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Nasmyth focus


Nasmyth focus
The Nasmyth foci of the Subaru Telescope
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 (1808–1890), best known for his invention of the steam hammer.


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   • OPTICS AND OPTICAL PHENOMENA
   • TELESCOPE EQUIPMENT AND TECHNIQUES


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