Nasmyth focus(nay -smith) Either of two fixed focal points on the altitude axis of a telescope with an altazimuth mounting. Light reflected from the secondary mirror of the telescope is diverted sideways by a third mirror mounted at 45° to the altitude axis. The light is brought to a focus at one of the two Nasmyth points on each side of the telescope, chosen by turning the third mirror. Bulky equipment can be mounted on observation platforms to use these foci. The arrangement was devised by James Nasmyth in the 1840s but came to prominence only in the late 20th century.
Nasmyth focus[′nā‚smith ‚fō·kəs]
One of two locations in a telescope with an altitude-azimuth mounting, located on the horizontal axis on either side of the telescope structure, to which light can be reflected to come to a focus; at such a focus there is usually an observing platform to carry instrumentation.