The Würzburg antennas
Although the radio waves emitted by celestial bodies had been discovered at the beginning of the XXth century, their importance would only be recognized with the advent of radar during the Second World War. In fact, the first steps in radioastronomy were taken by the physicists and engineers specializing in radio waves.
From the war in the air, to the study of the cosmos
A certain number of large radars used by the German army were recycled by the first European radioastronomers. Three Würzburg-type antennas were the first large instruments of this discipline to be installed in France. One of them was installed at Meudon Observatory and used essentially for the study of the Sun, while two others were installed in a vast area specially bought for this purpose in the Cher, in the commune of Nançay.
These two 7m50 diameter antennas were adapted between 1957 and 1959, for equatorial mountings; each was on a platform which could move along two several hundred meters long rails arranged in the form of the letter T. Specially designed electronics enabled the two antennas to work together as an interferometer (a so-called adjustable interferometer because the two antennas could move) at a wavelength of 21 cm: the best angular resolution obtainable was 17,4 seconds of arc, limited by the distance separating the two antennas.
The birth of interferometry
The first computer installed at Meudon, an IBM 650, was used to solve numerous numerical problems.
From 1959 to 1962, the Nançay team worked under the aegis of the Paris Observatory; its director André Danjon having recognized quite rightly the importance of this new disciple. The team established the radio characteristics of certain discrete galactic radio sources (among them Sagittarius A) and also extragalactic ones (among them Cygnus A and Virgo A).
In a climate of intense international competition, the Würzburg antennas, pioneering instruments, showed the way to new interferometers and other types of huge radiotelescopes.
The three Würzburg antennas can still be seen. One has remained at the Nançay site, but is no longer used. Its twin has been restored to its original state and is now a key element of the radar museum of Douvres-la-Délivrande. The Meudon antenna was moved to Bordeaux Observatory: it still works and is used for teaching.
The large Nançay radiotelescope
At a time when radioastronomy was developing rapidly and needed increasingly large instruments, France equipped itself with a very large radiotelescope to study objects other than the Sun. With a surface area of 7000m2, the large Nançay radiotelescope, built in the years 1958 to 1966, was the equal of the largest instruments in the world at that time.
Once the discovery in 1955 in the radio spectrum of a line which constitutes an undeniable signature of neutral hydrogen (at a wavelength of 21cm) had been made, radioastronomers began to exploit this signature in all kinds of ways.
A wavelength of 21cm
Thanks to this emission line, it is possible to determine the distribution of hydrogen in galaxies, to « weigh » this hydrogen, and to estimate the total mass of galaxies, and, moreover, to assess the dynamics of galaxies, which constitutes a major step forward for cosmology.
Applying this instrument to study other wavelengths, such as the 18cm line of the OH radical, radioastronomers have been able to study relatively close celestial objects, such as highly evolved stars and comets. Such observations, made both a night and during the day, and whatever the weather, have enabled us to measure the gaseous activity of comets, even at times when their nuclei break up as they approach the Sun. And it has even been possible to study the structure and motion of matter around stars as they transform to planetary nebulae.
This instrument is also part of a world-wide network of 5 major radiotelescopes whose mission is to search for and monitor systematically pulsars, and in particular the hard to find millisecond pulsars.
A meridian telescope
The Kraus concept used for the large radiotelescope has enabled the telescope to have a very large surface area, at a cost, however of limited directionality. It is only possible to observe sources close to the meridian plane, the North-South plane. An initial flat, inclined mirror reflects the waves towards a fixed mirror, which is a portion of a sphere, which sends the waves to a focal point. A mobile « wagon » follows this focal point, and thus the point in the sky which is being observed, thereby compensating for terrestrial rotation.
These gigantic mirrors, 200 and 300 m long respectively, are built so well that their shape is accurate to within 5mm. They are made of a 12.5mm mesh, a size smaller than the wavelengths studied.