  WIRC+Pol is a near-infrared low-resolution spectropolarimeter on the 200-inch
Telescope at Palomar Observatory. The instrument utilizes a polarization
grating to perform polarimetric beam splitting and spectral dispersion
simultaneously. It can operate either with a focal plane slit to reduce sky
background or in a slitless mode. Four different spectra sampling four linear
polarization angles are recorded in the focal plane, allowing the instrument to
measure all linear polarization states in one exposure. The instrument has been
on-sky since February 2017 and we found that the systematic errors, likely
arising from flat fielding and gravity effects on the instrument, limit our
accuracy to ~1%. These systematic effects were slowly varying, and hence could
be removed with a polarimetric modulator. A half-wave plate modulator and a
linear polarizer were installed in front of WIRC+Pol in March 2019. The
modulator worked as expected, allowing us to measure and remove all
instrumental polarization we previously observed. The deepest integration on a
bright point source (J = 7.689, unpolarized star HD 65970) demonstrated
uncertainties in q and u of 0.03% per spectral channel, consistent with the
photon noise limit. Observations of fainter sources showed that the instrument
could reach the photon noise limit for observations in the slitless mode. For
observations in slit, the uncertainties were still a factor of few above the
photon noise limit, likely due to slit loss.
