Schematic of the three-channel dichroic design in the LAMOST spectrograph prototype. To extend the spectral coverage to the NIR, a new NIR dichroic mirror was added after the original blue dichroic. As illustrated, the collimated beam from the telescope is first split by the blue dichroic, which separates the short-wavelength component (λ < 570 nm). The remaining beam is then directed to the NIR dichroic, which transmits light at 570–900 nm to the red channel and reflects light at λ > 900 nm into the new NIR channel, enabling simultaneous three-channel spectroscopic observation.