Editing Reflecting telescope (section)

==Focal planes==

=== Prime focus ===

[[File:Prime focus telescope.svg|right|thumb|250px|A prime focus telescope design. The observer/camera is at the focal point (shown as a red X).]]

In a ''prime focus'' design no secondary optics are used, the image is accessed at the [[Focus (optics)|focal point]] of the [[primary mirror]]. At the focal point is some type of structure for holding a film plate or electronic detector. In the past, in very large telescopes, an observer would sit inside the telescope in an "observing cage" to directly view the image or operate a camera.<ref>{{cite book|author=W. Patrick McCray|title=Giant Telescopes: Astronomical Ambition and the Promise of Technology|url=https://books.google.com/books?id=9mUL9dGn9V4C&pg=PA27|year=2004|publisher=Harvard University Press|isbn=978-0-674-01147-2|page=27}}</ref> Nowadays [[Charge-coupled device|CCD]] cameras allow for remote operation of the telescope from almost anywhere in the world. The space available at prime focus is severely limited by the need to avoid obstructing the incoming light.<ref>{{cite web|url=http://www.ucolick.org/public/telescopes/shane.html#prime|title=Prime Focus}}</ref>

[[Radio telescope]]s often have a prime focus design. The mirror is replaced by a metal surface for reflecting [[radio wave]]s, and the observer is an [[Antenna (electronics)|antenna]].
{{See also|Schmidt camera}}

=== Cassegrain focus ===
[[File:Cassegrain-Teleskop.svg|250px|thumb|right|Cassegrain design]]
For telescopes built to the Cassegrain design or other related designs, the image is formed behind the primary mirror, at the focal point of the [[secondary mirror]]. An observer views through the rear of the telescope, or a camera or other instrument is mounted on the rear. Cassegrain focus is commonly used for amateur telescopes or smaller research telescopes. However, for large telescopes with correspondingly large instruments, an instrument at Cassegrain focus must move with the telescope as it slews; this places additional requirements on the strength of the instrument support structure, and potentially limits the movement of the telescope in order to avoid collision with obstacles such as walls or equipment inside the observatory.

=== Nasmyth and coudé focus ===

[[Image:Nasmyth-Telescope.svg|right|thumb|250px|Nasmyth/coudé light path.]]

====Nasmyth====

{{Main|Nasmyth telescope}}

The Nasmyth design is similar to the Cassegrain except the light is not directed through a hole in the primary mirror; instead, a third mirror reflects the light to the side of the telescope to allow for the mounting of heavy instruments. This is a very common design in large research telescopes.<ref>{{cite book|author=Geoff Andersen|title=The Telescope: Its History, Technology, and Future|url=https://archive.org/details/telescopeitshist00ande|url-access=registration|year=2007|publisher=Princeton University Press|isbn=978-0-691-12979-2|page=[https://archive.org/details/telescopeitshist00ande/page/103 103]}}</ref>

====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.

===Fibre-fed spectrographs===

For instruments requiring very high stability, or that are very large and cumbersome, it is desirable to mount the instrument on a rigid structure, rather than moving it with the telescope. Whilst transmission of the full field of view would require a standard coudé focus, spectroscopy typically involves the measurement of only a few discrete objects, such as stars or galaxies. It is therefore feasible to collect light from these objects with [[optical fiber]]s at the telescope, placing the instrument at an arbitrary distance from the telescope. Examples of fiber-fed spectrographs include the planet-hunting spectrographs [[High Accuracy Radial Velocity Planet Searcher|HARPS]]<ref name="HARPS description">{{cite web|url=https://www.eso.org/sci/facilities/lasilla/instruments/harps/inst/description.html|title=HARPS Instrument Description}}</ref> or [[ESPRESSO]].<ref name="ESPRESSO description">{{cite web|url=https://www.eso.org/sci/facilities/develop/instruments/espresso.html|title=ESPRESSO Instrument Description}}</ref>

Additionally, the flexibility of optical fibers allow light to be collected from any focal plane; for example, the HARPS spectrograph utilises the Cassegrain focus of the [[ESO 3.6 m Telescope]],<ref name="HARPS description" /> whilst the Prime Focus Spectrograph is connected to the prime focus of the [[Subaru Telescope|Subaru telescope]].<ref name="PFS Instrumentation">{{cite web|url=http://pfs.ipmu.jp/instrumentation.html|title=Subaru PFS Instrumentation}}</ref>