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An optical instrument that consists of an entrance slit, collimator, disperser, camera, and detector and that produces and records a spectrum. A spectrograph is used to extract a variety of information about the conditions that exist where light originates and along the paths of light. It reveals the details that are stored in the light's spectral distribution, whether this light is from a source in the laboratory or a quasistellar object a billion light-years away.

Spectrograph design takes into account the type of light source to be measured, and the circumstances under which these measurements will be made. Since observational astronomy presents unusual problems in these areas, the design of astronomical spectrographs may also be unique.

Astronomical spectrographs have the same general features as laboratory spectrographs (see illustration). The width of the entrance slit influences both spectral resolution and the amount of light entering the spectrograph, two of the most important variables in spectroscopy. The collimator makes this light parallel so that the disperser (a grating or prism) may properly disperse it. The camera then focuses the dispersed spectrum onto a detector, which records it for further study.

Basic optical components of a spectrographenlarge picture
Basic optical components of a spectrograph

Laboratory spectrographs usually function properly only in a fixed orientation under controlled environmental conditions. By contrast, most astronomical spectrographs are used on a moving telescope operating at local temperature. Thus, their structures must be mechanically and optically insensitive to orientation and temperature.

The brightness, spectral characteristics, and geometry of laboratory sources may be tailored to experimental requirements and to the capabilities of a spectrograph. Astronomical sources, in the form of images at the focus of a telescope, cannot be manipulated, and their faintness and spectral diversity make unusual and difficult demands on spectrograph performance.

Typical laboratory spectrographs use either concave gratings, which effectively combine the functions of collimator, grating, and camera in one optical element, or plane reflection gratings with spherical reflectors for collimators and cameras.

Optical path in spectrographclick for a larger image
Optical path in spectrograph


(spek -trŏ-graf, -grahf) An optical instrument used in separating and recording the spectral components of light or other radiation. Spectrographs are a major astronomical tool for analysing the emission and absorption spectra of stars and other celestial objects. They are used with reflecting (or refracting) telescopes, usually mounted at the Cassegrain focus. If the telescope has a coudé or Nasmyth facility a spectrograph can be permanently positioned at these foci: such spectrographs are used for high-dispersion work. The dispersion of a spectrograph is the linear separation of spectral lines per unit wavelength difference, often quoted in millimeters per nanometer or per angstrom. The wavelength range over which the instrument will operate depends on the recording medium, and also on the optical elements of the device, but is generally in the region 300–1300 nanometers – i.e. light, near-infrared, and near-ultraviolet wavelengths.

The radiation is focused on and enters the instrument through a narrow rectangular slit. The diverging beam is made parallel by a collimator – a converging mirror or lens – and falls on or passes through a diffraction grating or prism. The beam is thus split into its component wavelengths. This spectrum is focused on a photographic plate or an electronic imaging device. Large quantities of digital information from highly sensitive electronic devices, such as CCD detectors, can be fed into a computer for rapid analysis and manipulation. The chromatic resolution or resolving power of a spectrograph is a measure of the detectable separation of wavelengths that are very nearly equal. It is given by the ratio λ/δλ, if at a wavelength λ it is just possible to distinguish between two spectral lines of wavelength difference δλ. In a large spectrograph, such as that in the illustration, there is often a choice of mirrors, of different focal lengths, for focusing the spectral image on the recording medium and also a choice of diffraction gratings.



a spectroscopic device in which a radiation detector records virtually simultaneously the entire spectrum that is spread out in the focal plane of the optical system. The radiation detectors in spectrographs may be photographic materials, multicomponent photodetectors, or image tubes. If the recording device is suited for the study of spectra that vary rapidly with time, then the spectrographs are known as fast spectrographs. In Soviet usage, fast spectrographs are divided, according to their design, into kinospectrographs, spectrochronographs, and chronospectrographs.


A spectroscope provided with a photographic camera or other device for recording the spectrum.
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