INTERFEROMETRY AND MONOCHROMATIC IMAGING AT THE MARSEILLE OBSERVATORY

We first give a brief history of the astronomical observatory in Marseilles, which was founded in 1702. Then, we describe the first attempt to measure in this Observatory the angular diameter of stars by interferometry, in 1873–1874. Because the size of the remarkable Foucault telescope that was used by édouard Stéphan for this program was only 80 cm, none of the bright stars was resolved, and the upper limit to their diameters was given as 1/6 of an arc second. This result was however a very significant advance, as only fancy figures had been given previously for stellar diameters. The next incursion of the Marseilles Observatory in interferometry took place in 1911–1914, when Charles Fabry and Henri Buisson measured with the same telescope the radial velocity and the temperature of the Orion Nebula, using the Pérot-Fabry interferometer developed at the Marseilles University. After WW2, the Observatory experienced a complete renewal. Then Georges Courtès used interference filters to obtain deep photographs of HII regions, and Pérot-Fabry interferometers for measuring their radial velocities. We describe the very important instrumental advances realized for this program, in particular the focal reducers that allowed a considerable increase in sensitivity. The final result obtained by Courtès and his collaborators was a complete H survey of the Milky Way, which was the basis for a new description of the structure of our Galaxy, with four spiral arms, and a detailed H survey of the Magellanic Clouds. The distribution of HII regions in the closest galaxies was also observed and their velocity field determined. In 1963, Courtès built the first integral field spectrograph, based on an array of micro-lenses; it had a great success, so that similar instruments are mounted at the focus of the largest present and future telescopes.