Abstract

Echelle spectrographs produce a two-dimensional array of spectral lines, and therefore present a new problem in the determination of wavelengths. One approach to this problem consists of delineation of the spectrum into its component cyclic orders and the subsequent identification of an unknown spectral line in terms of position within its order cycle, using the spectrographic optical axis as a fiduciary line. A system has been developed for determining the wavelengths of spectral lines in an echellogram produced on the Littrow echelle spectrograph to an accuracy on the order of 1 part in 100 000, an accuracy which is more than sufficient for routine line identification. The principles of the method are applicable to wavelength identification in any two-dimensional spectrum, and additional effort will make possible improvement in wavelength precision. Three methods of applying this system are described as follows:

  • 1. An order grid is superposed on the unknown spectrum and wavelengths are determined by arithmetical interpolation between known lines in the spectrum. A conventional spectrum viewer is used. Each order containing an unknown spectrum line to be determined must also contain at least two lines whose wavelengths are accurately known. For high accuracy, second-order interpolation is necessary.
  • 2. An order grid containing locus lines and optic axis line is positioned on the unknown spectrum. A conventional spectrum viewer is used in conjunction with mental arithmetical interpolation to determine the wavelength of an unknown spectrum line.
  • 3. An order grid containing locus lines and optic axis line is positioned on the unknown spectrum. A conventional spectrum viewer on which the 20 mm scale has been replaced by an interpolation scale is used to automatically interpolate positions between the optic axis line and the locus lines. Alternatively, the spectrum can be projected in a viewing box containing the interpolation scale.

© 1953 Optical Society of America

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