Windows to the Universe: Improving the Sensitivity of High Throughput Millimeter Telescopes

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. I 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. The larger apertures also produce a challenging anti-reflection (AR) design problem for refracting and transmissive optics. AR layers are required to minimize the light lost at each transmissive interface within a receiver, which therefore improves the optical throughput of the instrument. The plastic optics require consistently thin polymer coats across a wide area, while wide bandwidths require multilayer designs. I present multilayer AR coats for plastic optics of the BICEP Array receivers (30--300 GHz) utilizing an expanded polytetrafluoroethylene (ePTFE) membranes. These ePTFE membranes can be fine-tuned to the ideal quarter wavelength solutions with heated compression, and some types of ePTFE can be layered and compressively heat-bonded to generate thicker stacks. This heated compression process allows for a range of densities (from 0.3 g/cc to 1 g/cc) and thicknesses ($>$0.05 mm) over a wide radius (33 cm), opening the parameter space of potential AR coats in interesting directions. The ePTFE anti-reflection coats have produced band average reflections close to the ideal for the polyethylene optics on all four receivers in BICEP Array.

The four receivers that make up BICEP Array target different frequencies to aid in measuring and separating the polarized astrophysical components along the line-of-sight to the CMB. We measure the frequency dependent optical throughput of these receivers with a Fourier transform spectrometer (FTS), and our ability to constrain cosmological parameters is dependent on our understanding of the systematics of this optical throughput measurement. I report the biases induced in the eight parameter cosmological model by the bandpass shifts within our maximum likelihood search framework; the maximum shift in the tensor to scalar ratio $r$ was found to be $9.1 \pm 3.8\pow{-4}$ with conservative 2\% shifts to bandcenters.