Cassegrainstyle

Cassegrainstyle refers to a family of folded-optics telescope configurations based on the Cassegrain design, named after Laurent Cassegrain, a 17th‑century French priest who described the idea. In this style, light from a primary mirror is reflected to a secondary mirror and then redirected through a hole in the primary to the eyepiece or camera, allowing a long effective focal length within a compact tube.

In the classical Cassegrain arrangement, the primary is a concave mirror (often parabolic) and the secondary is a convex mirror (typically hyperbolic). The light path travels from the front, reflects off the primary, then off the secondary, and exits through a central hole in the primary toward the focus behind the primary. Variants of the Cassegrain family optimize different aspects of performance. Schmidt-Cassegrain and Maksutov-Cassegrain designs add corrective optics at the entrance to improve wide-field performance or ease of manufacturing. Ritchey-Chretien is another widely used form, employing hyperbolic primary and secondary mirrors to reduce optical aberrations, and is common in professional telescopes such as space observatories. Gregorian-Cassegrain configurations, using a concave secondary, represent another related approach with different optical characteristics.

Advantages of Cassegrainstyle designs include a long focal length in a relatively short tube, and flexible back focus that benefits camera placement and instrumentation. Limitations include central obstruction from the secondary, which can reduce contrast and light throughput, along with potential diffraction effects and sensitivity to alignment and thermal conditions.

Cassegrainstyle configurations remain common in both amateur and professional astronomy, especially for planetary imaging, astrophotography, and long-focus applications. Common examples include Schmidt-Cassegrain and Maksutov-Cassegrain telescopes, as well as Ritchey-Chretien variants used in larger observatories.