The Nançay Radio Observatory (in French: ''Station de Radioastronomie de Nançay''), opened in 1956, is part of
Paris Observatory
The Paris Observatory (french: Observatoire de Paris ), a research institution of the Paris Sciences et Lettres University, is the foremost astronomical observatory of France, and one of the largest astronomical centers in the world. Its hist ...
, and also associated with the
University of Orléans
The University of Orléans (french: Université d'Orléans) is a French university, in the Academy of Orléans and Tours. As of July 2015 it is a member of the regional university association Leonardo da Vinci consolidated University.
History
...
. It is located in the
department of
Cher
Cher (; born Cherilyn Sarkisian; May 20, 1946) is an American singer, actress and television personality. Often referred to by the media as the "Goddess of Pop", she has been described as embodying female autonomy in a male-dominated industr ...
in the
Sologne
Sologne (; ) is a natural region in Centre-Val de Loire, France, extending over portions of the departements of Loiret, Loir-et-Cher and Cher. Its area is about . To its north is the river Loire, to its south the river Cher, while the districts ...
region of
France
France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of overseas regions and territories in the Americas and the Atlantic, Pacific and Indian Oceans. Its metropolitan ar ...
. 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
Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an astronomical object was in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation comi ...
emerged after the
Second World War
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the World War II by country, vast majority of the world's countries—including all of the great power ...
, when experts and surplus equipment became available for civilian use. The
École Normale Superieure
École may refer to:
* an elementary school in the French educational stages normally followed by secondary education establishments (collège and lycée)
* École (river), a tributary of the Seine flowing in région Île-de-France
* École, Savo ...
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
Marcoussis () is a commune in the southern suburbs of Paris, France. It is located from the center of Paris.
Marcoussis is the location of the CNR (National Centre of Rugby) where the French national rugby union team prepare for internation ...
.

It was recognised that radio astronomy required a large, flat and remote site to accommodate
antennas
In radio engineering, an antenna or aerial is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver. In transmission, a radio transmitter supplies a ...
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
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from '' angular frequency''. Frequency is measured in hertz (Hz) which is ...
observed was 169 MHz (1.77 m
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...
).
After the discovery of the
21 cm line
The hydrogen line, 21 centimeter line, or H I line is the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation has a precise frequency of , w ...
in 1951 and the prospect of observing interstellar and extragalactic
line emission and
absorption, the need for more
sensitive radio telescopes
A radio telescope is a specialized antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the radio frequency ...
arose; their larger size would also deliver higher
angular resolution
Angular resolution describes the ability of any image-forming device such as an Optical telescope, optical or radio telescope, a microscope, a camera, or an Human eye, eye, to distinguish small details of an object, thereby making it a major det ...
. The plan for this "large radio telescope" was derived from a 1956 design by
John D. Kraus
John Daniel Kraus (June 28, 1910 – July 18, 2004) was an American physicist known for his contributions to electromagnetics, radio astronomy, and antenna theory. His inventions included the helical antenna, the corner reflector antenna ...
. This design made possible a large collecting area and high resolution, with only moderate need for moving parts. Disadvantages were the restriction to the
meridian
Meridian or a meridian line (from Latin ''meridies'' via Old French ''meridiane'', meaning “midday”) may refer to
Science
* Meridian (astronomy), imaginary circle in a plane perpendicular to the planes of the celestial equator and horizon
* ...
and the asymmetric angular resolution that would be much coarser in
altitude
Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...
than in
azimuth
An azimuth (; from ar, اَلسُّمُوت, as-sumūt, the directions) is an angular measurement in a spherical coordinate system. More specifically, it is the horizontal angle from a cardinal direction, most commonly north.
Mathematicall ...
. 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
Primary or primaries may refer to:
Arts, entertainment, and media Music Groups and labels
* Primary (band), from Australia
* Primary (musician), hip hop musician and record producer from South Korea
* Primary Music, Israeli record label
Work ...
and
secondary
Secondary may refer to: Science and nature
* Secondary emission, of particles
** Secondary electrons, electrons generated as ionization products
* The secondary winding, or the electrical or electronic circuit connected to the secondary winding i ...
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
In astronomy, a transit instrument is a small telescope with extremely precisely graduated mount used for the precise observation of star positions. They were previously widely used in astronomical observatories and naval observatories to measu ...
of the
Kraus-type
The Kraus-type radio telescope design was created by Dr. John D. Kraus (1910–2004).
Kraus-type telescopes are transit instruments, where the flat primary mirror reflects radio waves towards the spherical secondary mirror, which focuses it ...
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
Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...
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
Focal point may refer to:
* Focus (optics)
* Focus (geometry)
* Conjugate points, also called focal points
* Focal point (game theory)
* Unicom Focal Point, a portfolio management software tool
* Focal point review, a human resources process for ...
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
Meridian or a meridian line (from Latin ''meridies'' via Old French ''meridiane'', meaning “midday”) may refer to
Science
* Meridian (astronomy), imaginary circle in a plane perpendicular to the planes of the celestial equator and horizon
* ...
.
