Performances of an integral field unit for FOCAS on the Subaru telescope

We have developed an integral field unit (IFU) for the existing optical imaging spectrograph, Faint Object Camera And Spectrograph (FOCAS), on the Subaru telescope. FOCAS IFU finally saw a first light on March 2nd, 2018, and started the common use from 2019. In order to observe faint targets like distant galaxies, our IFU has a coarse sampling comparable to the best seeing size and high throughput. The field of view is 13.4 10.0 arcsec2 which is divided into 23 slices with the width of 0.435 arcsec. Our IFU has a slit separated by about 5.2 arcmin from an object field in order to simultaneously obtain a sky spectrum. We confirmed that the image quality is good enough for the 0.435-arcsec slice width and the best seeing size of 0.4 arcsec. Mean and median throughput of the IFU are respectively 85.0% and 87.3%. However some fields show lower throughput due to misalignment of the IFU optics and the worst throughput is 61.9% at one field corner. Flat fielding error is almost within ±3%, but worse errors are found at the low-throughput region. The worst error is 9% at the lowest throughput region.