We have undertaken the development of high-efficiency and wide spectral coverage grisms for MOIRCS, the near-infrared imager and multi-object spectrograph at the Subaru Telescope. Our prior medium-resolution gratings, incorporating Volume Phase Holographic (VPH) gratings, offered very high efficiency at their peak. However, their narrow transmission curves significantly limited their scientific application, especially in the MOS mode. In response to the demand for better medium-dispersion grisms from the community, we developed new high-sensitivity and wide-spectral-coverage grisms (“LightSmyth grisms”) for the J & H windows in 2019. It was a great success, with significant improvement on both peak efficiency and the wide bandwidth coverage. Following this achievement, we embarked on the development of a comparable medium-dispersion grism for the K-band (“VB-K grism”), incorporating our proprietary Volume-Binary (VB) grating. The fabrication was completed by the summer of 2023. The basic performance test inside the dome confirmed its excellent performance as planned. The first scientific use by open-use observers was achieved in March 2024. A more detailed on-sky performance evaluation is scheduled this summer.
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