The astronomical photography of the sky had become good enough that one could envisage mapping the entire sky.  This was the objective of the Carte du Ciel project, initiated by Mouchez. To do this, the Observatory’s opticians, the brothers Paul and Prosper Henry (1848-1905 and 1849-1903, respectively) built a prototype equatorial photographic refractor, which was reproduced many times to equip the French and foreign observatories participating in the project. But the project that would occupy the Observatory for years was never terminated. The photographic plates are of considerable interest for the determination of the proper motions of stars, by comparing their positions with those from modern observations.

caption : Equatorial of the Carte du Ciel - credits : Observatoire de Paris