Schematic layout of the entire optical system for the LAMOST NIR spectrograph prototype. The system consists of the slit, collimator, blue dichroic mirror, NIR dichroic mirror, NIR channel collimator corrector, VPH grating, NIR camera lens group, and detector. The VPH grating, refractive camera, and detector are integrated into a modular unit (NIR rotary stage) that rotates about the grating center.