What optical design is used by the Hubble Space Telescope?

The Hubble Space Telescope uses a Ritchey–Chrétien reflector optical design.

This is a specialized form of Cassegrain reflector, using two hyperbolic mirrors rather than lenses to collect and focus light. The design provides excellent image quality across a relatively wide field of view, making it well suited to Hubble’s work in visible, ultraviolet, and near-infrared astronomy.

Hubble’s primary mirror is 2.4 meters across. Its optics were built to extremely demanding specifications because the telescope was intended to achieve diffraction-limited performance above Earth’s atmosphere. Soon after launch in 1990, engineers discovered that the primary mirror had been ground incorrectly, producing spherical aberration. Servicing Mission 1 in 1993 installed corrective optics and restored Hubble’s intended imaging capability.

The design is often confused with a Gregorian telescope, another two-mirror arrangement. The crucial distinction is that Ritchey–Chrétien systems use hyperbolic primary and secondary mirrors to reduce coma and improve off-axis images. Hubble is therefore both a Cassegrain reflector and, more specifically, a Ritchey–Chrétien reflector.