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

TL;DR

The paper presents the design, calibration, and performance evaluation of a new adaptive optics imaging polarimeter for near-infrared observations, enhancing contrast and resolution for studying faint circumstellar material.

Contribution

It introduces a novel dual-channel imaging polarimetry mode for IRCAL, detailing its design, data reduction algorithms, and on-sky performance improvements.

Findings

Achieved two orders of magnitude reduction in stellar PSF halo.

Demonstrated high contrast imaging capabilities in the near-infrared.

Provided insights into factors limiting contrast and lessons for future instruments.

Abstract

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 two 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.