Building the 25m Dwingeloo antenna
In November 1951 was made the decision 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.
Construction and inauguration
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.
Start of observations
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 others 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.
Characteristic parameters of the telescope
Diameter: 25 m
Pointing accuracy: approximately 1 arcminute
Surface accuracy: 2 – 2.5 mm
Aperture efficiency: 0.64 ( = 18 or 21 cm)
0.40 ( = 6 cm)
Frontend receivers were available for 21 cm and for 18 cm wavelengths (and, upon request for 6 cm). Their parameters were:
System temperature: 36 K
1375-1425 MHz ( = 21 cm)
1580-1725 MHz ( = 18 cm)
Sensitivities (5 x rms noise) in 60 min integration time:
continuum, bandwidth 10 MHz: 20 mJy (2 mK)
line channel, 78 kHz wide: 150 mJy (17 mK)
As a backend a prototype of the Dwingeloo Autocorrelation Spectrometer (DAS), developed for the JCMT, was used. It had 1024 channels (if desired to be used with two IFs as 2*512 channels). It operated at overall bandwidths of 10, 5, 2.5 …. 0.067 MHz. If desired observations with a time resolution of 0.1 sec could be done.
Our 25-m Dwingeloo antenna came into operation in as the world’s largest radio astronomy antenna. In April 1956 the telescope was inaugurated by Her Majesty Queen Juliana.
Its main contribution was discovering the size, distribution of neutral hydrogen gas and its motions in our own Milky Way Galaxy.
Building the world’s most powerful radio imager for astronomy
After several years negotiation led by Jan Oort to build the Benelux Cross Antenna Project, Belgium finally decided to pull out of the project. It was 1963, and never one to abandon hope, Jan Oort led the project into its new incarnation, the Westerbork Synthesis Radio Telescope (WSRT). One year later, in 1964, we were already breaking ground to build the world’s most powerful radio imager for astronomy!
The Westerbork Synthesis Radio Telescope (WSRT) was designed and built.
WSRT begins operations
The most powerful radio telescope in the world
The WSRT began operations and for over a decade was the most powerful radio telescope in the world.
To mark the widening of the science made possible, the organization’s name was changed to the Netherlands Foundation for Radio Astronomy.
For the first time the neutral gas in other galaxies was brought into focus, the existence of large amounts of dark matter in galaxies was demonstrated and ultra-relativistic jets of out flowing plasma from galactic nuclei was observed (and now known to derive from massive black holes at galaxy centres).
Surveying the radio sky
A mere four years after first breaking ground, WSRT began surveying the radio sky.
Taking advantage of the WSRT
Jan Oort was quick to take advantage of the new instrument and his observations with WSRT showed that the well known X-ray source SCO-X1, was, in fact, three sources! It was the fine angular resolution of WSRT which made this observation possible.
Gas, stars and spiral arms
Jan Oort continued to make discoveries with WSRT, showing for the first time a radio image of the spiral arms of a galaxy. This meant that both gas and stars follow the structure of the spirals, but the gas goes much further from the centre than the stars.
Revolutionizing data processing
Jan Högbom, one of the pioneer developers of the WSRT, continues improvements with the invention of the CLEAN algorithm for making images using aperture synthesis. This was a revolution in data processing which was quickly adopted by all the major radio astronomy facilities in the world leading to a huge improvement in the quality of images from all aperture synthesis telescopes, with WSRT leading the way!
A WSRT image of 3C236 before and after subtracting the Point Spread Function (PSF) of the synthesised beam from the “dirty” image (top) to produce the sharp image (bottom) in this case of the Seyfert galaxy 3C236. Note the rings and radial spikes caused by the WSRT PSF.
Less dependent on Leiden
After it becomes possible to buy minicomputers, computer facilities in Groningen and Dwingeloo are updated to become less dependent on the mainframe computer in Leiden. Up until then, the WSRT relied on the IBM mainframe located at Leiden University for inspecting and calibrating observations.
Credit: IBM system 360-50 console – MfK Bern. Photograph by Sandstein [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
Approaching the first decade of operations with WSRT, a great wealth of expertise has accumulated at WSRT and its associated labs at Dwingeloo, Leiden, and Groningen. In particular, the aperture synthesis and computer software guru Wim Brouw publishes his book chapter “Aperture Synthesis” which becomes the standard reference for radio astronomers around the world.
