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SCExAO at the Subaru telescope is a visible and near-infrared high-contrastimaging instrument employing extreme adaptive optics and coronagraphy. Theinstrument feeds the near-infrared light (JHK) to the integral-fieldspectrograph CHARIS. The spectropolarimetric capability of CHARIS is enabled bya Wollaston prism and is unique among high-contrast imagers. We present adetailed Mueller matrix model describing the instrumental polarization effectsof the complete optical path, thus the telescope and instrument. Frommeasurements with the internal light source, we find that the image derotator(K-mirror) produces strongly wavelength-dependent crosstalk, in the worst caseconverting ~95% of the incident linear polarization to circularly polarizedlight that cannot be measured. Observations of an unpolarized star show thatthe magnitude of the instrumental polarization of the telescope varies withwavelength between 0.5% and 1%, and that its angle is exactly equal to thealtitude angle of the telescope. Using physical models of the fold mirror ofthe telescope, the half-wave plate, and the derotator, we simultaneously fitthe instrumental polarization effects in the 22 wavelength bins. Over the fullwavelength range, our model currently reaches a total polarimetric accuracybetween 0.08% and 0.24% in the degree of linear polarization. We proposeadditional calibration measurements to improve the polarimetric accuracy to<0.1% and plan to integrate the complete Mueller matrix model into the existingCHARIS post-processing pipeline. Our calibrations of CHARIS'spectropolarimetric mode will enable unique quantitative polarimetric studiesof circumstellar disks and planetary and brown dwarf companions.

Full characterization of the instrumental polarization effects of the spectropolarimetric mode of SCExAO-CHARIS