Four gratings of line spacings 300, 600, 1200 and 2400 L/mm provide nominal dispersions of 160, 80, 40 and 20 A/mm. The 600 and 1200 L/mm gratings are conventional replicas of ruled gratings and are blazed for 4400,A. The 2400 L/mm grating is a replica of a holographic grating and, although not blazed shows higher efficiency in the blue.
Each grating is mounted in a prealigned cell so that grating interchange is straightforward. The grating is accessed by removing the main casing lid.
Grating rotation is accomplished by a worm gear system with scales calibrated in 0.25 degree divisions.