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
Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference aris ...
from the receiver and thereby to improve
sensitivity
Sensitivity may refer to:
Science and technology Natural sciences
* Sensitivity (physiology), the ability of an organism or organ to respond to external stimuli
** Sensory processing sensitivity in humans
* Sensitivity and specificity, statisti ...
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 A real time interferometry, interferometric autocorrelator is an electronic tool used to examine the autocorrelation of, among other things, optical beam intensity and spectral components through examination of variable beam path differences. ''See ...
spectrometer
A spectrometer () is a scientific instrument used to separate and measure spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomenon where th ...
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
A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...
to study their rotation, distance, clustering and movement. This includes galaxies obscured in visible light by the
Milky Way
The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked ey ...
, blue compact galaxies, galaxies of low surface brightness (in visible light), and
active galactic nuclei
An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a much-higher-than-normal luminosity over at least some portion of the electromagnetic spectrum with characteristics indicating that the luminosity is not prod ...
.
*
Pulsars
A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward Ea ...
, including pulse timing, distance, and the
interstellar medium
In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...
on the lightpath to Earth. Nançay is part of the
European Pulsar Timing Array
The European Pulsar Timing Array (EPTA) is a European collaboration to combine five 100-m class radio-telescopes to observe an array of pulsars with the specific goal of detecting gravitational waves. It is one of three pulsar timing array proje ...
* Stellar envelopes,
eruptive stars and
red giants.
* 18 cm OH emission and absorption in
comets
A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ...
to determine their loss rate of water and gas.
The radio heliograph

The heliograph is a T-shaped
interferometer
Interferometry is a technique which uses the '' interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber o ...
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
Angular resolution describes the ability of any image-forming device such as an Optical telescope, optical or radio telescope, a microscope, a camera, or an Human eye, eye, to distinguish small details of an object, thereby making it a major det ...
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
A solar flare is an intense localized eruption of electromagnetic radiation in the Sun's atmosphere. Flares occur in active regions and are often, but not always, accompanied by coronal mass ejections, solar particle events, and other sola ...
and
coronal mass ejections
A coronal mass ejection (CME) is a significant release of plasma and accompanying magnetic field from the Sun's corona into the heliosphere. CMEs are often associated with solar flares and other forms of solar activity, but a broadly accepted t ...
.
The Nançay observations complement simultaneous observations by space probes in visible and
ultraviolet
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiati ...
light and in
X rays
X-rays (or rarely, ''X-radiation'') are a form of high-energy electromagnetic radiation. In many languages, it is referred to as Röntgen radiation, after the German scientist Wilhelm Conrad Röntgen, who discovered it in 1895 and named it ' ...
.
The decametric array

The decametric array was constructed between 1974 and 1977. It consists of 144
spiral antennas, which are made from conducting cables spun in spiral curves around conical support structures. At their base the cones are 5 m in diameter and they are 9 m tall; they are inclined 20° to the South. The cones are spread over an area of about a hectare. Half the cones are coiled in the opposite sense than the other, permitting the distinction between left and right
circularly polarised
In electrodynamics, circular polarization of an electromagnetic wave is a polarization state in which, at each point, the electromagnetic field of the wave has a constant magnitude and is rotating at a constant rate in a plane perpendicular to t ...
radio waves. In each polarisation the collecting area is about 3500 m
2, equivalent to a 67 m diameter dish. The instrument is sensitive to wavelengths between 3 m and 30 m, which are the longest radio waves observable through the
ionosphere. The instrument is not an interferometer, but a
phased array
In antenna theory, a phased array usually means an electronically scanned array, a computer-controlled array of antennas which creates a beam of radio waves that can be electronically steered to point in different directions without moving t ...
. A single dish antenna for these long wavelengths would have to be infeasibly large. Further, a phased array can be re-pointed at a different direction of observation instantaneously by changing the electronic signal delays between the individual antennas.
The angular resolution is about 7° by 14°. The decametric array does not create images, but observes a single spectrum from the sky position observed and records its change with time. The two principal objects are the upper corona of the
Sun and the
magnetosphere of Jupiter
The magnetosphere of Jupiter is the cavity created in the solar wind by the planet's magnetic field. Extending up to seven million kilometers in the Sun's direction and almost to the orbit of Saturn in the opposite direction, Jupiter's magnetosp ...
, which have both been observed almost daily since 1977. The temporal changes of signals from the Sun and Jupiter are very rapid, so that at Nançay very fast receivers have been developed for these observations.
The Nançay observations of Jupiter complement the results from space missions like
Voyager
Voyager may refer to:
Computing and communications
* LG Voyager, a mobile phone model manufactured by LG Electronics
* NCR Voyager, a computer platform produced by NCR Corporation
* Voyager (computer worm), a computer worm affecting Oracle ...
and
Galileo.
