Nasmyth focus


Nasmyth focus. A third mirror in the optical system of a telescope is used to bring the light to a focus at F, where in practice it is possible to mount heavy equipment on a platform forming part of the altazimuth mounting of a large telescope.
A focal point to one side of the tube of an altazimuth-mounted reflecting telescope, created by placing a third deflecting mirror in the optical system. This extra mirror is arranged to direct the beam along the altitude axis, and through a hole in the supporting trunnions. It was first used by the inventor James Nasmyth in the nineteenth century.
With the new generation of large altazimuth telescopes, made possible by computer control of the drive mechanism, the use of the Nasmyth focus has been revived. It has the advantage of remaining at a fixed position relative to the telescope wherever the instrument is pointed, and bulky or heavy instruments can be mounted there on a permanent platform, which rotates only in azimuth. In practice, there are two possible Nasmyth foci, one either side of the telescope tube. In the William Herschel Telescope, for example, the Nasmyth flat can be motor-driven into position to deflect the light beam to either of the Nasmyth foci.

See also: altazimuth mounting.