably overly ambitious, when Belgium quit the project it had to be scaled down, and in the meantime Ryle's group in Cambridge had demonstrated the potential of the earth-rotation aperture-synthesis technique.
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Prof. Oort, the driving force behind the project, had as his primary scientific goal the study of cosmology using the extragalactic radio source population. The original design was for twelve 25 m telescopes on an east- west baseline, extending over 1.5 km. Construction began in 1966/67, the first telescope was in place by August 1967, and the twelfth before the end of 1968.
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The drive system, cables, receivers and other electronics for the entire array were installed by the spring of 1970, and the telescope was officially opened by Queen Juliana in June of that year.
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The original WSRT was designed to operate at 1400 MHz, and initially there was only a continuum receiver system for this frequency. The mechanical, electronic and operational aspects of the original array were described in a series of articles (Baars and Hooghoudt, 1974; Casse and Muller, 1974; Högbom and Brouw, 1974).In normal operation at that time, the two movable dishes on a 300 m track at the east of the array were correlated with each of the ten fixed dishes, producing 20 interferometers.
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By setting the movable dishes to other locations and reobserving, the sampling density of the visibility plane can be increased, for proper imaging of very extended objects. It soon became clear that other receiver systems would be a desirable extension, and within 3 years a modest filter-bank line receiver, and 0.6 and 5 GHz frontends were added. The sensitivity of all frontend systems has been regularly upgraded throughout the lifetime of the WSRT.

The period 1975-1980 saw the biggest extension of the telescope, with the addition of two new movable dishes, first on the existing rails, but ultimately on a new track which extended the array to 2.7 km. By the early 1980s, new digital back ends greatly increased the number of line channels (to typically 32 per polarization channel), the instantaneous bandwidth (to a maximum of 80 MHz), and the number of interferometers (permitting the correlation of practically all dish combinations, not just the 40 unique baselines).

Doubling the number of interferometers improved sensitivity, but it mainly facilitated an increase in dynamic range by a better determination of telescope errors (``self-calibration"). The 1990s have seen the last major upgrade: the multifrequency frontends (MFFEs), the extended digital backend (DZB), the pulsar machine (PuMa), new online software (TMS), refurbishing of the steelwork of the dishes, with increased instantaneous bandwidth (to 160 MHz) still in the offing.

If you want to read more, there is a book celebrating the first 25 years of the WSRT with all the details about the history of the WSRT and of radio astronomy in The Netherlands, see the full paper of Raimond Ernst published in The Westerbork Observatory, Continuing Adventure in Radio Astronomy (eds. E. Raimond & R. Genee, Kluwer Academic Publ. 1996)


Baars and Hooghoudt, 1974. A&A, 31, 323
Casse and Mller, 1974. A&A, 31, 333
Högbom and Brouw, 1974. A&A, 33, 289 Raimond and Genee (eds.), 1996. ``The Westerbork Observatory, Continuing Adventure in Radio Astronomy," Kluwer (Dordrecht)

See animation: de WSRT (NL)

 

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