Advantages of an off-Littrow mounting of an echelle grating

A phenomenon is described in which the size of the exit-slit image of the entrance slit is influenced by the angle at which an echelle grating is used. The phenomenon becomes significant only for large deviations from Littrow conditions. A mathematical treatment and experimental results are presented to show that proper exploitation of the effect results in simultaneous improvements in bandpass, range, and intensity for an echelle spectrometer.