2025PASJ...77.1065M

Optical follow-up spectroscopy for newly discovered transients is essential to unveiling their nature in the era when large survey projects will find various transient objects. Combining an optical fiber-type integral field unit (IFU) and a large-aperture rapid-driving telescope is helpful for prompt follow-up spectroscopy. We developed a fiber-type IFU to connect to an existing optical spectrograph, KOOLS-IFU. The IFU is installed at a Nasmyth focus of the Seimei 3.8 m telescope, which can quickly point to targets thanks to its lightness. The main observational parameters of KOOLS-IFU with the new IFU are as follows: ${0{_{.}^{\prime \prime }}83}$ fiber pitch, ${8{_{.}^{\prime \prime }}3}$ × ${8{_{.}^{\prime \prime }}0}$ field of view with a ∼100% filling factor, observable wavelengths of 4100-10200 Å, a spectral resolution of ∼600-2000, and a limiting magnitude of ∼18 AB mag in a 10 min exposure, depending on the grism. KOOLS-IFU is the first instrument of the Seimei telescope to be used during the open-use time of the National Astronomical Observatory of Japan and the observation time of Kyoto University.