The use of two separated diffraction gratings or prisms doubles the spectroscopic resolution compared to that obtained with a single equivalent grating or prism. Ultra-high resolution is possible with widely spaced gratings. Varying the spacing between the two gratings or prisms produces useful modulation in the spectrum.
© 1980 Optical Society of America
Robert C. Hilborn, "Increased spectroscopic resolution with separated gratings and prisms," J. Opt. Soc. Am. 70, 126-128 (1980)