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The Expanded Very Large Array -- a New Telescope for New Science
R. A. Perley, C. J. Chandler, B. J. Butler, J. M. Wrobel
TL;DR
The paper addresses the VLA’s aging sensitivity, frequency coverage, and spectral-resolution capabilities by describing the EVLA upgrade, its goals, status, and access programs. The upgrade expands observing capability substantially, while commissioning continues to address data transmission, interference, and processing requirements.
Problem
The VLA’s fundamental capabilities changed little after 1980, limiting sensitivity, frequency coverage, and spectral resolution as astronomical science demands expanded.
Method
The EVLA project combines modern high-speed wide-band digital and receiver technologies with the VLA’s existing infrastructure, alongside commissioning and algorithm development.
Results
The EVLA expands bandwidth to 8 GHz per polarization, covers 1–50 GHz, and adds a highly flexible correlator, producing orders-of-magnitude greater scientific capability than the VLA.
Takeaways & Limitations
The expanded capabilities support investigations of radio transients, evolving radio-emitting objects, cosmic magnetic fields, and dust-obscured regions.
Takeaways & Limitations
The current maximum antenna-to-correlator transfer is 4 GHz, while the full 16 GHz capability was not expected for science observing until at least mid-2012.
Abstract
from arXiv · showhide
Since its commissioning in 1980, the Very Large Array (VLA) has consistently demonstrated its scientific productivity. However, its fundamental capabilities have changed little since 1980, particularly in the key areas of sensitivity, frequency coverage, and velocity resolution. These limitations have been addressed by a major upgrade of the array, which began in 2001 and will be completed at the end of 2012. When completed, the Expanded VLA -- the EVLA -- will provide complete frequency coverage from 1 to 50 GHz, a continuum sensitivity of typically 1 microJy/beam (in 9 hours with full bandwidth), and a modern correlator with vastly greater capabilities and flexibility than the VLA's. In this paper we describe the goals of the EVLA project, its current status, and the anticipated expansion of capabilities over the next few years. User access to the array through the OSRO and RSRO programs is described. The following papers in this special issue, derived from observations in its early science period, demonstrate the astonishing breadth of this most flexible and powerful general-purpose telescope.
1. INTRODUCTION
The VLA is a flexible, reconfigurable radio interferometer whose original 1980-era technology limited bandwidth, frequency coverage, and spectral resolution. The EVLA project responds by applying modern transmission, receiver, and digital technologies to expand those capabilities.
- 1. INTRODUCTION: The VLA uses 27 antennas arranged along three 21 km arms and can be reconfigured across four maximum-baseline configurations.These configurations provide different resolutions and image surface-brightness sensitivities.
- 1. INTRODUCTION: At completion in 1980, the VLA supported four frequency bands, 100 MHz per polarization, and up to 512 spectral channels.Its correlator could instead provide full-Stokes visibilities without spectral resolution.
- 1. INTRODUCTION: For two decades, largely unchanged signal-transmission and correlation systems froze the VLA’s bandwidth and spectral resolution at 1980 levels.The array’s physical scale and observing modes remained important sources of flexibility.
- 1. INTRODUCTION: Modern technologies promised at least an order-of-magnitude improvement in sensitivity, frequency coverage, and spectral resolution at modest cost.These advances addressed expanding demands for rapid response, fast imaging, precision polarimetry, higher sensitivity, wider frequency coverage, and higher spectral resolution.
- 1. INTRODUCTION: The EVLA project began in 2001 and was scheduled for completion by the end of 2012 as a U.S., Canadian, and Mexican partnership.The project had an inflation-adjusted budget of $96M in 2011 dollars.
2. KEY EVLA GOALS AND CAPABILITIES
The EVLA project targets order-of-magnitude capability improvements organized around four science themes, supported by expanded frequency coverage, high-bandwidth electronics, and the flexible WIDAR correlator.
- Science themes: The EVLA’s science goals span magnetic fields, obscured objects, rapidly evolving transients, and the formation and evolution of cosmic objects.These themes frame the anticipated science enabled by the expanded array.
- Array capabilities: 1 to 50 GHz complete frequency coverage is provided through eight new or improved receiver bands.The receivers use state-of-the-art technology to extend the array’s frequency access.
- Array capabilities: 8 GHz/polarization processing is supported by new antenna electronics and high-speed samplers.The electronics process eight signal channels of up to 2 GHz each, while samplers handle up to 8 GHz per polarization.
- WIDAR correlator: 10 peta-32-bit ops/sec WIDAR correlator produces cross-power spectral visibilities for all array baselines.Its design supports wide instantaneous bandwidth, extensive spectral-channel capacity, full polarization, and reconfigurable spectral windows.
- WIDAR correlator: 0.12 Hz to 2000 kHz adjustable spectral resolution enables resources to be reallocated among spectral windows, polarization products, and resolution.The correlator provides at least 16384 channels per baseline and can exceed 4 million channels.
