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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. Recently, a Wollaston prism was added to CHARIS' opticalpath, giving CHARIS a spectropolarimetric capability that is unique amonghigh-contrast imaging instruments. We present a detailed Mueller matrix modeldescribing the instrumental polarization effects of the complete optical path,thus the telescope and instrument. The 22 wavelength bins of CHARIS provide aunique opportunity to investigate in detail the wavelength dependence of theinstrumental polarization effects. From measurements with the internal lightsource, we find that the image derotator (K-mirror) produces strongwavelength-dependent crosstalk, in the worst case converting ~95% of theincident linear polarization to circularly polarized light that cannot bemeasured. Theoretical calculations show that the magnitude of the instrumentalpolarization of the telescope varies with wavelength between approximately 0.5%and 0.7%, and that its angle is exactly equal to the altitude angle of thetelescope. We plan to more accurately determine the instrumental polarizationof the telescope with observations of a polarization standard star, and fitmore comprehensive physical models to all experimental data. In addition, weplan to integrate the complete Mueller matrix model into the existing CHARISpost-processing pipeline, with the aim to achieve a polarimetric accuracy of<0.1% in the degree of linear polarization. Our calibrations of CHARIS'spectropolarimetric mode will enable unique quantitative polarimetric studiesof circumstellar disks and planetary and brown dwarf companions.

Calibration of the instrumental polarization effects of SCExAO-CHARIS' spectropolarimetric mode