Murchison Widefield Array
|Murchison Widefield Array|
An individual MWA-32T tile
|Location||Murchison Radio-astronomy Observatory in Western Australia|
|Wavelength||3.75 to 1 m (radio)|
|Telescope style||128 16-element dual-polarisation antennas|
|Collecting area||512 m2|
The Murchison Widefield Array (MWA) is a joint project between an international consortium of universities to build a low-frequency radio array operating in the frequency range 80–300 MHz. The main scientific goals of the MWA are to detect neutral atomic Hydrogen emission from the cosmological Epoch of Reionization (EoR), to study the sun, the heliosphere, the Earth's ionosphere, and to study radio transient phenomena. The total cost of the project is A$51 million.
The MWA is the first so-called large-N array, fully cross-correlating signals from 128 phased tiles, each of which consist of 16 crossed dipoles arranged in a 4x4 square. The field of view is large by the standard of astronomical instruments, being on the order of 30 degrees across.
The MWA was to be situated at Mileura Station where initial testing had been conducted then moved southwest Boolardy station in outback Western Australia, at the Murchison Radio-astronomy Observatory (MRO), 800 kilometres north of Perth. This location offers a quiet radio environment and stable climate for observations. The MRO is also the site of CSIRO's Australian Square Kilometre Array Pathfinder (ASKAP) and one of two candidate sites for the Square Kilometre Array (SKA). In addition to the geographic link, the MWA is a technology and science pathfinder for the SKA.
The MWA is an inherently versatile instrument with a wide range of potential science goals. Scientific priorities during the early science phase will be determined partly by the evolving instrumental capabilities, and partly by the potential of such studies to accelerate commissioning and the initiation of the key science projects.
In astronomy, the highest priority key science project is detection of red-shifted 21 cm signals from HI during the EoR, using power spectral techniques, direct detection of quasar ionised "bubbles", or both. The MWA will be one of the most sensitive EoR instruments yet constructed: its observations should characterise the properties of the sources that are responsible for ionizing the intergalactic medium, chart the evolution of the global neutral fraction, and probe the nature of quasar emissions by constraining the properties of their ionized proximity zones.
In solar, heliospheric and ionospheric (SHI) research, the highest priority is characterisation of the heliospheric magneto-ionic medium via interplanetary scintillation and Faraday rotation propagation effects using background astronomical radio sources.
Secondary key science projects include radio transient detection and monitoring, solar burst imaging, studies of ionospheric phenomena, and a variety of astronomical studies using all-sky survey data. Examples of the latter include Faraday tomography of the interstellar medium, the galactic distribution of cosmic rays, the hidden population of galactic supernova remnants, pulsar emission mechanisms and population statistics, and the low-frequency cosmic web. Most of these secondary projects can be conducted using data collected during or in support of the two highest-priority key science projects, in part because accurate calibration of the MWA requires comprehensive characterisation of the sky across wide instantaneous fields-of-view, as well as accurate characterisation of the behaviour of the ionosphere.
An MWA antenna comprises four by four regular grid of dual-polarisation dipole elements arranged on a 4m x 4m steel mesh ground plane. Each antenna (with its 16 dipoles) is known as a "tile". Signals from each dipole pass though a low noise amplifier (LNA) and are combined in an analogue beamformer to produce tile beams on the sky. Beamformers sit next to the tiles in the field. The radio frequency (RF) signals for the tile-beams are transmitted to a receiver, each receiver being able to process the signals from a group of eight tiles. Receivers therefore sit in the field, close to groups of eight tiles; cables between receivers and beamformers carry data, power and control signals. Power for the receivers is provided from a central generator. The receiver contains analogue elements to condition the signals in preparation for sampling and digitisation. The frequency range 80–300 MHz is Nyquist-sampled at high precision. Digital elements in the receiver (after the digitiser) are used to transform the time-series data to the frequency domain with a 1.28 MHz resolution – 5 bits real and 5 bits imaginary for each resolution element. Sets of 1.28 MHz coarse frequency channels are transmitted via an optical fibre connection to the correlator subsystem, located in the CSIRO Data Processing Facility near the MWA site. MWA shares the CSIRO facility with the ASKAP program.
The majority of the tiles (112) will be scattered across a roughly 1.5 km core region, forming an array with very high imaging quality, and a field of view of several hundred square degrees at a resolution of several arcminutes. The remaining 16 tiles will be placed at locations outside the core, yielding baseline distances of about 3 km to allow higher angular resolution for solar burst measurements.
The correlator subsystem comprises Poly-phase Filter Bank (PFB) boards that convert the 1.28 MHz coarse frequency channels into channels with 10 kHz frequency resolution in preparation for cross-correlation. Correlator boards then cross-multiply signals from all tiles to form visibility data. A distributed clock signal drives the coherence of receivers in the field and maintains timing for the correlator.
Data from the correlator subsystem are transmitted to a RealTime Computer Processing Array (RTC), which is also located in the CSIRO Data Processing Faciltity. The primary function of the RTC is to run the RealTime Software (RTS); a software suite that performs realtime calibration and imaging of the correlator output. The output information within the RTC/RTS is then further processed, depending on the science mode in operation at a given time. The RTS also writes out calibration data, including bright source measurements, tile gain solutions, and parameters for the properties of the ionosphere above the MWA site. The science output files and calibration data are written to an off-site archive for further analyses.
The MWA will be operated remotely through an interface to a Monitor and Control (M&C) software package resident on a dedicated computer located within the CSIRO Data Processing Facility at the MWA site. The M&C software maintains a state-based description of the hardware and an event-driven database describing the observation scheduling of the Instrument. M&C software commands several elements of the system including pointing and tracking of the beamformers, frequency selection of the receivers, correlation parameters for the correlator, and RTC/RTS functions, amongst others. The M&C system contributes to the MWA archive by storing instrument "metadata" into an external database. This includes both the instrument configurations for each observation and also housekeeping information collected from various hardware components.
Data will be transferred from the RTC storage disks to the MWA archive located at the end of a high-bandwidth network connection. The primary MWA data archive(s) will likely be located in Perth, with copies in other locations in Australia and the US. The resultant qualified data will then be provided to, and stored by, the various scientific databases for subsequent distribution to the respective scientific communities for analysis and interpretation.
A 32-tile prototype (MWA-32T) was constructed and operated with increasing capability over the period 2007–2011, testing telescope hardware and making preliminary science observations, including initial observations of EoR fields. A 512-tile instrument (512T) was planned but de-scoped due to funding issues. The 128-tile instrument (MWA) was constructed in 2012, commencing science operations in early 2013. The infrastructure on-site at MRO will allow an eventual build-out to 256 tiles, increasing the sensitivity and resolution of the instrument.
The MWA Project is composed of the following project partners, in no particular order:
Funding for the MWA to date has been provided by partner institutions and by allocations from national funding agencies: the US National Science Foundation, the Australian Research Council (ARC), National Collaborative Research Infrastructure Strategy (NCRIS), Australia-India Strategic Research Fund Overview (AISRF) and support to the Raman Research Institute (RRI) for MWA in India.
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