  SCExAO at the Subaru telescope is a visible and near-infrared high-contrast
imaging instrument employing extreme adaptive optics and coronagraphy. The
instrument feeds the near-infrared light (JHK) to the integral-field
spectrograph CHARIS. The spectropolarimetric capability of CHARIS is enabled by
a Wollaston prism and is unique among high-contrast imagers. We present a
detailed Mueller matrix model describing the instrumental polarization effects
of the complete optical path, thus the telescope and instrument. From
measurements with the internal light source, we find that the image derotator
(K-mirror) produces strongly wavelength-dependent crosstalk, in the worst case
converting ~95% of the incident linear polarization to circularly polarized
light that cannot be measured. Observations of an unpolarized star show that
the magnitude of the instrumental polarization of the telescope varies with
wavelength between 0.5% and 1%, and that its angle is exactly equal to the
altitude angle of the telescope. Using physical models of the fold mirror of
the telescope, the half-wave plate, and the derotator, we simultaneously fit
the instrumental polarization effects in the 22 wavelength bins. Over the full
wavelength range, our model currently reaches a total polarimetric accuracy
between 0.08% and 0.24% in the degree of linear polarization. We propose
additional calibration measurements to improve the polarimetric accuracy to
<0.1% and plan to integrate the complete Mueller matrix model into the existing
CHARIS post-processing pipeline. Our calibrations of CHARIS'
spectropolarimetric mode will enable unique quantitative polarimetric studies
of circumstellar disks and planetary and brown dwarf companions.
