Geometrical and physical optics analysis for mm-wavelength refractor telescopes designed to map the cosmic microwave background

TL;DR

This paper compares two mm-wavelength refractor telescope designs, using geometrical and physical optics, revealing that plastic lenses can perform comparably to silicon lenses with relaxed design constraints, impacting future cosmic microwave background mapping instruments.

Contribution

It provides a detailed optical analysis of plastic versus silicon lens systems for mm-wavelength telescopes, highlighting performance trade-offs and limitations of geometrical optics.

Findings

Plastic lenses can match silicon lenses in performance with relaxed telecentricity.

Beam ellipticity varies with pixel position and size, challenging standard models.

Silicon refractors exhibit higher cross-polarization than HDPE designs.

Abstract

We present a compact two-lens HDPE f/1.6 refractor design that is capable of supporting a 28-deg diffraction-limited field of view at 1-mm wavelengths and contrast it to a similar two-lens refractor using silicon lenses. We compare the optical properties of these two systems as predicted by both geometrical and physical optics. The presented analysis suggests that by relaxing telecentricity requirements, a plastic two-lens refractor system can perform comparably to a similar silicon system across a wide field of view and wavelengths up to 1 mm. We show that for both telescope designs, cold stop spillover changes significantly across the field of view in a way that is somewhat inconsistent with Gaussian beam formalism and simple f-number scaling. We present results that highlight beam ellipticity dependence on both pixel location and pixel aperture size --- an effect that is challenging to reproduce in standard geometrical optics. We show that a silicon refractor design suffers from larger cross-polarization compared to the HDPE design. Our results address the limitations of solely relying on geometrical optics to assess relative performance of two optical systems. We discuss implications for future refractor designs.