SchmidtCassegrain

Schmidt-Cassegrain telescope (SCT) is a catadioptric optical telescope that combines a Schmidt correcting plate with a Cassegrain-style light path. Light enters through the thin Schmidt plate at the tube's front, strikes a concave primary mirror, and is reflected to a small convex secondary mirror that is mounted on the inner surface of the corrector plate. The light is then reflected back through a central hole in the primary to the eyepiece or camera. The folded optical path yields a long effective focal length in a compact tube, which makes SCTs popular for both planetary and deep-sky observing and astrophotography.

Design and performance: The Schmidt corrector plate corrects spherical aberration of the primary, while the secondary magnifies and redirects the light to the focal plane. Typical SCTs have long focal ratios, commonly around f/10, though faster versions exist. Apertures range from about 3.5 inches (90 mm) to 14 inches (350 mm) in amateur models. The central obstruction improves rigidity and reduces length but slightly lowers contrast. The closed-tube design minimizes air currents, though thermal equilibration and dew on the corrector can affect performance.

History and use: The Schmidt-Cassegrain concept was commercialized in the mid-20th century and became widely adopted by mass-market manufacturers. Celestron introduced one of the first mass-produced SCTs (the C8) in the early 1970s, with Meade and others following. Today SCTs remain popular for their combination of portability, long focal length, and versatility in both visual astronomy and astrophotography. They require occasional collimation and proper cooling and dew control for best results.