Polarization rotation through differential transmission in refractive CMB telescopes identified using a hybrid physical optics method

TL;DR

This paper reveals a polarization rotation systematic in refractive CMB telescopes caused by differential transmission in AR coatings, modeled using a hybrid physical optics method, which can impact polarization measurements if uncorrected.

Contribution

The paper introduces a hybrid physical optics method that accurately models polarization response, identifying a new systematic effect in refractive CMB telescopes due to AR coating transmission.

Findings

Polarization rotation varies across the focal plane with amplitudes of 0.05-0.5 degrees.

Differential transmission causes temperature to polarization leakage.

Ignoring this effect can lead to Stokes Q/U mixing in measurements.

Abstract

We identify a polarization rotation systematic in the far field beams of refractive cosmic microwave background (CMB) telescopes caused by differential transmission in anti-reflection (AR) coatings of optical elements. This systematic was identified following the development of a hybrid physical optics method that incorporates full-wave electromagnetic simulations of AR coatings to model the full polarization response of refractive systems. Applying this method to a two-lens CMB telescope with non-ideal AR coating, we show that polarization-dependent transmission can produce a rotation of the far-field polarization angle that varies across the focal plane with a typical amplitude of 0.05-0.5 degrees. If ignored in analysis, this effect can produce temperature to polarization leakage and Stokes Q/U mixing.