FOCAS: The Faint Object Camera and Spectrograph for the Subaru Telescope

Abstract The Faint Object Camera and Spectrograph (FOCAS) is a Cassegrain optical instrument for the Subaru Telescope. Its capabilities include $6^\prime \phi$ FOV direct imaging, low-resolution spectroscopy ($R=250 \hbox{--} 2000$ with ${0\rlap {.}{}^{\mathrm {\prime \prime }}4}$ slitwidth), multi-slit spectroscopy and polarimetry. We describe the overall design of FOCAS, its observing functions, and the performance verification procedures that have been carried out.