LOFAR and NenuFAR
LOFAR
The Low-Frequency Array, or LOFAR, is a large radio telescope, with an antenna network located mainly in the Netherlands, and spreading across 7 other European countries as of 2019. Originally designed and built by ASTRON, the Netherlands Instit ...
consists of about 50 antenna arrays, or "stations", throughout Europe. These are connected by high-speed Internet link to a computer in the Netherlands. It is optimised for 110 MHz to 250 MHz (2.7 m to 1.2 m), but still has modest performance at 30 MHz to 80 MHz (10 m to 3.7 m).

NenuFAR (New Extension in Nançay Upgrading LOFAR) is a very low-frequency
phased arra
A phaser is an electronic sound processor used to filter a signal, and it has a series of troughs in its frequency-attenutation graph. The position (in Hz) of the peaks and troughs are typically modulated by an internal low-frequency oscill ...
optimised for the frequency range from 10 MHz to 85 MHz (30 m to 4 m). These are the longest radio waves not blocked by the
ionosphere. Early science operations should begin in 2019. The main scientific objectives are:
* detection and study of (magnetospheres of)
exoplanets
An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...
at radio wavelengths,
* detection of the epoch of formation of the first stars and galaxies some 100 million years after the
Big Bang
The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolution of the observable universe from t ...
, when the neutral atomic hydrogen was
reionised,
* the study of
pulsars
A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward Ea ...
, including spectroscopy, across the
Milky Way
The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked ey ...
, at low frequencies.
When complete, there will be 1938 antennas. Most will be in a core of 400 m diameter, but 114 antennas will be spread to up to 3 km distance.
NenuFAR will be a triple instrument:
* a radio telescope observing multiple positions simultaneously,
* an autonomous radio imager, building radio images of 1° resolution in seconds and of 10' in hours,
* a LOFAR "superstation", i.e. a large extension of the Nançay LOFAR station allowing the combination of NenuFAR and LOFAR to make radio images of sub-arcsecond resolution.
Other instruments and collaborations
In recent years and decades, projects of astronomical observation have become international co-operations, due to the necessary pooling of expertise and funding. In some cases, telescopes also extend across multiple countries. As such, developments at Nançay in the 21st century tend to be the provision of a site for parts of larger instruments, such as
LOFAR
The Low-Frequency Array, or LOFAR, is a large radio telescope, with an antenna network located mainly in the Netherlands, and spreading across 7 other European countries as of 2019. Originally designed and built by ASTRON, the Netherlands Instit ...
, and contribution of expertise to international collaborations such as LOFAR and the
Square Kilometre Array
The Square Kilometre Array (SKA) is an intergovernmental international radio telescope project being built in Australia (low-frequency) and South Africa (mid-frequency). The combining infrastructure, the Square Kilometre Array Observatory (SK ...
(SKA).
EMBRACE
Located at Nançay and
Westerbork,
EMBRACE (Electronic Multibeam Radio Astronomy Concept) is a prototype installation for phase 2 of the SKA. It is a phased array of 4608 antennas that operate between 900 MHy and 1500 MHz. These are sheltered in a 70 m
2 radio dome. With multiple beams, several sky locations can be observed at the same time.
ORFEES
ORFEES (Observation Radiospéctrale pour FEDOME et les Etudes des Eruptions Solaires) is a 5 m diameter antenna dedicated to space weather and prediction of solar flares. It observes the solar corona daily between 130 MHz and 1 GHz and can monitor the radio emission of the Sun in near real time.
CODALEMA
CODALEMA (Cosmic ray Detection Array with Logarithmic ElectroMagnetic Antennas) is a set of instruments to try and detect ultra-high energy
cosmic rays
Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our ow ...
, which cause cascades of particles in the atmosphere. These
air showers generate very brief electromagnetic signals that are measured in a wide frequency band from 20 MHz to 200 MHz. An array of about 50 antennas is spread over a large area of the site.
Monitoring antenna
An antenna, situated above the treetops on a 22 m high mast, has been monitoring the radioelectric quality of the Nançay site for 20 years. It allows to identify interference that affects the observations by the radio heliograph and the decametric array. The bands from 100 MHz to 4000 MHz are observed in their entirety and in multiple directions.
Pôle des Étoiles
The large radio telescope, a number of display panels about the observatory, and one or two of the heliograph antennas can be seen from the car park of the visitor centre ''Pôle des Étoiles''. During opening times, the visitor centre offers a permanent exhibition about astronomy and the work of the observatory. Once daily there is also a planetarium show and a guided tour of the large radio telescope and the radio heliograph.
See also
*
List of astronomical observatories
This is a list of astronomical observatories ordered by name, along with initial dates of operation (where an accurate date is available) and location. The list also includes a final year of operation for many observatories that are no longer in ...
*
List of radio telescopes
This is a list of radio telescopes – over one hundred – that are or have been used for radio astronomy. The list includes both single dishes and interferometric arrays. The list is sorted by region, then by name; unnamed telescopes are in r ...
References
Further reading
*
*
*
*
External links
*
Nançay Radio Observatory(official site, in French)
Pôle des Étoiles(official site of the visitor centre, in French)
{{DEFAULTSORT:Nancay Radio Telescope
Astronomical observatories in France
Low-Frequency Array (LOFAR)
Radio telescopes
Paris Observatory