Achromatizing a liquid-crystal spectropolarimeter: Retardance vs Stokes-based calibration of HiVIS

Abstract

Astronomical spectropolarimeters can be subject to many sources of systematic error which 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 >10Hz. 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 the a newly upgraded HiVIS spectropolarimeter on the 3.67m AEOS telescope. The HiVIS spectropolarimeter has also been expanded to include broad-band 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.

Accepted in PASP

References in corpus (6)

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Cited by in corpus (7)

• Deriving Telescope Mueller Matrices Using Daytime Sky Polarization Observations
• Design of a Full-Stokes Polarimeter for VLT/X-shooter
• An Inexpensive Liquid Crystal Spectropolarimeter for the Dominion Astrophysical Observatory Plaskett Telescope
• Correcting systematic polarization effects in Keck LRISp spectropolarimetry to <0.05%
• Calibrating and Stabilizing Spectropolarimeters with Charge Shuffling and Daytime Sky Measurements
• Daytime Sky Polarization Calibration Limitations
• Polarization Modeling and Predictions for DKIST Part 4: Calibration Accuracy Over Field of View, Retardance Spatial Uniformity And Achromat Design Sensitivity