The Nançay Radio Observatory (in French: Station de Radioastronomie de Nançay), opened in 1956, is part of Paris Observatory, and also associated with the University of Orléans. It is located in the department of Cher in the Sologne region of France. The station consists of several instruments. Most iconic of these is the large decimetric radio telescope, which is one of the largest radio telescopes in the world. Long established are also the radio heliograph, a T-shaped array, and the decametric array operating at wavelengths between 3 m and 30 m.
History Radio astronomy emerged worldwide after the Second World War, when radar experts and surplus equipment became available for civilian use. The physics department of the École Normale Superieure was given three 7.5 m diameter Würzburg Riese that the British had seized from the Germans during the war. These were initially deployed at a research centre of the French navy at Marcoussis.
It was recognised that radio astronomy required a large, flat and remote site to accommodate antennas spread over distances of 1.5–2 km or of considerable size, and to avoid unwanted radio waves from human technology. A 150 ha plot of woodland near Nançay became available and was purchased in 1953. Initially, various small instruments – single dishes and interferometers – were installed. 6 m wide railway tracks, one running east–west and one north–south were constructed, which would carry the equatorially mounted 40 t Würzburg antennas. A predecessor to the current heliograph had 16 antennas of 5 m diameter spread equally along a 1500 m long east–west baseline, while eight antennas of 6 m in diameter were aligned north–south. The frequency observed was 169 MHz (1.77 m wavelength). After the discovery of the 21 cm line in 1951 and the prospect of observing interstellar and extragalactic line emission and absorption, the need for more sensitive radio telescopes arose; their larger size would also deliver higher angular resolution. The plan for this "large radio telescope" was derived from a 1956 design by John D. Kraus. This design made possible a large collecting area and high resolution in one direction, with only moderate need for moving parts. Disadvantages were the restriction to the meridian and the asymmetric angular resolution that would be much coarser in altitude than in azimuth. The altitude control initially proved very difficult.
The large radio telescope
The large radio telescope (in French: le Grand Radiotélescope, or affectionately le Grand Miroir) was constructed between 1960 and 1965. Initially, only the central 20% of the primary and secondary mirrors were erected as a proof of concept. The mirrors were extended to their full, current size in 1964 and the telescope was officially opened in 1965 by Charles de Gaulle. Scientific observations began in 1967. The large radio telescope is a transit telescope of the Kraus-type design. The primary mirror at the north end of the installation is a planar mirror measuring 200 m in width and 40 m in height. This is tiltable to adjust to the altitude of the observed object. It consists of five 20 m wide segments, each of 40 t mass. The radio waves are reflected horizontally into the secondary mirror 460 m to the South. The shape of the secondary is that of a segment of a sphere 300 m wide and 35 m high. The secondary reflects the radio waves back into its focal point 280 m to its North and about 60% the distance back to the primary. A cabin with further mirrors and the receiver is located at the focus. During an observation, the cabin is moved west to east to track the observed object for about an hour around its transit through the meridian. The primary and secondary mirrors are formed by metal wire mesh with holes of 12.5 mm. The reflecting surfaces are accurate to 4 mm, permitting use at wavelengths upwards of about 8 cm. The telescope is thus designed for decimeter waves, including the 21 cm spectral line of neutral atomic hydrogen (HI) and the 18 cm spectral line of the OH radical. The radio wave detector is cooled to 20 K to reduce noise from the receiver and thereby to improve sensitivity to the celestial radiation. The large radio telescope observes at frequencies between 1.1 GHz and 3.5 GHz, continuum emission as well as spectral emission or absorption lines. The autocorrelator spectrometer can observe eight spectra at different frequencies with 1024 channels each and a spectral resolution of 0.3 kHz. The instrument is particularly suited to large statistical surveys and the monitoring of objects of variable brightness. Observational projects include:
21 cm HI emission of galaxies to study their rotation, distance, clustering and movement. This includes galaxies obscured in visible light by the Milky Way, blue compact galaxies, galaxies of low surface brightness (in visible light), and active galactic nuclei. Pulsars, including pulse timing, distance, and the interstellar medium on the lightpath to Earth. Nançay is part of the European Pulsar Timing Array Stellar envelopes, eruptive stars and red giants. 18 cm OH emission and absorption in comets to determine their loss rate of water and gas.
The radio heliograph
The heliograph is a T-shaped interferometer made up of equatorially mounted antennas of several metres (mostly 5 m) diameter. 19 antennas are located on an east–west baseline 3.2 km long, 25 antennas are on a north–south baseline 2.5 km long. The instrument observes the Sun seven hours a day to produce images of the corona in the frequency range 150 MHz to 450 MHz (wavelengths of 2 m to 0.67 m). The angular resolution is then similar to that of the naked eye in visible light. Up to 200 images per second can be taken. This allows the systematic study of the quiet corona, solar flares and coronal mass ejections. The Nançay observations complement simultaneous observations by space probes in visible and ultraviolet light and in X rays.
The decametric array
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