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echelle grating
An echelle grating (from French, ''échelle'', meaning stairs or ladder) is a type of diffraction grating characterised by a relatively low groove density, but a groove shape which is optimized for use at a high incidence angle and therefore in high diffraction orders. Echelle gratings are, like other types of diffraction grating, used in spectrometers and similar instruments. They are most useful in cross-dispersed high resolution spectrographs, such as HARPS, PARAS (PRL Advanced Radial Velocity Abu Sky Search) and numerous other astronomical instruments.
== History ==
The concept of a coarsely-ruled grating used at grazing angles was discovered by Albert Michelson in 1898,〔A. A. Michelson, "(The Echelon Spectroscope )," ''Astrophysical Journal'' 8: 37-47 (1898)〕 where he referred to it as an "echelon". However, it was not until 1923 that echelle spectrometers began to take on the form that is now characteristic of an echelle spectrometer, in which the high-resolution grating is used in tandem with a crossed low-dispersion grating. This configuration was discovered by Nagaoka and Mishima〔H. Nagaoka and T. Mishima, "(A combination of a concave grating with a Lummer-Gehrcke plate or an echelon grating for examining fine structure of spectral lines )," ''Astrophysical Journal'' 57: 92-97 (1923)〕 and has been used in a similar layout ever since.
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