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

WIRC+Pol is a J and H-band spectropolarimetric mode for the prime-focus WIRC camera at the Palomar 200-inch Hale telescope. The mode has been enabled with the installation of a split-pupil polarization grating in Feburary 2017. The polarization grating was designed for the simultaneous measurement of Stokes Q and U without the use of a modulator at a seeing-limited spectral resolving power of 100. WIRC+Pol can operate in both a slitless mode and with a 3" slit. 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 this design the primary mirror is the only optic in front of the modulator. The new upgrade has allowed us to calibrate the instrumental polarization to less than 0.1%. Here we present a detailed description of the upgrade and report on WIRC+Pol's post-upgrade on-sky performance based on observations of unpolarized and polarized standard stars. We have used the observations of polarized standard stars to develop a wavelength-dependent polarimetric instrument model that we characterize as a function of time and focal-plane location.