An Etymological Dictionary of Astronomy and Astrophysics
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In an altazimuth-mounted telescope, a focal point to one side of the tube, created by placing a third deflecting mirror in the optical path. This extra mirror directs the beam along the altitude axis, and through a hole in the supporting trunnions. Nasmyth focus has the advantage of remaining at a fixed position relative to the telescope wherever the instrument is pointed. Moreover, bulky or heavy instruments can be mounted there on a permanent platform, which rotates only in azimuth.

Named after the inventor James Nasmyth (1808-1890), who first used it in the 19-th century.