Near-infrared polarimeter for the Subaru telescope Motohide Tamura *a , Misato Fukagawa b , Koji Murakawa c , Hiroshi Suto c , Yoichi Itoh d , Yoshiyuki Doi c a National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan b University of Tokyo, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan c Subaru Telescope, National Astronomical Observatory of Japan, 650 N. A’ohoku Place, Hilo, HI 96720 d Kobe University, Rokkodai 1-1, Nada, Hyogo 606-8502, Japan ABSTRACT We report the development and performance of a near-infrared polarimeter for the Subaru 8.2-m telescope. The polarimeter is currently used with one of the Subaru instruments, CIAO, the stellar coronagraphic imager with adaptive optics. CIAO is the instrument specialized to obtain high contrast images of faint objects in the vicinity of bright objects. For achieving both high spatial resolution and high dynamic range, the instrument is used with the Subaru adaptive optics and has a dedicated cold coronagraphic capability. The polarimeter comprises two components. One component consists of an achromatic (1 - 2.5 micron) half-waveplate, an achromatic quarter-waveplate, and a calibration wire grid. Both half- and quarter-waveplates are rotatable and retractable, while the calibrator is only retractable. This component is placed upstream of any optical components including adaptive optics system, which minimizes the effect of various mirrors on instrumental polarization. The other component consists of two analyzers, a cold wire grid and a cold Wollaston prism. These are placed in the filter wheels of CIAO cryostat and can be chosen. The whole system is remotely controlled. The instrument has been commissioned on the Subaru telescope and its linear-polarization performance has been tested with/without the adaptive optics. Keywords: infrared, polarimetry, coronagraph, adaptive optics, Subaru telescope 1. CIAO AND POLARIMETER CIAO is a near-infrared stellar coronagraphic imager for the Subaru 8.2 meter telescope. The purpose of this instrument is to obtain diffraction limited (~0.06 arcsec at wavelength of 2 micron) images of faint objects in close vicinity of bright objects. For achieving both high spatial resolution and high dynamic range, the instrument is used with the Subaru Cassegrain adaptive optics 1 and designed to have a cold coronagraphic capability. Polarimetric capability is very important with this instrument: scientific applications of high spatial resolution (sub-arcsecond) polarimetry with or without coronagraph are vast. Besides the scientific merits, there are some technical advantages. Extended structures around bright objects such as circumstellar disks and envelopes are often of scattered origin and expected to show a large polarization, while the PSF halo associated with bright point-sources should not show regular pattern of polarization vectors and their degrees of polarization are very small. Polarimetry makes it possible to distinguish the small extended structures and the PSF halo, which is often difficult by simple imaging. Therefore, we have developed a * hide@subaru.naoj.org; phone +81-422-34-3513; fax +81-422-34-3527