Editing Reflecting telescope (section)

====Coudé====

Adding further optics to a Nasmyth-style telescope to deliver the light (usually through the [[declination]] axis) to a fixed focus point that does not move as the telescope is reoriented gives a ''coudé'' focus (from the French word for elbow).<ref name="The Coude Focus">{{cite web|url=http://mthamilton.ucolick.org/public/tele_inst/3m/coude.html|title=The Coude Focus}}</ref> The coudé focus gives a narrower field of view than a Nasmyth focus<ref name="The Coude Focus"/> and is used with very heavy instruments that do not need a wide field of view. One such application is high-resolution [[spectrograph]]s that have large collimating mirrors (ideally with the same diameter as the telescope's primary mirror) and very long focal lengths. Such instruments could not withstand being moved, and adding mirrors to the light path to form a ''coudé train'', diverting the light to a fixed position to such an instrument housed on or below the observing floor (and usually built as an unmoving integral part of the observatory building) was the only option. The [[60-inch Hale telescope]] (1.5 m), [[Hooker Telescope]], [[200-inch Hale Telescope]], [[Shane Telescope]], and [[Harlan J. Smith Telescope]] all were built with coudé foci instrumentation. The development of [[echelle grating|echelle]] spectrometers allowed high-resolution spectroscopy with a much more compact instrument, one which can sometimes be successfully mounted on the Cassegrain focus. Since inexpensive and adequately stable computer-controlled alt-az telescope mounts were developed in the 1980s, the Nasmyth design has generally supplanted the coudé focus for large telescopes.