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.
