Editing Optical spectrometer (section)

==Spectrographs==
[[Image:simple spectroscope.jpg|right|thumb|200px|A very simple spectroscope based on a prism]]
[[File:The KMOS spectrograph before shipping to Chile.jpg|thumb|The [[K-band Multi-Object Spectrograph|KMOS]] spectrograph.<ref>{{cite news|title=Powerful New VLT Instrument Arrives in Chile|url=http://www.eso.org/public/announcements/ann12071/|access-date=11 October 2012|newspaper=ESO Announcement}}</ref>]]
[[File:Solar Spectrograph 2, Ondřejov Astronomical.jpg|thumb|Horizontal Solar Spectrograph at the Czech Astronomical Institute in Ondřejov, Czech Republic]]

A spectrograph is an instrument that separates light into its wavelengths and records the data.<ref name=spie>{{cite web| url = http://spie.org/x32350.xml| title = Spectrometer, Spectroscope, and SpectrographExcerpt from Field Guide to Spectroscopy}}</ref> A spectrograph typically has a multi-channel detector system or camera that detects and records the spectrum of light.<ref name=spie/><ref>{{GoldBookRef |title=spectrograph |file=S05836 }}</ref>

The term was first used in 1876 by [[Henry Draper|Dr. Henry Draper]] when he invented the earliest version of this device, and which he used to take several photographs of the spectrum of [[Vega]]. This earliest version of the spectrograph was cumbersome to use and difficult to manage.<ref>{{citation | title = Memoir of Henry Draper, 1837-1882 | author = George Barker | page = 103 | url = http://www.nasonline.org/publications/biographical-memoirs/memoir-pdfs/draper-henry.pdf}}</ref>

There are several kinds of machines referred to as ''spectrographs'', depending on the precise nature of the waves. The first spectrographs used [[photographic paper]] as the detector. The plant pigment [[phytochrome]] was discovered using a spectrograph that used living plants as the detector. More recent spectrographs use electronic detectors, such as [[Charge-coupled device|CCD]]s which can be used for both visible and [[ultraviolet|UV]] light. The exact choice of detector depends on the wavelengths of light to be recorded.

A spectrograph is sometimes called [[polychromator]], as an analogy to [[monochromator]].

===Stellar and solar spectrograph=== 
The star [[stellar classification|spectral classification]] and discovery of the [[main sequence]], [[Hubble's law]] and the [[Galaxy morphological classification|Hubble sequence]] were all made with spectrographs that used photographic paper. [[James Webb Space Telescope]] contains both a near-infrared spectrograph ([[NIRSpec (Near-Infrared Spectrograph)|NIRSpec]]) and a mid-infrared spectrograph ([[MIRI (Mid-Infrared Instrument)|MIRI]]).

===Echelle spectrograph===
An [[echelle grating|echelle]]-based spectrograph uses two [[diffraction grating]]s, rotated 90 degrees with respect to each other and placed close to one another. Therefore, an entrance point and not a slit is used and a CCD-chip records the spectrum. Both gratings have a wide spacing, and one is [[Blazed grating|blazed]] so that only the first order is visible and the other is blazed with many higher orders visible, so a very fine spectrum is presented to the CCD.

===Slitless spectrograph===
In conventional spectrographs, a slit is inserted into the beam to limit the image extent in the dispersion direction.  A [[slitless spectrograph]] omits the slit; this results in images that [[convolution|convolve]] the image information with spectral information along the direction of dispersion.  If the field is not sufficiently sparse, then spectra from different sources in the image field will overlap.  The trade is that slitless spectrographs can produce [[spectral imaging|spectral images]] much more quickly than scanning a conventional spectrograph.  That is useful in applications such as [[solar physics]] where time evolution is important.