Nasmyth focus

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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.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006

Nasmyth focus

[′nā‚smith ‚fō·kəs]
(astronomy)
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.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
The instrument he built--later to be known as the Nasmyth telescope --had a 10-inch diameter speculum which he said was so brilliant that it made his friend William Lassell's "mouth water".
After a while the intention is to convert it to a Nasmyth--Cassegrain form, when it will be renamed the 'James Nasmyth Telescope'.