Conventional double-grating monochromators used in pure rotational Raman (PRR) temperature LIDAR usually adopt the Littrow configuration to separate backscattered PRR lines. A mirror is added to this configuration, thereby producing the Littman configuration. The dispersion of the modified configuration is greater than thrice that of the Littrow configuration for incident angles larger than 60 deg. A spectroscopy system with a linear dispersion better than 0.9  mm  /  nm is designed for the 532-nm excitation laser wavelength, and the selected N₂ PRR lines are effectively separated. The overall system is simulated using only off-the-shelf optical components, and the results show that the volume is smaller than 240  ×  200  ×  80  mm³.