Journal of Astronomical History and Heritage, 24(2), 247– 284 (2021). INTERFEROMETRY AND MONOCHROMATIC IMAGING AT THE MARSEILLE OBSERVATORY Yvon Georgelin Observatoire de Marseille/LAM 38 rue Frédéric Joliot-Curie, 13013 Marseille, France E-mail:
[email protected] and James Lequeux LERMA, Observatoire de Paris-PSL-Sorbonne Université, 61 Avenue de l’Observatoire, 75014 Paris, France E-mail:
[email protected] ABSTRACT: 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 at 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 were resolved, and the upper limit to their diameters was given as 1/6 of an arcsecond. This result was however a very significant advance, as only fancy figures had been given previously for stellar diameters. The next incursion in interferometry of the Marseille Observatory 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 Marseille University. After World War II, the Observatory underwent 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.