Nasmyth focus | |
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. |