Cassegrain

The Cassegrain telescope is a type of reflecting telescope that uses two mirrors to form an image. Light from a distant object strikes a concave primary mirror, is reflected to a convex secondary mirror, and is then directed back through a hole in the primary to an eyepiece or detector. This arrangement yields a long effective focal length in a relatively short tube, making the design compact for given magnification.

The design is named after the French astronomer Laurent Cassegrain, who described the concept in the 17th century. In a classical Cassegrain, the primary mirror is concave and the secondary mirror is convex. The light path runs from the front through the telescope, into the hole in the primary, and to the focal plane behind the primary. A central obstruction created by the secondary mirror reduces light throughput and can affect contrast, with typical obscuration percentages ranging from about 30% to 40% of the aperture.

Variants of the Cassegrain include the Schmidt-Cassegrain telescope, which adds a Schmidt corrector plate to correct spherical aberration over a wide field, and the Ritchey-Chrétien design, which uses hyperbolic primary and secondary mirrors to improve off-axis performance and reduce coma, making it a popular choice for large research telescopes. Other derivatives adjust mirror shapes or use additional corrective optics to tailor field curvature and aberrations.

Cassegrain-type telescopes are widely used in both amateur and professional astronomy due to their long focal lengths in compact tubes, suitability for high-m magnification, and compatibility with photographic and digital detectors. Limitations include central obstruction-related light loss and diffraction effects, which can influence image contrast and resolution.