Two methods for calibration and wavelength calculation for echelle spectrograms are discussed. The rough method employing ruler measurements on photographic enlargements has an accuracy of about 1 in 10<sup>5</sup>. The accurate method using comparator measurements of Th standard lines and a computer routine has given an accuracy of 2 in 10<sup>7</sup> and promises greater precision for spectrograms obtained with modern echelle gratings.
FRANK L. MOORE, JR. and BENJAMIN FURST, "Echelle Calibration and Wavelength Calculation," J. Opt. Soc. Am. 62, 762-766 (1972)