POLARIMETRIC CALIBRATION OF MIMIR AND THE GALACTIC PLANE INFRARED POLARIZATION SURVEY (GPIPS)

Methods and observations are described for the full field of view (FOV) polarimetric calibration of the Mimir near-infrared imaging polarimeter in support of the Galactic Plane Infrared Polarization Survey and other applications. Polarimetric calibration consisted of three steps: (1) flat fielding using in-dome images obtained with the compound zero-order half-wave-plate (HWP) in the 16 position angles employed in polarimetric observations, (2) mapping and removing the remaining instrumental polarization via observations of globular cluster stars, and (3) using polarization standard stars to convert instrument-based polarization position angles to equatorial and to determine the linear polarimetric efficiency. For Mimir, the polarization flat fielding reduced systematic variations of the polarization efficiency across the FOV by a factor of 20. The 151 observations of eight globular clusters yielded 40,000 stars for measuring the remaining 0.05%-0.45% instrumental polarization to uncertainties of 0.02%-0.04%. After these corrections, the 444 observations of 23 polarized stars enabled equatorial polarization position angles to be determined to typical uncertainties of 0 degrees.5, and the on-axis (corrected to full FOV) polarization efficiency of 91.1% +/- 0.4% to be determined. Fully corrected standard star observations showed excellent agreement with published values of polarization percentage and position angle. Additionally, the observations of four sky fields containing Whittet et al. primary polarization standard stars were analyzed to yield 30 new secondary standards. These are fainter than the primaries, allowing use with larger telescope apertures. The secondary standards have polarization position angle uncertainties under 5 degrees and range in degree of polarization from 0.4% to 8.5%.