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The Phase II Murchison Widefield Array: Design Overview
Randall B. Wayth, Steven J. Tingay, Cathryn M. Trott, David Emrich, Melanie Johnston-Hollitt, Ben McKinley, B. M. Gaensler, A. P. Beardsley, T. Booler, B. Crosse, T. M. O. Franzen, L. Horsley, D. L. Kaplan, D. Kenney, M. F. Morales, D. Pallot, G. Sleap, K. Steele, M. Walker, A. Williams, C. Wu, Iver. H. Cairns, M. D. Filipovic, S. Johnston, T. Murphy, P. Quinn, L. Staveley-Smith, R. Webster, J. S. B. Wyithe
TL;DR
The MWA faced demanding EoR calibration and sensitivity requirements alongside imaging limits imposed by angular resolution and antenna layout. Phase II addresses these needs by adding compact regular and long-baseline tiles while exploring calibration architectures. The upgrade is expected to improve EoR power-spectrum sensitivity by 3.5, double angular resolution, and improve continuum-image sensitivity by an order of magnitude.
Problem
EoR detection requires stringent calibration and dynamic range, while MWA imaging is constrained by angular resolution and the spatial scales sampled by its layout.
Method
Phase II adds regular hexagonal core tiles and long-baseline tiles, with alternative array architectures supporting improved calibration and imaging.
Results
The compact configuration improves EoR power-spectrum sensitivity by a factor of 3.5, while the extended configuration doubles angular resolution and improves continuum-image sensitivity by an order of magnitude.
Takeaways & Limitations
The upgrade enhances angular resolution, brightness sensitivity, and EoR power-spectrum sensitivity while retaining the MWA’s large field of view and snapshot imaging capability.
Takeaways & Limitations
Compact-array imaging retains grating-like beam features, so visibility weighting requires application-dependent selection and can cost approximately 50% of theoretical sensitivity.
Abstract
from arXiv · showhide
We describe the motivation and design details of the "Phase II" upgrade of the Murchison Widefield Array (MWA) radio telescope. The expansion doubles to 256 the number of antenna tiles deployed in the array. The new antenna tiles enhance the capabilities of the MWA in several key science areas. Seventy-two of the new tiles are deployed in a regular configuration near the existing MWA core. These new tiles enhance the surface brightness sensitivity of the MWA and will improve the ability of the MWA to estimate the slope of the Epoch of Reionisation power spectrum by a factor of ~3.5. The remaining 56 tiles are deployed on long baselines, doubling the maximum baseline of the array and improving the array u,v coverage. The improved imaging capabilities will provide an order of magnitude improvement in the noise floor of MWA continuum images. The upgrade retains all of the features that have underpinned the MWA's success (large field-of-view, snapshot image quality, pointing agility) and boosts the scientific potential with enhanced imaging capabilities and by enabling new calibration strategies.
1 INTRODUCTION
Low-frequency radio astronomy is expanding through new facilities motivated by astrophysical and cosmological problems, while creating demanding imaging, calibration, data, and propagation challenges. The MWA’s next evolution doubles its antennas and maximum baseline while exploring alternative calibration architectures.
- 1 INTRODUCTION: Low-frequency radio astronomy has undergone a major renaissance through new large-scale facilities enabled by signal processing and computing advances.
- 1 INTRODUCTION: The redshifted H I signal from the Cosmic Dawn and Epoch of Reionisation is a prominent low-frequency science challenge.
- 1 INTRODUCTION: General-purpose instruments such as LOFAR and the MWA combine EoR motivations with broad investigative capabilities.
- 1 INTRODUCTION: Large datasets, wide-field and wide-band imaging, calibration, and complex propagation effects create practical challenges for these instruments.
- 1 INTRODUCTION: The MWA’s experience and infrastructure are directly relevant to SKA Low, and its low-frequency design is naturally suited to upgrades.
- 1 INTRODUCTION: The MWA’s Phase II evolution doubles the antenna count and maximum baseline while exploring calibration techniques based on alternative array architectures.
2 SCIENTIFIC MOTIVATION FOR THE UPGRADE
The upgrade targets two central needs: stronger EoR power-spectrum sensitivity and better imaging through improved angular resolution, spatial sampling, and reduced confusion. Its design combines a regular compact deployment with 56 long-baseline tiles.
- 2 SCIENTIFIC MOTIVATION FOR THE UPGRADE: The Phase II design focuses on improving EoR power-spectrum sensitivity and imaging through better calibration, angular resolution, u,v coverage, and reduced confusion.
- 2.1 Epoch of Reionisation: EoR detection requires not only sensitivity but also stringent calibration and sky-model accuracy to meet dynamic-range requirements.
- 2.1 Epoch of Reionisation: Regular antenna arrangements provide identical redundant baselines that boost power-spectrum sensitivity and enable redundant calibration.
