Polarization offset through differential transmission in refractive CMB telescopes identified using a hybrid physical...

We identify a polarization offset 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 effect 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 a single-layer dielectric AR coating, we show that polarization-dependent transmission can produce an offset of the far-field polarization angle that varies across the focal plane with a typical amplitude of 0.05-0.5^∘. If ignored in analysis, this effect can produce temperature-to-polarization leakage and Stokes Q/U mixing.