The great "Fixing Project"
The years inflict their wear-and-tear on the WSRT and the reflector surface from the 25m antennas begin to separate from their frames. All the staff of WSRT and astronomers participated in the great “Fixing Project” to re-attach the reflector mesh of the telescopes. Over 175000 screws were installed in only 17 days!
The first digital correlator
Albert Bos, WSRT’s “Mr. Correlator” develops the first digital correlator using integrated circuit chips developed. This would be the first in a series to be used not only at WSRT, but at radio facilities worldwide.
First VLBI observations with WSRT
VLBI (Very-Long-Baseline-Interferometry) is a technique that uses an array of telescopes to study small radio sources. Much increased VLBI sensitivity quickly led to plans to phase up the WSRT to create the equivalent of a single telescope with a 93-metre diameter. Arnold van Ardenne succeeded in designing a first generation tied-array backend.
WSRT is on its own!
Software is developed which makes it possible to judge the overall quality of the observations with the WSRT on site. No more waiting for results from the labs in Dwingeloo, Leiden and Groningen; from 1980 onward the WSRT was on its own!
Credit: HP 1000 E-Series minicomputer. Photograph by Autopilot [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
Confirming earlier optical measurements
Radio images from WSRT of the galaxy NGC3198 show convincingly that Dark Matter dominates the galaxy, confirming earlier, less precise, measurements by optical telescopes.
Hydrogen Maser frequency standard
The Wideband VLBI-adding system and MKIII tape recorder using the DCB continuum correlator and new hydrogen maser. The hydrogen maser allowed the longer integration times essential for 6cm and shorter wavelength observations.
The Westerbork Northern Sky Survey (WENSS)
The Westerbork Northern Sky Survey (WENSS) begins, leading to a large catalog of sources used by many researchers around the world.
The Measurement Equation
WSRT radio astronomers Hamaker, Bregman, and Sault develop the Measurement Equation technique for calibrating data and removing interference coming from non astronomical sources, including from imperfections in the telescope and from man-made emissions. This is another leap forward in data processing since the invention of the CLEAN algorithm two decades earlier. The Measurement Equation is now the standard method at all radio observatories in the world.
Pulsar timing with PuMa
The WSRT enters the time domain with its first observations of pulsars using the newly installed Pulsar Machine (PuMa). The PuMa system analyses the data time stream from WSRT in order to count the pulses from these fast rotating neutron stars. The nearly perfect regularity of the spinning neutron stars make them appear as natural clocks allowing us to measure their orbits, and test the limits of Einstein’s General Theory of Relativity.
Multi Frequency Front End (MFFE)
The Multi Frequency Front Ends are installed at WSRT making a vast improvement in performance. Not only is the WSRT now more sensitve than ever, but it can change rapidly between receiving frequencies. A change in configuration which used to take days now only takes minutes!
Helping out NASA
The WSRT is used in an experiment to determine if the missing Mars Polar Lander (MPL) from NASA is still “alive”. Nothing has been heard from the lander since the start of the descent towards Mars.
After several experiments it is concluded that the MPL no longer broadcasts signals. A few years later NASA confirms that the lander has crashed on the Martian surface. A very successful experiment for the WSRT!
WSRT steals the show
WSRT is the star of the film “Discovery of Heaven” directed by Jeroen Krabbé, and based on the book by Harry Mulish. Many scenes are shot at WSRT, and the radio telescope can be said to steal the show!
Studying early-type galaxies
The WSRT Programme Committee decides to allocate a considerable amount of observing time to observe neutral hydrogen in a large sample of early-type galaxies, until then believed to be depleted of gas. The increased sensitivity and capabilities of the new MFFE receivers offer an ideal opportunity to begin studying this class of galaxies in detail.
Rotation Measure Synthesis
WSRT astronomers Michiel Brentjens and Ger de Bruyn invent the method of Rotation Measure Synthesis opening up a new view of the polarized sky. Polarized light tells us about magnetic fields in the Universe, which is an essential ingredient in astrophysical processes. We can’t understand the stars and galaxies without knowing about magnetic fields!
The new method of Brentjens and de Bruyn is more precise than any observations beforehand, and allows us to measure large areas of the sky in a single snapshot, while at the same time reducing the confusion from sources located in front and behind one another.
