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.
