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echelle grating(ay-shel ) A diffraction grating with very fine lines ruled much farther apart than in other gratings. Such a grating has very high resolution but only over a fairly narrow wavelength band. The resolving power at a given wavelength depends primarily on the total ruled width and the angles of incidence and refraction; a high resolution can be obtained with coarse ruling if the grooves are properly shaped (see blazed grating). The echelle grating of the echelle spectrograph is used at high-order diffraction to give larger angular dispersion. Since the high orders overlap, cross dispersion with, for example, a second grating must be used. The second grating is a standard grating with its direction of dispersion at right angles to that of the echelle; the spectral images of the different orders can then be separated and stacked above one another at the detector.
echelle grating[ā′shel ‚grād·iŋ]
A diffraction grating designed for use in high orders and at angles of illumination greater than 45° to obtain high dispersion and resolving power by the use of high orders of interference.