KOOLS--IFU: Kyoto Okayama Optical Low-dispersion Spectrograph with Optical-Fiber Integral Field Unit

Observations of transient objects, such as short gamma-ray bursts (GRBs) and electromagnetic counterparts of gravitational wave sources, require prompt spectroscopy. To carry out prompt spectroscopy, we developed an optical-fiber integral field unit (IFU) and connected it with an existing optical spectrograph KOOLS. KOOLS with IFU, hereafter KOOLS–IFU, was mounted on the Okayama Astrophysical Observatory 188-cm telescope. The fiber core and cladding diameters of the fiber bundle are 100 $\mu$m and 125 $\mu$m, respectively, and 127 fibers are hexagonally close-packed in the sleeve of the two-dimensional (2D) fiber array. We conducted test observations to measure the KOOLS–IFU performance and obtained the following conclusions: (1) the spatial sampling is 2’‘.34 $\pm$ 0’‘.05 per fiber, and the total field of view (FoV) is 30’‘.4 $\pm$ 0’'.65 with 127 fibers; (2) the observable wavelength and the spectral resolving power of the grisms of KOOLS are 4030–7310 A and 400–600; 5020–8830 A and 600–900; 4160–6000 A and 1000–1200; and 6150–7930 A and 1800–2400, respectively; and (3) the estimated limiting magnitude is 18.2–18.7 AB mag during 30-min exposure under the optimal condition.