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Spectroscopy with the echelle

Vistas in Astronomy
Publication Date
DOI: 10.1016/0083-6656(55)90052-0


Abstract The echelle is a type of dispersing element intermediate in properties between the blazed plane diffraction grating and the reflection etalon of Michelson and Williams. Echelle spectrographs have now been produced in six forms which for many spectroscopic problems give advantages in speed, compactness, and stigmatic properties, and compete effectively with much larger instruments in resolving power and dispersion. An echelle is crossed with a small high-speed stigmatic prism or grating spectrograph for the production of cyclic spectra, giving the equivalent of a large concave grating instrument. A special advantage of the echelle spectrograph lies in its free spectral range, which is sufficient to reveal hyperfine structures and Zeeman patterns effectively without overlapping. Echelles are now produced by the Bausch and Lomb Optical Company by three methods, involving respectively ruling in deep aluminium layers on glass as with ordinary gratings, cutting laps in metal with a fairly precise tool room dividing engine and using these to grind into an optical flat the desired set of flat-faced grooves with appropriate procedures to average out periodic errors, and replicating in plastic of either ruled or lapped echelles. It has been found that wavelength precision of about one part in five million, intermediate between that given by the usual large concave gratings and that given by the reflection echelon or etalon, is obtainable with echelles.

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