Spot diagrams of the LAMOST NIR spectrograph prototype under three operational modes at room temperature (20°C). The rectangle in each panel represents a 2 × 2 pixel area (40 × 40 μm). All spots are calculated for key object points (|x| = 33, 52, and 64.15 mm) within the scientific field of view. Top left: mode 1 (grating angle 10﹩\mathop{.}\limits^{\unicode{x000b0}}﹩2, covering 0.9 ∼ 1.015 μm and 1.192 ∼ 1.305 μm); top right: mode 2 (grating angle 11﹩\mathop{.}\limits^{\unicode{x000b0}}﹩5, covering 1.015 ∼ 1.125 μm and 1.301 ∼ 1.41 μm); bottom: mode 3 (grating angle 12﹩\mathop{.}\limits^{\unicode{x000b0}}﹩5, covering 1.1 ∼ 1.21 μm and 1.385 ∼ 1.495 μm). The root mean square (rms) spot diameter is below 40 μm for all field points and modes, which is well within the geometric image extent of the fiber and thus demonstrates uniform image quality sufficient for spectroscopic requirements.