The Observatory at Meudon

The solar tower of Meudon

High resolution solar instruments are generally installed on the tops of towers; these have to be tens of metres high so that the instruments are above the most turbulent layers of the atmosphere.

Building of the solar tower at the Meudon Observatory began in 1963; it was commissioned in 1969. The 35m high building houses a vertical coudé telescope which has a 60cm mirror with a focal length of 45 m. On the roof, a set of two large flat mirrors, called a cœlostat, sends the light of the Sun towards the primary mirror of the telescope, which is in the lower part of the building. The beam is then "sent back", finally making a 42 cm wide image of the Sun at the focal point of the telescope.

Cœlostat and spectrograph

At this point, a large (14 m long) spectrograph enables the light to be dispersed into a spectrum with a resolution of 300000. For many years, Meudon’s solar tower was a reference for the measurement of the speeds of chromospheric structures, thanks to its subtracting double-pass system, installed in 1973.

The solar tower is still used today for scientific work; however, it is also an outstanding instrument for teaching the methods of solar physics. It is also an excellent platform for developing new instruments, and an ideal telescope for routine observations of solar magnetic activity; these latter are important for space meteorology, whose purpose is to protect us from the effects of solar eruptions.

The Meudon château

The existing château of Meudon is called the «château Neuf» (the «new château») in contrast to the «château Vieux» (the «old château») which was seriously damaged by fire in 1795 and was finally demolished in 1806.

The Château Neuf has had en eventful history. It was born as a small summer palace, designed by Le Primatice (1504-1570), an artist from Bologna: a pavilion overlooked a richly decorated artificial grotto, embellished with numerous fountains.

When Meudon became the Grand Dauphin’s royal residence in 1695, the grotto was replaced by a new château, and so the number of apartments increased. Jules Hardouin-Mansart (1646-1708) finished there one of his last creations.

From the pleasure grotto to the château Neuf

After the château Vieux, which had been taken over by the committee of public safety (the comité de Salut Public) for military uses, was destroyed following some unfortunate artillery experiments, the château Neuf became the residence of the King of Rome.

It was still an imperial residence under Napoléon III on the eve of the 1870 war.

With the encirclement of Paris and the retreat of the National Guard from Meudon to the Capital, the second bavarian corps established its quarters on the Meudon hills. The terrace, with its unequalled view, was transformed into a fortified battery. The park was changed radically and the trees were cut down. The inhabitants of Meudon were expelled and sent to Versailles.

Artillery battles

The artillery battles between the Bavarians and the Issy and Vanves fortresses did not affect the château. However, two days after the armistice, a fire ignited there. The fire lasted for three whole days. Finally, the gunners of the Commune launched a few projectiles towards the Château.

With the return of peace, and the fact that the walls of the château had survived relatively well, various projects for its restoration were proposed, but came to nothing. The park became the headquarters of the 3rd infantry division of the 2nd corps; the ruins were pillaged and completely abandoned.

What remained of the château Neuf was saved by the addition of an enormous dome housing a monumental astronomical refractor. Starting in1876, Jules Janssen had in effect already established his astrophysical observatory in a part of the park. The reconstruction of the building was entrusted to the architect Constant Moyeux. The dome itself was designed by Janssen and the Cail workshops. The work itself lasted ten years, from 1880 to 1890. The first observation with the large refractor took place on the 15th of December 1893.

The Meudon Observatory park

When Servien, superintendent of finances and baron of Meudon, bought the château perched above Meudon, in 1655, there was no park. He spent his fortune to buy the land, to create the large terrace held up by the magnificent wall which can still be seen today, to lay out the paths, to dig out the pools, right up to his death four year later.

French style gardens

Louvois bought the domain in 1679; he employed architects, gardeners, fountain designers etc. who had worked at Versailles, and in particular Le Nôtre who was asked to arrange the gardens and the pools, and to set up a complete hydraulic system to supply water for the decorative fountains. This was the apogee of French style gardens.

Enamoured by the marvellous gardens, by the magnificent layout, by the effects induced by the changes in height and by the view of Paris, Louis XIV bought the domain for his son in 1695. He planted tens of thousands of flowers and 8000 chestnut trees. The forest, traversed by alleyways wide enough for horse-drawn carriages, became a place where the Court could go and hunt.

Royal hunts

After the death of Louis XIV, the Royal domain was increasingly neglected. Louis XV and Louis XVI used it for hunting. The latter sold the lower gardens to the peasants. To separate the pleasure gardens from the hunting grounds, he built a wall, which is still there, and destroyed the water reservoirs and pools.
After 1800 the domain was divided up. Napoleon bought a part of the grounds ; he tore out the boxwood hedges and the old arbours so that his troops could parade on the large terrace.