- 2.1 Epoch of Reionisation: 72 new tiles were deployed in a regular hexagonal configuration in response to experience with existing EoR telescopes and experiments.
- 2.2 Image-based science: Improved angular resolution enables finer source and structure separation and directly affects classical and sidelobe confusion in continuum images.
- 2.2 Image-based science: 56 new tiles beyond the Phase I circumference double the maximum baseline and produce more uniform u,v coverage.
3 THE PHASE II UPGRADE
The Phase II upgrade adds 128 tiles while retaining existing receivers and correlator, creating compact and extended configurations with distinct scientific and operational roles. The compact configuration emphasizes short, redundant baselines, while the extended configuration doubles the maximum baseline and improves u, v coverage.
- Upgrade overview: 128 new tiles expand the deployed array to 256, comprising 72 compact hexagonal tiles and 56 long-baseline tiles.Existing receivers and correlator were retained, limiting simultaneous operation to 128 tiles and requiring periodic reconfiguration.
- Implementation constraints: The expansion was constrained by receiver capacity, reconfiguration requirements, environmental clearances, and restrictions on buried infrastructure.Long-baseline equipment was placed on the surface, with infrastructure work limited primarily to extending existing access tracks.
- Compact configuration: The compact configuration combines 56 existing core tiles with 72 new hexagonal tiles and concentrates most baselines below 200 m.Its regular hexagonal component provides many redundant baselines, while the overall configuration remains hybrid with the existing pseudo-random core.
- Compact configuration: The hexagonal arrangement was selected over a rectangular grid because it reduces pronounced grating features, matches HERA, and fits the existing Phase I core region.
- Long-baseline infrastructure: Long-baseline tiles use RF-over-fibre and self-contained solar power because their distances exceed feasible coaxial-cable reach.The supporting system mimics receiver functionality and connects the tiles to the array through optical fibre.
4 CAPABILITY OF THE UPGRADED MWA
Phase II improves MWA power-spectrum sensitivity and continuum imaging through enhanced small-scale sensitivity, longer baselines, better u,v coverage, and reduced confusion. The compact and extended configurations serve different applications, with compact-array imaging requiring visibility-weighting trade-offs.
- Power spectrum sensitivity: A factor of four raw sensitivity increase is achieved over EoR-relevant scales for a 1000-hour observation.The comparison uses a typical 21 cm signal model at 150 MHz.
- Power spectrum sensitivity: A factor of 3.5 improvement in power-spectrum slope and offset parameter estimation is expected for Phase II.The Fisher analysis includes realistic foregrounds.
- Imaging: The extended configuration doubles the maximum baseline and more uniformly fills the u,v plane, improving the synthesised beam.The compact and extended configurations each contain 128 tiles but have different spatial-resolution and coverage properties.
- Imaging: The Phase II extended array has similar beams under natural and uniform-like weighting, with very low sidelobes away from the phase centre.This contrasts with the large halo around the naturally weighted Phase I beam.
- Imaging: An order of magnitude improvement in continuum-image noise floor is expected from improved sensitivity, lower classical confusion, and reduced sidelobe confusion.The improved angular resolution is expected to reduce classical confusion by a factor of 5–10 for Phase II.
- Imaging: Compact-array imaging remains constrained by grating-like sidelobes, and suppressing them with uniform-like weighting costs approximately 50% theoretical sensitivity.The appropriate visibility weighting depends on the imaging application.
5 CONCLUSION
The Phase II upgrade expands the MWA’s capabilities in angular resolution, brightness sensitivity, EoR power-spectrum sensitivity, and calibration while retaining its wide field of view and snapshot imaging. Further digital and hardware upgrades remain planned.
- 5 CONCLUSION: The upgrade retains the MWA’s large field of view and excellent snapshot imaging capability while enhancing angular resolution, brightness sensitivity, and EoR power-spectrum sensitivity.These features underpin the originally deployed array’s success.
- 5 CONCLUSION: The Phase II upgrade doubles the array’s maximum baseline, doubles angular resolution, and improves snapshot u,v coverage.These changes support improved imaging while retaining the MWA’s large field of view and snapshot imaging capability.
- 5 CONCLUSION: An order of magnitude improvement in continuum image sensitivity is expected from reduced classical and sidelobe confusion.The improved image resolution and synthesised beam produce this expected reduction in the noise floor.
- 5 CONCLUSION: A factor of 3.5 improvement in EoR power-spectrum sensitivity is expected from the compact configuration’s regularly spaced hexagonal arrays.The hybrid array layout also supports improved and novel calibration techniques using redundant and non-redundant baselines.
- 5 CONCLUSION: Ongoing work includes an upgraded correlator, a new digital receiver, commensal Breakthrough Listen processing, and possible future baseline and frequency extensions.The upgraded correlator is intended to correlate all 256 tiles with increased bandwidth.