Off-axis pivot mounting for aberration-corrected concave gratings at normal incidence

The off-axis pivot-point mounting for toroidal uniform line-space (TULS) gratings and spherical varied line-space (SVLS) is presented: One scans the spectrum by rotating the grating about the pivot point. The best choice for the location of the point is discussed: For a monochromator the location is chosen such that the grating is moved approximately along the bisector of the subtended angle; for a spectrometer, it is chosen such that the grating is moved approximately along the direction of the incident ray. Minimizing the spectral defocusing caused by the rotation of the grating determines the optimum length of the pivot arm. The pivot points for TULS and SVLS gratings are found to be located on opposite sides with respect to the normal to the grating. In a comparison of the optical performance, the spectral focusing is similar, but the spatial aberrations can be fewer for a SVLS grating.