Methods are discussed for finding the optimum configuration of an inplane Czerny–Turner-type spectrograph. An asymmetry parameter is used which involves a lateral movement of the grating, perpendicular to the rulings. This displacement improves image quality at the center of a fixed photographic plate, and we find that the grating position for best imaging varies with angle. The optimum relationship between grating angle and position is derived by ray tracing for a specific 4-m instrument and the image widths are found to be superior to those calculated for configurations based on analytical expressions. Ray tracing is used to derive a compromise configuration with a fixed plate and a fixed grating for which the image quality is maximized at the photographic plate-center for all useful grating angles, and falls off symmetrically toward each end of the plate. For this configuration, it is necessary only to refocus the entrance slit as the grating is rotated to scan wavelengths.
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