Automatic precision measurement of spectrograms

A fully automatic comparator has been designed and implemented to determine precision wavelengths from high-resolution spectrograms. The accuracy attained is superior to that of an experienced operator using a semiautomatic comparator with a photoelectric setting device. The system consists of a comparator, slightly modified for simultaneous data acquisition from two parallel scans of the spectrogram, interfaced to a minicomputer. The software which controls the system embodies three innovations of special interest. (1) Data acquired from two parallel scans are compared and used to separate unknown from standard lines, to eliminate spurious lines, to identify blends of unknown with standard lines, to improve the accuracy of the measured positions, and to flag lines which require special examination. (2) Two classes of lines are automatically recognized and appropriate line finding methods are applied to each. This provides precision measurement for both simple and complex line profiles. (3) Wavelength determination using a least-squares fitted grating equation is supported in addition to polynomial interpolation. This is most useful in spectral regions with sparsely distributed standards. The principles and implementation of these techniques are fully described.