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MIMIZUKU is the first-generation mid-infrared instrument for the TAO 6.5-m telescope. It has three internal optical channels to cover a wide wavelength range from 2 to 38 µm. Of the three channels, the NIR channel is responsible for observations in the shortest wavelength range, shorter than 5.3 µm. The performance of the NIR channel is evaluated in the laboratory. Through the tests, we confirm the followings: 1) the detector (HAWAII 1RG with 5.3-µm cutoff) likely achieves ∼80% quantum efficiency; 2) imaging performance is sufficient to achieve seeing-limit spatial resolution; 3) system efficiencies in imaging mode are 2.4–31%; and 4) the system efficiencies in spectroscopic modes is 5–18%. These results suggest that the optical performance of the NIR channel is achieved as expected from characteristics of the optical components. However, calculations of the background levels and on-sky sensitivity based on these results suggest that neutral density (ND) filters are needed to avoid saturation in L ′ - and M′ -band observations and that the ND filters and the entrance window, made of chemical-vapor-deposition (CVD) diamond, significantly degrade the sensitivity in these bands. This means that the use of different window materials and improvements of the detector readout speed are required to achieve both near-infrared and long-wavelength mid-infrared (>30 µm) observations.
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