Preparing for the Low Frequency Array (LOFAR)
A Low Frequency Frontend is added and WSRT undertakes a survey of the sky at low frequencies. This is in preparation for the new Low Frequency Array (LOFAR) designed and built by ASTRON engineers with antennas distributed throughout The Netherlands and Europe. The WSRT equipped with the Low Frequency Frontend surveys the sky and finds the most appropriate location on the sky to later look for the first stars in the Universe with LOFAR.
Preparations start to suitably modify one of the telescopes of the WSRT array in order to support the early assessment of Galileo, the Global Navigation Satellite Service.
On the 12th of June 2010, the International LOFAR telescope was officially opened by Queen Beatrix of the Netherlands during a special ceremony. This ceremony took place in the central LOFAR area of about 400 hectare between Exloo and Buinen in the eastern part of Drenthe, the Netherlands.
In a multimedia presentation, scientists, politicians, captains of industry and local entrepreneurs involved in the project, explained how important the LOFAR project has been for them and will be in the future. Subsequently, the Queen, surrounded by a group of school children, officially inaugurated LOFAR by pushing a button which started the observations with the telescope. The observations resulted after a few seconds in the first official scientific results with this super radio telescope.
Representatives from consortia in France, Germany, the Netherlands, Sweden, and the United Kingdom then officially signed the memorandum that kicks off their scientific collaboration in LOFAR. The all-electronic, ‘next generation’ telescope, developed by ASTRON, now offers astronomers the joint use of a network of antennae that spreads from its core region in the northeast of the Netherlands to distances of a thousand kilometres across Europe.
LOFAR, the Low Frequency Array, is designed and built by ASTRON. The 25,000 antennas are spread over, eventually, 40 fields in the north of the Netherlands and in Germany, Sweden, France and England. Glass fibres connect the antennas with a supercomputer at the University of Groningen. In this way, a giant telescope is created with a diameter of one hundred to one thousand kilometres.
The giant telescope will enable scientists to study how distant galaxies take shape, to find out when the early Universe was first lit up, to probe the properties of energetic cosmic particles, to map magnetised structures all across the sky, and to monitor the sun’s activity as well as a wide range of variable and explosive celestial objects. LOFAR uses sophisticated computing and high speed internet to combine all the signals to survey the sky in great detail. It is a pathfinder for the development of a global telescope, the Square Kilometre Array (SKA).
The LOFAR sensor network is also used for research in the fields of geophysics, precision agriculture and ICT. While the antennas observe the Universe, underground sensors collect data about the structure of the Earth. These data contribute to better models for subsidence, water management and gas exploration.
The LOFAR project is financed by the BSIK agreement, by NWO, ASTRON, the Northern Netherlands Provinces (SNN), the European Union and the project partners. The total investment is about 100 million euro. The advanced glass fibre network is also being used by about sixty schools in the region for extremely fast internet access.
Astronomers and engineers are already exploring the possibilities for a successor of the LOFAR telescope: the Square Kilometre Array (SKA). The SKA will be a revolutionary radio telescope made of thousands of receptors linked together across an area the size of a continent. The total collecting area of all the receptors combined will be approximately one square kilometre, making the SKA the largest and most sensitive radio telescope ever built. The SKA will be constructed through a global cooperation. Thanks to innovative technologies developed by ASTRON and the experiences with LOFAR, ASTRON and the Dutch astronomical community play an important role in the realisation of the SKA.
Credits: Images Opening LOFAR: The queen officially inaugurated the LOFAR telescope on 12 June 2010. Credit: Hans Hordijk. SKA: artist impressions of the SKA. Credit: SPDO and Swinburne Astronomy Productions.
Verifying Einstein's Principle of Equivalence
After the discovery of a pulsar in a three body system, WSRT participates in a long campaign of high precision timing of the pulses. This project continues for over five years, leading to the most precise verification of Einstein’s Principle of Equivalence.
Einstein stated that the mass of a body measured by its reaction to gravity, or by its movement, must be the same. This has never been proved, but in 2018 WSRT astronomer Anne Archibald showed, using data from WSRT and other radio telescopes, that Einstein’s Principle of Equivalence is correct to within one part in millions!
Credit: Einstein in 1947. Photograph by Oren Jack Turner, Princeton, N.J. [Public domain]
Expanding field of view
After years of pioneering development by ASTRON engineers, the new technology of Phased Array Feeds are installed in 12 of the WSRT dishes. This system, called the Aperture Tile in Focus, (Apertif) greatly increases the amount of sky which can be imaged in a single snapshot.
WSRT begins a new era of discovery!