2025AJ....170..349Z

The Large Sky Area Multi-Object Fiber Spectroscopy Telescope (LAMOST) has demonstrated exceptional capability in optical spectroscopic surveys. To overcome its inherent limitations in observing late-type stars and regions of high extinction, we present the optical design of a near-infrared (NIR) spectrograph prototype. This instrument is designed to expand LAMOST's spectral coverage into the J band, specifically targeting 900-1400 nm. Driven by key science goals-precise parameter determination for late-type stars and chemical abundance analysis of red supergiants-the design specifications require a spectral resolution R ≥ 1500, a throughput exceeding 35%, and a multiplexing capability of 100 fibers. Confronted with constraints on large-format NIR detector availability, the design employs a 2 × 2 mosaic detector configuration. This necessitates three complementary observing modes, achieved through synchronized rotations of a volume phase holographic grating and the entire NIR optomechanical module, to seamlessly cover the required wavelength range. A critical innovation is the adoption of a fast (F/1.3), fully refractive camera, which avoids the central obscuration of a Schmidt design, thereby enhancing throughput. Optical performance analysis demonstrates that the design meets all requirements: it accommodates 106 fibers, achieves a resolution of 1400-2300 (with a two-thirds slit width), and provides a theoretical peak throughput of 65%. This spectrograph prototype will effectively extend LAMOST's discovery potential into the NIR, enabling groundbreaking studies of Galactic structure and the chemical evolution of nearby galaxies.