Abstract
A method for the accurate measurement of wavelength in echellegrams is described. It differs from previous methods in being applicable to a spectrograph with a continuously variable projection-length, such as a variable wavelength Littrow lens spectrograph. Any line in an echelle spectrum is characterized by the ratio l/L, where L is the distance between two arbitrary marks at opposite ends of the prism spectrograph slit, and l is the distance of the echelle line from one of them. By fitting a quadratic curve to the known spectrum of neon, the “unknown” wavelengths of a mercury-cadmium spectrum are derived with an accuracy of better than 1 in 106. It is shown that a linear curve is adequate for certain complex spectra such as the absorption spectrum of bromine. Allowance can be made for line shifts due to thermal, mechanical, or pressure causes.
© 1959 Optical Society of America
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