Nasmyth focus

Nasmyth focus

The Nasmyth foci of the Subaru Telescope.

A Nasmyth focus is a focal point to one side of the tube of a telescope with an altazimuth mount. It is formed by placing a third mirror (tertiary) so as to direct the beam along the altitude axis and through a hole in the supporting trunnions.


Nasmyth foci enable bulky instruments to be mounted on a permanent platform that needs to rotate only in azimuth; they are commonly used with large modern telescopes, especially for spectrographic work. Named after the Scottish engineer James Nasmyth, best known for his invention of the steam hammer.