Understanding Polarization Accuracy: The Effect of Dielectric Mirror Coatings on Instrument Polarization Behavior at a System Level

White, Amanda J.

Graduate Thesis Or Dissertation

Understanding Polarization Accuracy: The Effect of Dielectric Mirror Coatings on Instrument Polarization Behavior at a System Level Público Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/nk322f747

Abstract

In astronomy, the ability to make in situ measurements is rare and incredibly limited. As many physical inferences as possible are squeezed out of every photon observed by astronomers, making the resulting models and theories only as good as the data collected and the methods used to calibrate those data. As science requirements drive tighter constraints on the sensitivity and accuracy of measurements, calibration efforts must also be improved.

Astronomical mirror coatings are often metals protected by multiple layers of dielectrics. Varying the thickness and layering of the dielectrics can lead to significant changes in the polarization properties (retardance, diattenuation, and depolarization) of reflected light across all wavelengths. The polarized response also varies with the angle of incidence, the shape of the mirror, and the uniformity of the coating. In models that predict the polarized response of simple or complex optical systems, assumptions are usually made on the properties and uniformity of coated optical surfaces.

The National Science Foundation’s Daniel K. Inouye Solar Telescope (DKIST) aims to take polarization measurements of unprecedented polarimetric sensitivity and accuracy in order to achieve its science goals. This requires detailed knowledge of all sources of instrumental-induced polarization changes, particularly from the various coatings on optics, to accurately predict both polarization performance through the entire DKIST system in all possible configurations, and to predict any polarization errors across the telescope field of view.

This thesis seeks to improve current polarization calibration methods by characterizing different dielectric mirror coatings and providing more realistic inputs to the system models being used. We first assess the impact of refractive index source data on the accuracy of model outcomes. We then present how a non-uniformly applied coating affects polarization performance and causes depolarization across an aperture. Using DKIST as an application, we compare the depolarization effects of mirror coating non-uniformity to other known sources of systematic polarization error. We find for the majority of wavelengths that estimates of depolarization are well within the tolerance limits set by DKIST’s error budget. However, the depolarization estimates at the 396 nm and 854 nm Ca II lines were found to be on the order of 10⁻², which is above the allowed tolerance for error in DKIST without accounting for any other sources of systematic error. We conclude that more work is needed at the Ca II wavelengths to determine if DKIST’s current wavelength-independent error matrix is sufficient for understanding polarization accuracy requirements across the entire DKIST wavelength bandpass or if a wavelength-dependent error matrix must be developed for the best prediction of system performance.

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