The Atacama Cosmology Telescope: The polarization-sensitive ACTPol instrument (1605.06569v1)

Abstract: The Atacama Cosmology Telescope (ACT) is designed to make high angular resolution measurements of anisotropies in the Cosmic Microwave Background (CMB) at millimeter wavelengths. We describe ACTPol, an upgraded receiver for ACT, which uses feedhorn-coupled, polarization-sensitive detector arrays, a 3 degree field of view, 100 mK cryogenics with continuous cooling, and meta material anti-reflection coatings. ACTPol comprises three arrays with separate cryogenic optics: two arrays at a central frequency of 148 GHz and one array operating simultaneously at both 97 GHz and 148 GHz. The combined instrument sensitivity, angular resolution, and sky coverage are optimized for measuring angular power spectra, clusters via the thermal Sunyaev-Zel'dovich and kinetic Sunyaev-Zel'dovich signals, and CMB lensing due to large scale structure. The receiver was commissioned with its first 148 GHz array in 2013, observed with both 148 GHz arrays in 2014, and has recently completed its first full season of operations with the full suite of three arrays. This paper provides an overview of the design and initial performance of the receiver and related systems.

An Overview of the Atacama Cosmology Telescope's ACTPol Instrument

The paper provides an in-depth examination of the ACTPol instrument, an enhanced receiver for the Atacama Cosmology Telescope (ACT), specifically designed to perform high angular resolution measurements of anisotropies in the Cosmic Microwave Background (CMB) at millimeter wavelengths. ACTPol distinguishes itself through its use of feedhorn-coupled, polarization-sensitive detector arrays, a 3-degree field of view, advanced cryogenics operating at 100 mK, and state-of-the-art metamaterial anti-reflection coatings.

Instrument Specifications

ACTPol comprises three arrays with differentiated cryogenic optics: two arrays functioning at a central frequency of 148 GHz and a third array equipped to operate simultaneously at 97 GHz and 148 GHz. This configuration is optimized for the precise measurement of angular power spectra, detection of clusters via thermal and kinetic Sunyaev-Zel'dovich effects, and CMB lensing attributable to large-scale structures. Initial commissioning of the instrument occurred with the first 148 GHz array in 2013, expanded to both 148 GHz arrays in 2014, and culminated in a full-suite operation with all three arrays.

Observational Strategy

The ACTPol instrument’s observational strategy combines both wide and deep field observations. The deep surveys target specific regions that overlap with existing multi-wavelength observations while wide surveys cover extensive sky regions to maximize cosmological parameter constraints. This dual strategy supports a comprehensive analysis of both the CMB temperature and polarization anisotropies, allowing ACTPol to place independent constraints on cosmological models like $\Lambda$CDM.

Technological Advancements

This paper details a series of technological advancements integrated into the ACTPol design. The polarization-sensitive arrays employ orthomode transducers (OMTs) to direct radiation from feedhorn-coupled methods to the transition-edge sensor (TES) bolometers. ACTPol utilizes metamaterial AR coatings developed through micromachining techniques for superior performance and a new cryogenic receiver considerably enhances cooling efficiency and detector sensitivity, enabling long-term continuous operation without reliance on liquid cryogens.

Performance Metrics

The sensitivity and resolution metrics of ACTPol have undergone rigorous evaluation. The beam profiles, characterized through planet observations, reveal robust determinations suitable for high-precision measurements essential for cosmological studies. Furthermore, polarization calibration contributes accurately to the separation of E and B modes, crucial for examining gravitational lensing and primordial gravitational waves potentially induced during an inflationary epoch of the universe.

Implications and Future Directions

The ACTPol instrument's contributions extend to both theoretical simulations and practical applications by enhancing our understanding of the CMB's polarization properties. The results promise significant implications for our grasp of the underlying physics of the universe, from fundamental tests of relativity and quantum field theory to specific parameters within the standard model of cosmology.

Looking forward, the outlined methodologies and technological capabilities bear significance for future CMB polarization instruments. As ACTPol continues its observational campaigns, data outputs will bolster theoretical models, possibly informing future missions aimed at deeper and more exacting CMB examinations.

This comprehensive suite of capabilities, coupled with the ACT's strategic observation point in the Atacama Desert with its characteristically low precipitable water vapor and high altitude, positions the ACTPol as a prominent tool within the astrophysical community. The outcomes of ongoing and future analyses underscore the dynamic interfacing of observational cosmology with fundamental physics through instruments like ACTPol.