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This paper presents the design and deployment of the Hydrogen Epoch of Reionization Array (HERA) phase II system. HERA is designed as a staged experiment targeting 21 cm emission measurements of the Epoch of Reionization. First results from the phase I array are published as of early 2022, and deployment of the phase II system is nearing completion. We describe the design of the phase II system and discuss progress on commissioning and future upgrades. As HERA is a designated Square Kilometre Array pathfinder instrument, we also show a number of “case studies” that investigate systematics seen while commissioning the phase II system, which may be of use in the design and operation of future arrays. Common pathologies are likely to manifest in similar ways across instruments, and many of these sources of contamination can be mitigated once the source is identified.

We present a framework for identifying and flagging malfunctioning antennas in large radio interferometers. We outline two distinct categories of metrics designed to detect outliers along known failure modes of large arrays: cross‐correlation metrics, based on all antenna pairs, and auto‐correlation metrics, based solely on individual antennas. We define and motivate the statistical framework for all metrics used, and present tailored visualizations that aid us in clearly identifying new and existing systematics. We implement these techniques using data from 105 antennas in the Hydrogen Epoch of Reionization Array (HERA) as a case study. Finally, we provide a detailed algorithm for implementing these metrics as flagging tools on real data sets. Monitoring the performance of a many‐element system is essential to acquiring science‐quality data and requires an automated approachWe define metrics based on cross‐correlations that assess the health of the whole array and its component subsystems in an automated wayWe outline metrics based on autocorrelations that identify systematics and provide a generalizable approach to automated antenna flagging Monitoring the performance of a many‐element system is essential to acquiring science‐quality data and requires an automated approach We define metrics based on cross‐correlations that assess the health of the whole array and its component subsystems in an automated way We outline metrics based on autocorrelations that identify systematics and provide a generalizable approach to automated antenna flagging

The Hydrogen Epoch of Reionization Array (HERA) is a staged experiment to measure 21 cm emission from the primordial intergalactic medium (IGM) throughout cosmic reionization (z= 6-12), and to explore earlier epochs of our Cosmic Dawn (z? 30). During these epochs, early stars and black holes heated and ionized the IGM, introducing fluctuations in 21 cm emission. HERA is designed to characterize the evolution of the 21 cm power spectrum to constrain the timing and morphology of reionization, the properties of the first galaxies, the evolution of large-scale structure, and the early sources of heating. The full HERA instrument will be a 350-element interferometer in South Africa consisting of 14 m parabolic dishes observing from 50 to 250 MHz. Currently, 19 dishes have been deployed on site and the next 18 are under construction. HERA has been designated as an SKA Precursor instrument. In this paper, we summarize HERA's scientific context and provide forecasts for its key science results. After reviewing the current state of the art in foreground mitigation, we use the delay-spectrum technique to motivate high-level performance requirements for the HERA instrument. Next, we present the HERA instrument design, along with the subsystem specifications that ensure that HERA meets its performance requirements. Finally, we summarize the schedule and status of the project. We conclude by suggesting that, given the realities of foreground contamination, current-generation 21 cm instruments are approaching their sensitivity limits. HERA is designed to bring both the sensitivity and the precision to deliver its primary science on the basis of proven foreground filtering techniques, while developing new subtraction techniques to unlock new capabilities. The result will be a major step toward realizing the widely recognized scientific potential of 21 cm cosmology.

Multiple instruments are pursuing constraints on dark energy, observing reionization and opening a window on the dark ages through the detection and characterization of the 21 cm hydrogen line for redshifts ranging from ?1 to 25. These instruments, including CHIME in the sub-meter and HERA in the meter bands, are wide-field arrays with multiple-degree beams, typically operating in transit mode. Accurate knowledge of their primary beams is critical for separation of bright foregrounds from the desired cosmological signals, but difficult to achieve through astronomical observations alone. Previous beam calibration work at low frequencies has focused on model verification and does not address the need of 21 cm experiments for routine beam mapping, to the horizon, of the as-built array. We describe the design and methodology of a drone-mounted calibrator, the External Calibrator for Hydrogen Observatories (ECHO), that aims to address this need. We report on a first set of trials to calibrate low-frequency dipoles at 137 MHz and compare ECHO measurements to an established beam-mapping system based on transmissions from the Orbcomm satellite constellation. We create beam maps of two dipoles at a 9° resolution and find sample noise ranging from 1% at the zenith to 100% in the far sidelobes. Assuming this sample noise represents the error in the measurement, the higher end of this range is not yet consistent with the desired requirement but is an improvement on Orbcomm. The overall performance of ECHO suggests that the desired precision and angular coverage is achievable in practice with modest improvements. We identify the main sources of systematic error and uncertainty in our measurements and describe the steps needed to overcome them.

In order to study the "Cosmic Dawn" and the Epoch of Reionization with 21 cm tomography, we need to statistically separate the cosmological signal from foregrounds known to be orders of magnitude brighter. Over the last few years, we have learned much about the role our telescopes play in creating a putatively foreground-free region called the "EoR window." In this work, we examine how an interferometer's effects can be taken into account in a way that allows for the rigorous estimation of 21 cm power spectra from interferometric maps while mitigating foreground contamination and thus increasing sensitivity. This requires a precise understanding of the statistical relationship between the maps we make and the underlying true sky. While some of these calculations would be computationally infeasible if performed exactly, we explore several well-controlled approximations that make mapmaking and the calculation of map statistics much faster, especially for compact and highly redundant interferometers designed specifically for 21 cm cosmology. We demonstrate the utility of these methods and the parametrized trade-offs between accuracy and speed using one such telescope, the upcoming Hydrogen Epoch of Reionization Array, as a case study. [ABSTRACT FROM AUTHOR]