Sidelobe modeling and mitigation for a three mirror anastigmat cosmic microwave background telescope

Abstract

Telescopes measuring cosmic microwave background (CMB) polarization on large angular scales require exquisite control of systematic errors to ensure the fidelity of the cosmological results. In particular, far-sidelobe contamination from wide angle scattering is a potentially prominent source of systematic error for large aperture microwave telescopes. Here we describe and demonstrate a ray-tracing-based modeling technique to predict far sidelobes for a three mirror anastigmat telescope designed to observe the CMB from the South Pole. Those sidelobes are produced by light scattered in the receiver optics subsequently interacting with the walls of the surrounding telescope enclosure. After comparing simulated sidelobe maps and angular power spectra for different enclosure wall treatments, we propose a highly scattering surface that would provide more than an order of magnitude reduction in the degree-scale far-sidelobe contrast compared to a typical reflective surface. We conclude by discussing the fabrication of a prototype scattering wall panel and presenting measurements of its angular scattering profile.

Freeform three-mirror anastigmatic large-aperture telescope and receiver optics for CMB-S4

“…This article is part of a series of development studies to evaluate performance for the South Pole three-mirror large-aperture telescope for CMB-S4 including sidelobes performance [11] and seamless mirror manufacture [12]. This work presents a three- ) describing the three freeform mirrors (M1, M2, M3 for primary, secondary and tertiary) surfaces in their local coordinate system, which is centered on each mirror (see Table 2).…”