Daytime Sky Polarization Calibration Limitations

TL;DR

This paper investigates the limitations of using daytime sky polarization for calibrating large astronomical telescopes, emphasizing atmospheric and instrumental factors affecting calibration accuracy.

Contribution

It develops algorithms and observational strategies to improve the robustness of polarization calibration using daytime sky measurements for large telescopes.

Findings

Calibration is robust against multiple-scattering if high polarization regions are used.

High reflectivity mirror coatings reduce sensitivity to telescope-induced polarization.

On-sky measurements agree with Zemax polarization models.

Abstract

The daytime sky has been recently demonstrated as a useful calibration tool for deriving polarization cross-talk properties of large astronomical telescopes. The Daniel K Inouye Solar Telescope (DKIST) and other large telescopes under construction can benefit from precise polarimetric calibration of large mirrors. Several atmospheric phenomena and instrumental errors potentially limit the techniques accuracy. At the 3.67m AEOS telescope on Haleakala, we have performed a large observing campaign with the HiVIS spectropolarimeter to identify limitations and develop algorithms for extracting consistent calibrations. Effective sampling of the telescope optical configurations and filtering of data for several derived parameters provide robustness to the derived Mueller matrix calibrations. Second-order scattering models of the sky show that this method is relatively insensitive to multiple-scattering in the sky provided calibration observations are done in regions of high polarization degree. The technique is also insensitive to assumptions about telescope induced polarization provided the mirror coatings are highly reflective. Zemax-derived polarization models show agreement between the functional dependence of polarization predictions and the corresponding on-sky calibrations.