Anti-reflection coated vacuum window for the Primordial Inflation Polarization ExploreR (PIPER) balloon-borne instrument

Abstract

Measuring the faint polarization signal of the cosmic microwave background (CMB) not only requires high optical throughput and instrument sensitivity but also control over systematic effects. Polarimetric cameras or receivers used in this setting often employ dielectric vacuum windows, filters, or lenses to appropriately prepare light for detection by cooled sensor arrays. These elements in the optical chain are typically designed to minimize reflective losses and hence improve sensitivity while minimizing potential imaging artifacts such as glint and ghosting. The Primordial Inflation Polarization ExploreR (PIPER) is a balloon-borne instrument designed to measure the polarization of the CMB radiation at the largest angular scales and characterize astrophysical dust foregrounds. PIPER's twin telescopes and detector systems are submerged in an open-aperture liquid helium bucket dewar. A fused-silica window anti-reflection (AR) coated with polytetrafluoroethylene is installed on the vacuum cryostat that houses the cryogenic detector arrays. Light passes from the skyward portions of the telescope to the detector arrays through this window, which utilizes an indium seal to prevent superfluid helium leaks into the vacuum cryostat volume. The AR coating implemented reduces reflections from each interface to <1% compared to ∼10% from an uncoated window surface. The AR coating procedure and room temperature optical measurements of the window are presented. The indium vacuum sealing process is also described in detail, and test results characterizing its integrity to superfluid helium leaks are provided.

QUBIC II: Spectral polarimetry with bolometric interferometry

“…Some of them are planned to be upgraded: CLASS [10], PO-LARBEAR 2 + Simons Array [11], AdvACT [12], and BICEP3/BICEP array [13]. Planned experiments include Simons Observatory [14], PIPER [15], LSPE [16], CMB-S4 [17] and LiteBIRD [18].…”

Experiment for cryogenic large-aperture intensity mapping: instrument design

“…The receiver window (Sec. 2.1) employs an indium seal 141 to remain superfluid tight, preventing LHe from entering the receiver volume, where superfluid films could spoil sub-K cooler operation. The receiver lid is lightweighted to control deflection at the window and increase its strength-to-weight ratio.…”

Polarización del Fondo Cósmico de Radiación: Buscando los modos B utilizando el experimento QUBIC de interferometría bolométrica

“…Se espera alcanzar σ(r) < 0.003. PIPER [73] El Primordial Inflation Polarization ExploreR es un instrumento que será colocado en un globo. Observará grandes escalas angulares operando en frecuencias 200, 270, 350 y 600 GHz.…”

“…Algunos de estos serán actualizados: CLASS [69], POLARBEAR 2 + Simons Array (POLARBEAR + SO) [61], AdvACT [105], y BICEP3/BICEP-array (BICEP-Arr) [71]. Los experimentos que están en planificación son Simons Observatory (los puntos correspondientes a SO incluye los telescopios de pequeña, mediana y gran apertura) [72], PIPER [73], LSPE (STRIP y SWIPE incluidos) [74], CMB-S4 [75] y LiteBIRD [76] (se incluyen los telescopios de pequeña, mediana y gran apertura). Para las simulaciones utilizamos el software HEALPix [98] para realizar las proyecciones de los mapas.…”