In 1876 a part of the domain was given to the astronomer Janssen to create an observatory. In 1913 the domain was classed as a historical monument and registered as a protected site in 1930. This classification ensures the conservation of monuments and sites. In spite of the damage caused by the storms in 1990 and 1999, the forest still has a large number of very large trees, a significant variety of species, and is home to an important population of birds and insects.

Historical Monuments and protected areas

The ministry responsible for the environment, acting on a European directive (sand-chalk lawns) concerning protected zones with birds, insects and rare plants, declared in 2009 the «afforestations and prairies of the Meudon Observatory» to be a Nature Reserve for ecology, flora and fauna (Zone Naturelle d’Intérêt Ecologique Faunistique et Floristique).

The management of the Observatory, using the most appropriate methods, is doing its best to preserve this exceptional asset.

The Meudon spectroheliograph

Soon after the solar eclipse of August 18th 1868, Jules Janssen had the idea of selecting out the hydrogen Hα line (then called the C line). By isolating this narrow part of the optical spectrum, it becomes possible to see prominences in the chromosphere even without an eclipse.

In 1889, Henri Deslandres was integrated into the Observatoire de Paris in order to develop spectroscopy. Starting from one of Jules Janssen’s ideas, he built in 1892 an instrument which could register photographically images of the whole chromosphere, which were nevertheless limited to a few specific spectral lines. He had just invented the spectroheliograph – which had been invented independently by the American George Ellery Hale.

In 1897, Deslandres came to the Observatoire de Meudon, which he directed after the death of Janssen in 1907. He continued to work on chromospheric photography, and had built the existing spectroheliograph building, which was inaugurated in 1909.

A two mirror cœlostat sends the light of the Sun towards an objective lens placed in an opening in the southern wall of the building. This 25cm objective focusses an image on the entrance slit of the spectroheliograph.
A displacement of the objective lens causes the solar image to pass in front of the entrance slit. In time with this motion, a photographic plate slid behind a slit placed at the spectral line. The result was an image of the Sun taken at the given wavelength.

Since 2003, a CCD camera records the signal from a row of pixels placed along the spectral line. The set of signals recorded by the camera is then analyzed by software in order to reconstruct the monochromatic image of the chromosphere.
The images are made in the Hα line in order to see the filaments and prominences, but also in the calcium K line at 393.3nm in order to highlight the faculae. Another picture, taken in the wings of the calcium line, shows the brightest sunspots and faculae.

Starting from 1909, the Meudon spectroheliogram collection is the most important in the world. The systematic observation of the Sun is a service activity at the Observatoire de Paris.

The 1m telescope

The Meudon observatory is the fruit of Jules Janssen’s innovative ideas. He wished to create a centre for the physical study of celestial objects, using the then emerging techniques of photography and spectroscopy. Thus, in addition to its large refractor, the observatory was equipped with a 1 meter diameter photographic telescope.

Designed for the photography of dim objects

This revolutionary telescope had a focal length (the distance from the main mirror to the image) of only three meters. It was exceptionally luminous, and was designed for the photography of dim objects: nebulae, star clusters, and comets. It was remarkable in that it could be completely dismantled and taken on mission, but in fact it never was.

In front of the tube, a rigid mobile bar carried the photographic plate, while a small prism sampled the light and sent a fraction to an eyepiece for tracking during the exposure. First light was in 1891.

The dark green tube, the large wooden staircase, and the numerous varnished wood fittings in the dome, all exuded an air of comfort and calm. The dome, built by the Cail company, had four shutters furnishing a wide opening to the sky. This arrangement was later replaced by two horizontally opening shutters. A wooden shed housed a photographic laboratory, offices, and also rooms for the observers.

The polarization of light by planetary surfaces

By the end of the 19th century, the telescope had furnished large numbers of photographs of nebulae ; it was then used essentially for spectroscopy (the analysis of light by splitting it into its various colours), and in particular for cometary research.
Starting in 1930, Bernard Lyot studied the polarization of light from planetary surfaces using the T1m (the one meter telescope), and he changed its optics to make it more suitable for his work.

At the end of the sixties, Audouin Dollfus modernized the telescope by increasing its focal length to 22 m, and so make it more suitable for his polarimetric research. An auxiliary 32 cm refractor was attached to the tube, in addition to the existing large 16cm guide telescope.
Students working for the Observatory’s Master diploma now use the telescope for practical exercises in observation.