Cassegrain reflector telescope

Laurent Cassegrain proposed a reflecting telescope design in 1672 that would become the basis for most modern large telescopes, though he likely never built one himself. His innovation was placing a small convex secondary mirror in front of the primary mirror, reflecting light back through a hole in the primary to an eyepiece behind it. This folded the optical path, producing a compact instrument with a long effective focal length.

The design competed with Newton's reflecting telescope, which used a flat secondary mirror to deflect light to a side-mounted eyepiece. Newton's arrangement was simpler to construct; Cassegrain's produced less optical distortion and placed the eyepiece in a more convenient position.

Manufacturing the curved secondary mirror proved difficult with 17th-century techniques. Newton dismissed Cassegrain's design, and it languished for over a century. Only when reflecting telescope construction improved in the 19th century did the Cassegrain configuration's advantages become practical.

Modern variants dominate large telescope design. The Ritchey-Chrétien configuration, used by the Hubble Space Telescope and most professional observatories, is a Cassegrain derivative with hyperbolic mirrors that eliminate coma aberration. The Schmidt-Cassegrain, popular with amateur astronomers, adds a corrector plate for wide-field imaging.

The optical principle Cassegrain described—using a secondary mirror to fold the light path and create virtual focal lengths longer than the physical tube—proves more versatile than Newton's side-mounted eyepiece. The observer looks along the telescope's axis rather than perpendicular to it, and the secondary mirror can be shaped to correct aberrations that simpler designs cannot address.

Cassegrain's idea was centuries ahead of the manufacturing capability to realize it. The design survived as a theoretical curiosity until technology caught up—a reminder that the adjacent possible constrains invention not just by what can be conceived but by what can be built.