The French national astronomical research institute, based at the original site in Paris where it was founded in 1667. This is the oldest astronomical observatory still in use for research. There is an astrophysics section, located at the Observatoire de Meudon, just outside Paris, and a radio astronomy station at Nançay. Research is carried out in many branches of astronomy. At the Paris site, there are three nineteenth-century instruments, including the telescope built for the Carte du Ciel project and a 38-centimetre 15-inch refractor, which is occasionally used for positional work. Systematic astrometric measurements are made with a prismatic astrolabe. The Observatoire de Meudon was founded in 1876. It became the Astrophysics Section of the Observatoire de Paris in 1926 when the two institutions were merged. The instruments there include an 83-centimetre 33-inch refractor dating from 1893, a 1-metre 40-inch reflector, also from 1893 but modernized in 1969, a solar tower telescope used for spectroscopic studies of the Sun, a spectroheliograph and a large siderostat used in conjunction with a solar magnetograph and instruments to monitor the solar chromosphere. The Nançay radio astronomy station, established in 1953, is a large site with many instruments. Observations of solar radio emission are made with a radioheliograph, a multi-channel instrument for spectral observations of the Sun in the radio band and telescopes for monitoring solar activity. A special array is used for observations of the Sun and the planet Jupiter at wavelengths between 3 and 300 metres. The largest radio telescope at the site is of a unique design, consisting of two immense reflecting surfaces, one flat and one concave, which face each other. The flat reflector consists of ten panels, each 20 × 40 metres 65 × 130 feet, which can be rotated about a horizontal axis. The concave reflector is 300 metres 980 feet long and 35 metres 115 feet high. Radio signals are reflected from the plane reflector on to the concave reflector, from which they are brought to a focus and collected by receivers mounted in a movable cabin. This telescope is used for studies of the 21-centimetre emission from neutral hydrogen, emission from the hydroxyl OH molecule, and other work.