The IRCAL Polarimeter: Design, Calibration, and Data Reduction for an Adaptive Optics Imaging Polarimeter

We have upgraded IRCAL, the near-infrared science camera of the Lick Observatory adaptive optics system, to add a dual-channel imaging polarimetry mode. This mode uses an optically contacted YLF (LiYF 4 ) Wollaston prism to provide simultaneous images in perpendicular linear polarizations, providing high-resolution high–dynamic-range polarimetry in the near-infrared. We describe the design and construction of the polarimeter, discuss in detail the data reduction algorithms adopted, and evaluate the instrument's on-the-sky performance. The IRCAL polarimeter is capable of reducing the stellar PSF halo by about 2 orders of magnitude, thereby increasing contrast for studies of faint circumstellar dust-scattered light. We discuss the various factors that limit the achieved contrast, and present lessons applicable to future high-contrast imaging polarimeters.