2021/08/10 by G. J. Joost `t Hart, R. G. van Holstein, Hart, G. J. Joost `t +41 · 1 citation
Physics and Astronomy · #Stellar, planetary, and galactic studies #Adaptive optics and wavefront sensing #Astrophysics and Star Formation Studies
paper · pdf · doi:10.48550/arxiv.2108.04833
SCExAO at the Subaru telescope is a visible and near-infrared high-contrast\nimaging instrument employing extreme adaptive optics and coronagraphy. The\ninstrument feeds the near-infrared light (JHK) to the integral-field\nspectrograph CHARIS. The spectropolarimetric capability of CHARIS is enabled by\na Wollaston prism and is unique among high-contrast imagers. We present a\ndetailed Mueller matrix model describing the instrumental polarization effects\nof the complete optical path, thus the telescope and instrument. From\nmeasurements with the internal light source, we find that the image derotator\n(K-mirror) produces strongly wavelength-dependent crosstalk, in the worst case\nconverting ~95% of the incident linear polarization to circularly polarized\nlight that cannot be measured. Observations of an unpolarized star show that\nthe magnitude of the instrumental polarization of the telescope varies with\nwavelength between 0.5% and 1%, and that its angle is exactly equal to the\naltitude angle of the telescope. Using physical models of the fold mirror of\nthe telescope, the half-wave plate, and the derotator, we simultaneously fit\nthe instrumental polarization effects in the 22 wavelength bins. Over the full\nwavelength range, our model currently reaches a total polarimetric accuracy\nbetween 0.08% and 0.24% in the degree of linear polarization. We propose\nadditional calibration measurements to improve the polarimetric accuracy to\n<0.1% and plan to integrate the complete Mueller matrix model into the existing\nCHARIS post-processing pipeline. Our calibrations of CHARIS'\nspectropolarimetric mode will enable unique quantitative polarimetric studies\nof circumstellar disks and planetary and brown dwarf companions.\n