BICEP/Keck XIX: Extremely Thin Composite Polymer Vacuum Windows for BICEP and Other High Throughput Millimeter Wave Telescopes

TL;DR

This paper presents the development and deployment of ultra-thin composite polymer vacuum windows for millimeter-wave telescopes, significantly improving detector sensitivity in CMB observations.

Contribution

Introduction of a novel extremely thin composite polyethylene window technology that enhances vacuum integrity and reduces optical loss for high-throughput millimeter-wave telescopes.

Findings

6% improvement in detector sensitivity on BICEP3

Successfully deployed over two observing seasons at the South Pole

Maintained mechanical robustness and vacuum integrity in extreme conditions

Abstract

Millimeter-wave refracting telescopes targeting the degree-scale structure of the cosmic microwave background (CMB) have recently grown to diffraction-limited apertures of over 0.5 meters. These instruments are entirely housed in vacuum cryostats to support their sub-kelvin bolometric detectors and to minimize radiative loading from thermal emission due to absorption loss in their transmissive optical elements. The large vacuum window is the only optical element in the system at ambient temperature, and therefore minimizing loss in the window is crucial for maximizing detector sensitivity. This motivates the use of low-loss polymer materials and a window as thin as practicable. However, the window must simultaneously meet the requirement to keep sufficient vacuum, and therefore must limit gas permeation and remain mechanically robust against catastrophic failure under pressure. We report on the development of extremely thin composite polyethylene window technology that meets these goals. Two windows have been deployed for two full observing seasons on the BICEP3 and BA150 CMB telescopes at the South Pole. On BICEP3, the window has demonstrated a 6% improvement in detector sensitivity.