WIRC-POL: 0.1% polarimetric precision on-sky with the installation of a HWP modulator

Abstract

WIRC+Pol is a J and H-band linear spectropolarimetric mode for the prime-focus WIRC camera at the Palomar 200-inch Hale telescope. The mode is enabled by a split-pupil polarization grating that allows for the simultaneous measurement of Stokes Q and U without the use of a modulator at a seeing-limited spectral resolving power of ~100. To overcome time-dependent instrumental polarization effects that limited the calibration of instrumental polarization to a ~1% level, in February 2019 we installed a linear polarizer and half-wave plate modulator in front of the instrument, both of which are rotatable. With these two new optics, we are now able to calibrate the instrumental polarization to less than 0.1%. Here we report on WIRC+Pol’s post-upgrade on-sky performance, including a polarimetric instrument model that we characterize as a function of time and focal-plane location.