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The HiZ-GUNDAM is a time-domain and multi-messenger astronomy mission by monitoring high-energy astronomical transient events such as gamma-ray bursts (GRBs). The HiZ-GUNDAM is designed to provide alerts of high-redshift GRBs with wide field X-ray monitors (WFXMs) and a co-onboard 30-cm optical and near-infrared telescope (NIRT) for immediate photometric follow-up observations. The HiZ-GUNDAM satellite automatically changes its attitude toward the discovered transient object, starts the follow-up observations with NIRT, and sends alert information including the detailed position, the apparent magnitude, and the photometric redshift of the transient object within one hour. This mission was selected as one of the mission concept candidates of the competitively-chosen medium-class mission of ISAS/JAXA. Aiming for launch in 2030s, conceptual studies of the satellite and onboard instruments are currently ongoing. The five-band simultaneous observation at 0.5-2.5 μm is realized by a beam splitter and a Kösters prism. The incoming beam is split into visible light (0.5-0.9 µm) and near-infrared light by the beam splitter, and visible light is received by an optical detector. The near-infrared light is additionally split into four bands (0.9-1.3 μm, 1.3-1.7 μm, 1.7-2.1 μm, and 2.1-2.5 μm, respectively) by the Kösters prism, and received by an infrared detector. The telescope, the beam-splitter, the Kösters prism, and the optical detector are cooled down to <200 K, and the infrared detector is additionally cooled down to <120 K by radiation cooling. All mirrors in the telescope are made of aluminum alloy to reduce alignment errors during cooling. In this presentation, we introduce the current status of the development of NIRT onboard HiZ-GUNDAM.
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