RitcheyChrétien

Ritchey-Chrétien is a two-mirror reflecting telescope design developed in the early 20th century by Georges Ritchey and Henri Chrétien. It is a Cassegrain-type configuration that uses a hyperbolic primary mirror and a hyperbolic secondary mirror to correct optical aberrations, most notably coma, over a relatively wide field of view. The design is widely adopted in professional astronomy for producing sharp images across a broad, coma-free field.

Optical properties and implementation: By employing hyperbolic shapes for both mirrors, the Ritchey-Chrétien telescope achieves a wide, coma-free field with reduced spherical aberration. However, the focal plane is inherently curved, so detectors or cameras often require field-flattening optics or a curved detector surface. Typical f-numbers for RC ground-based telescopes range roughly from f/8 to f/15, and the design is favored for large-aperture systems and for space telescopes due to its high image quality over a wide field.

History and notable examples: The Ritchey-Chrétien concept was developed independently by Ritchey and Chrétien in the 1910s–1920s. It has become a dominant design for many modern professional telescopes, including the Hubble Space Telescope, whose mirrors are hyperbolic, and several large ground-based facilities such as the Keck and VLT units. The design remains common in contemporary astronomy where wide, sharp fields are required for imaging and survey work.