Achromatizing a Liquid-Crystal Spectropolarimeter: Retardance vs. Stokes-Based Calibration of HiVIS

Astronomical spectropolarimeters can be subject to many sources of systematic error that limit the precision and accuracy of the instrument. We present a calibration method for observing high-resolution polarized spectra using chromatic liquid-crystal variable retarders (LCVRs). These LCVRs allow for polarimetric modulation of the incident light without any moving optics at frequencies≥10 Hz ≥ 10 Hz . We demonstrate a calibration method using pure Stokes input states that enables an achromatization of the system. This Stokes-based deprojection method reproduces input polarization even though highly chromatic instrument effects exist. This process is first demonstrated in a laboratory spectropolarimeter where we characterize the LCVRs and show example deprojections. The process is then implemented on the newly upgraded HiVIS spectropolarimeter on the 3.67 m Advanced Electro-Optical System (AEOS) telescope. The HiVIS spectropolarimeter has also been expanded to include broadband full-Stokes spectropolarimetry using achromatic wave plates in addition to the tunable full-Stokes polarimetric mode using LCVRs. These two new polarimetric modes in combination with a new polarimetric calibration unit provide a much more sensitive polarimetric package with greatly reduced systematic error.