A Ritchey–Chrétien reflecting telescope with an aspheric concave primary mirror and a concave ellipsoidal secondary. It has a short focal ratio and wide field of view free from coma, spherical aberration, and astigmatism, but the field of view is curved and located within the instrument, making it unsuitable for visual work. The design is rarely encountered. It is named after the French optician André (Joseph Alexandre) Couder (1897–1979).
Subjects: Astronomy and Astrophysics.