Example of residual sky removal in NAS observations, illustrating the performance of the sky-subtraction procedure described in Section 4.1. Adapted from Z. Lin et al. (2025) with updated residual statistics. Top: median sky spectrum measured in the sky panel (black) and the naive difference f_source − f_sky. Outside the masked [O I] λ5577 region, the sky continuum level is ∼7 × 10⁻¹⁸ erg cm⁻² s⁻¹ arcsec⁻² Å⁻¹. Bottom: naive differencing (red) leaves residual structure at the ∼(4–8) × 10⁻²⁰ erg cm⁻² s⁻¹ arcsec⁻² Å⁻¹ level. After polynomial matching of the sky spectrum, the residual (black) is statistically consistent with zero median (﹩{\tilde{S}}_{{\rm{res}}}\simeq 6\times 1{0}^{-22}﹩) and exhibits ﹩{\sigma }_{{\rm{res}}}\simeq 2\times 1{0}^{-20}﹩ erg cm⁻² s⁻¹ arcsec⁻² Å⁻¹, with fluctuations confined within ±3 × 10⁻²⁰. The [O I] λ5577 line (gray shaded region) is masked due to its independent and strong temporal variability.