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

Area of Science:

• Cosmology and astrophysics
• Optical engineering
• Electromagnetics

Background:

• Cosmic Microwave Background (CMB) observations require precise polarization measurements.
• Refractive optical systems in telescopes can introduce systematic errors.
• Anti-reflection (AR) coatings are crucial for minimizing light loss but can have polarization-dependent properties.

Purpose of the Study:

• To identify and characterize a polarization offset systematic in refractive CMB telescopes.
• To develop a method for modeling polarization effects in optical systems with AR coatings.
• To quantify the impact of this systematic on far-field polarization measurements.

Main Methods:

• Developed a hybrid physical optics method combining electromagnetic simulations.
• Incorporated full-wave simulations of AR coatings to model polarization response.
• Applied the method to a two-lens CMB telescope model with a single-layer dielectric AR coating.

Main Results:

• Identified a polarization offset systematic caused by differential transmission in AR coatings.
• Demonstrated that this effect produces a polarization angle offset varying across the focal plane (0.05-0.5°).
• Showed that polarization-dependent transmission is the underlying cause.

Conclusions:

• This systematic can lead to significant polarization angle errors in CMB telescope far-field beams.
• If uncorrected, the effect can cause temperature-to-polarization leakage and Stokes Q/U mixing.
• Accurate modeling of AR coatings is essential for precise CMB polarization measurements.