The disperser selector provides three positions for gratings with their mounts and tilt assemblies, three grisms and their mounts, a flat mirror for the long camera imaging mode, an open position for the short camera imaging mode, and provision for mounting a Fabry-Perot etalon. The selector consists of a frame, a drive assembly, and a carriage which carries the various optics and mounts.
The general requirement for removal and replacement of a dispersing element is that any spectral feature returns to within 5 pixels (1 pixel goal) after transfer. Flexure of the disperser relative to the CCD focal plane should be less than 0.5 pixels (0.1 pixel goal) over a 1 hour exposure.