Three-dimensional structure (left) and working principle (right) of the LAMOST NIR spectrograph prototype. The right panel illustrates the light path: light from the slit is collimated and then split by the blue and NIR dichroic mirrors, with NIR light being reflected into the NIR channel. The core of the NIR channel is an integrated rotary stage housing the VPH grating, camera, and dewar. A coordinated rotation of the grating (independent) and the entire stage enables the system to maintain the grating blaze angle at optimal efficiency for different wavelengths. Brakes at the rear of the stage lock the position after adjustment to ensure operational stability.