Resolution and Efficiency of Single- and Double-Pass Spectrographs*

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Abstract
The dispersion, resolution, and signal-to-noise ratio in single- and double-pass use of spectrographs are discussed. A theoretical ratio of single- to double pass-dispersion is derived for a non-Littrow spectrograph. The resolution that can be expected is computed by convolution of the appropriate Doppler, Lorentz, and and Schuster line profiles. A method for choosing between single- and double-pass operation to optimize the signal-to-noise ratio for a given resolving power is described. The measured dispersion, resolution, and signal-to-noise ratio of a 5-m vacuum spectrograph using a 12×25-cm grating are compared to the computed values. The results indicate that the resolution obtained is essentially the same as the computed values and that double-pass operation can be very efficient for high-resolution spectrographs.

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