The principle of tandem (series) arrangements of spectroscopic elements—used with prisms in 1861 by Kirchhoff and Bunsen, later applied to Fabry–Perot étalons, and more recently to concave gratings in the soft x-ray domain—has been successfully extended to high-resolution gratings and echelles. For example, when two gratings are used in series near autocollimation, the resolving power is doubled as compared with a single element at the same angle. Moreover, for a given linear dispersion, the luminosity may be increased up to a factor of 4 (for perfect reflectivity and blaze), since the doubled angular dispersion within a constant aperture permits the reduction of focal length by a factor of 2. The advantages pointed out here also characterize multiple-pass arrangements, first described for prisms by Wadsworth in 1895 and again emphasized by Couderc, Jacquinot, and Walsh, and for gratings by Hulthén, Rank, and others. These advantages indicate the possibility of extending multiple-pass arrangements to the use of echelles when the additional mirror reflection is acceptable. Design characteristics for tandem use of gratings in monochromators and for tandem use of high-order gratings (echelettes, echelles) in broad-range spectrograph arrangements, as well as experimental results, are presented. The single-echelette (echelle) spectrographic arrangement which is basic to the tandem extension is that first suggested by Shane and mounted, crossed with a prism, by Wood in 1947, and similarly mounted, crossed with a grating, by Harrison, Archer, and Camus in 1952. In the tandem arrangement only simple geometrical alignment, with no phase adjustment between the two gratings, is required, unlike the mounting for grating mosaics with several coplanar gratings.
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