coudé focus telescope

A coudé focus telescope is a Nasmyth telescope (reflector telescope with a tertiary mirror reflecting the light sideways) with additional optics to bring the light to a stationary focal plane. The term coudé focus refers to the position of the stationary focal plane location it provides. The purpose is to eliminate the need to move unwieldy (large and/or heavy) instruments any time the telescope is repointed: the instrument can sit motionless next to the telescope. Large research telescopes may have the option to switch to a coudé focus when their largest instruments are used. Examples:

• 60-inch Hale Telescope (aka 1.5m Hale Telescope, Mount Wilson Observatory, California).
• Hooker Telescope (100 inches, Mount Wilson Observatory).
• Hale Telescope (200 inches, Palomar Observatory).
• Shane Telescope (Lick Observatory).
• Harlan J. Smith Telescope (McDonald Observatory, Texas).

The Nasmyth design is common for bulky instruments on large telescopes using an altazimuth mount and the coudé design for those using an equatorial mount.

The word coudé is French for "elbow": the design has (at least) one more bend in the light path than typical.