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

Achieving a Spectropolarimetric Precision Better than 0.1% in the Near-Infrared with WIRC+Pol