- Specialized modes: Specialized modes support phased-array processing, pulsar binning, up to eight simultaneous subarrays, and external data capture.Pulsar binning provides up to 2000 phase bins and temporal resolution as short as 200 µsec with all spectral channels.
3. EVLA CAPABILITIES GROWTH
The EVLA’s capabilities expanded progressively during the VLA conversion, while continued operation was maintained. Receiver coverage, bandwidth, spectral-channel capacity, observing modes, and processing requirements all increased substantially.
- Antenna and correlator growth: The conversion preserved observing through backward compatibility, with only a single 7-week shutdown during construction.Capabilities were released incrementally before project completion.
- Antenna and frequency band capabilities: The current antenna-to-correlator transfer limit is 4 GHz, while the full 16 GHz capability was not expected for science observing until at least mid-2012.This bandwidth limitation was separate from receiver availability.
- Growth in correlator capabilities: WIDAR increased available bandwidth per polarization from 256 MHz to 2 GHz, with up to 8 GHz and 4 million channels expected by January 2013.The 256-MHz-to-2-GHz increase could increase dataset size by a factor of 8.
- Science commissioning: EVLA operations required new observing, calibration, post-processing procedures, and software, including automated handling of variable radio-frequency interference below roughly 10 GHz.Some specialized observing modes were expected to take longer to become available.
4. USING THE EVLA
EVLA observing is open worldwide through semester-based proposals and an online preparation workflow. Approved observations are configured into scheduling blocks, queued dynamically, monitored, and archived for later access.
- Access and scheduling: Observing time is open to astronomers worldwide, with no quotas or reserved time blocks.Beginning in 2011, scheduling used six-month semesters.
- Access and scheduling: Proposal deadlines are 5pm Eastern Time on February 1 and August 1.The deadlines nominally cover observing beginning in August or February, respectively.
- Proposal submission: Astronomers use the Proposal Submission Tool to submit observation requests, scientific and technical justifications, and available funding requests.Proposal evaluation includes technical review by NRAO staff and panels.
- Observation preparation: The Observation Preparation Tool converts approved proposals into Scheduling Blocks specifying sources, instrumental setups, and timing, which are submitted to a dynamic queue.NRAO staff examine queued blocks using the Observation Scheduling Tool.
- Observation operations: After observations, staff monitor array health, maintain logs, archive data, and provide astronomers with an archive-access link.Archived data remain proprietary to proposal authors for 12 months.
5. DATA POST-PROCESSING, PIPELINES, AND ALGORITHM DEVELOPMENT
EVLA data volumes and spectral complexity require scalable processing, algorithmic improvements, and faster computing and I/O. Planned support combines automated standard-mode reduction with flexible software for nonstandard or user-modified processing.
- Data scale: Large instantaneous bandwidth and flexibly arranged, non-contiguous spectral windows create substantially greater post-processing needs than the VLA.Spectral windows can vary in width and frequency resolution.
- Data scale: EVLA data commonly exceed 5 MB/s, while WIDAR can produce up to 350 GB/s and datasets larger than 1 TB.These rates drive requirements for scalable calibration and imaging.
- Pipelines: Standard observing modes such as continuum, HI spectral-line, and polarization were intended for mostly automated reduction using observation metadata and heuristics.An automated pipeline was planned but was not yet available at the time described.
- Pipelines: A general post-processing package was required for data unsuitable for reliable pipeline reduction or for astronomers wishing to modify pipeline operations.The package had to transform measured visibilities into final image cubes.
- Algorithm development: CASA lacked some EVLA reduction capabilities, prompting implementation work and algorithm development for flagging, wide-field polarization imaging, RFI excision, and ionospheric correction.Several listed areas lacked generally accepted algorithmic solutions.
- Computing infrastructure: A planned computing cluster of tens of nodes, together with faster CASA code, was intended to support pipeline and interactive processing.The cluster was planned for both automatic standard-mode reduction and astronomer-directed reduction.
6. SUMMARY
The EVLA expands the VLA with wider bandwidth, complete 1–50 GHz coverage, and a highly flexible high-resolution correlator. The resulting capabilities target a broad range of investigations across transient, evolving, magnetic, and obscured radio sources.
- Summary: The EVLA provides 8 GHz per polarization, complete 1–50 GHz frequency coverage, and a new correlator with high spectral resolution and flexibility.These changes are described as orders-of-magnitude improvements over the VLA.
- Summary: The expanded capabilities support investigations of celestial radio transients, evolving radio-emitting objects, cosmic magnetic fields, and dusty obscured regions.The paper frames the EVLA as a general-purpose centimeter-wave imaging telescope for the world user community.
- Acknowledgments: The project depended on contributions from hundreds of individuals and support from U.S., Canadian, and Mexican funding agencies.The acknowledgments identify the NSF, National Research Council, and Mexican Consejo Nacional De Ciencia y Tecnología.