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

The off-axis pivot-point mounting for toroidal uniform Line-space (TULS) gr atings and spherical varied line-space (SVLS) is presented: One scans the s pectrum 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 t he optimum length of the pivot arm. The pivot points for TULS and SVLS grat ings are found to be located on opposite sides with respect to the normal t o the grating. In a comparison of the optical performance, the spectral foc using is similar, but the spatial aberrations can be fewer for a SVLS grati ng. (C) 2000 Optical Society of America.