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| DiFX correlator | |
|---|---|
| Name | DiFX correlator |
| Developer | Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Australian National University, National Radio Astronomy Observatory |
| Released | 2000s |
| Programming language | C (programming language), Fortran |
| Platform | Linux, High-performance computing, Grid computing |
| License | GNU General Public License |
DiFX correlator
The DiFX correlator is a software-based astronomical correlator used for interferometric data processing in radio astronomy, developed to replace hardware correlators and enable flexible processing for arrays such as the Very Long Baseline Array, Australian Long Baseline Array, and international very long baseline interferometry projects involving institutions like the European VLBI Network, National Radio Astronomy Observatory, CSIRO, and the Max Planck Institute for Radio Astronomy. It provides real-time and post-processing correlation for observations from facilities including the Atacama Large Millimeter/submillimeter Array, MeerKAT, Square Kilometre Array pathfinders, and geodetic campaigns by agencies such as the International VLBI Service.
DiFX is a distributed, software-based correlator that implements the cross-correlation of digitized voltage streams from radio telescopes to produce visibility data for synthesis imaging and astrometry, integrating algorithms from digital signal processing research at institutions like MIT, Caltech, and Harvard–Smithsonian Center for Astrophysics. It supports heterogeneous arrays combining telescopes such as the Parkes Observatory, Westerbork Synthesis Radio Telescope, Effelsberg 100-m Radio Telescope, and the Green Bank Telescope, with scheduling and control interoperability alongside systems from Astronomical Image Processing System and Common Astronomy Software Applications.
Development began in the early 2000s at the Centre for Astrophysics and Supercomputing at Swinburne University of Technology with contributions from engineers and scientists affiliated with the Australian National University, Curtin University, and the National Radio Astronomy Observatory. Early deployments involved collaboration with the Long Baseline Array and testing on baseline campaigns with the Mark II VLBI System and experiments coordinated by the International VLBI Service. Subsequent enhancements were driven by requirements from projects led by the European VLBI Network and proposals associated with the Square Kilometre Array precursor initiatives involving CSRIO Astronomy and Space Science and partners such as ASTRON.
The software architecture leverages distributed computing nodes managed by middleware compatible with Message Passing Interface implementations and high-performance storage systems used at facilities like the Pawsey Supercomputing Centre and NCSA. Core components include data input handlers for disk-based and real-time streams from backends like the Roach boards and digitizers used at the Joint Institute for VLBI ERIC stations, an FX-style correlator engine implementing Fourier transforms and fringe rotation informed by techniques from Signal processing (DSP) research groups at University of Cambridge and Columbia University, and output modules producing standard formats ingestible by tools such as CASA and AIPS. Control software interfaces with scheduling systems and monitors developed in collaboration with observatory operations teams at CSIRO and NRAO.
DiFX supports multiple operational modes including real-time e-VLBI streams coordinated via high-speed networks like GEANT and the Internet2 research network, disk-based playback for post-observation processing, and hybrid modes for mixed real-time and recorded data involving partners such as Telstra and national research and education networks. Features include multi-band correlation, pulsar gating and binning developed with expertise from Jodrell Bank Observatory and CSIRO Astronomy and Space Science, spectral zoom windows for maser studies used by teams at the Max Planck Institute for Radio Astronomy, and robust clock model handling compatible with timekeeping services such as the International Bureau of Weights and Measures.
Performance scales with the number of compute nodes and network interconnects typical of clusters at centers like Murchison Widefield Array and the Pawsey Supercomputing Centre, with benchmarks demonstrating efficient throughput on commodity x86 servers running Linux and MPI libraries such as OpenMPI. DiFX has been used in high-bandwidth experiments achieving wide fractional bandwidths for continuum and spectral-line science carried out by consortia including the European VLBI Network and campaigns coordinated by the International VLBI Service for geodesy. Operational deployments have validated DiFX for precision astrometry applications undertaken by teams at Harvard–Smithsonian Center for Astrophysics and timing studies supported by groups associated with the Parkes Pulsar Timing Array.
Science enabled by DiFX includes imaging of active galactic nuclei studied by collaborations involving the Event Horizon Telescope partner institutions, maser mapping programs led by researchers at the Max Planck Institute for Radio Astronomy and University of Manchester, geodetic VLBI coordinated by the International VLBI Service, and pulsar timing projects associated with the European Pulsar Timing Array and Parkes Pulsar Timing Array. DiFX has been integrated into observational campaigns with arrays like the Asian VLBI Network and contributed to methodological advances reported by researchers at CSIRO, Swinburne University of Technology, and the National Radio Astronomy Observatory.
The correlator is distributed as open-source software under the GNU General Public License with source repositories and contribution practices influenced by norms at organizations such as GitHub and development workflows used by the Apache Software Foundation community. Packaging and deployment use build tools and continuous integration techniques common to projects supported by supercomputing centers like Pawsey Supercomputing Centre and collaborative infrastructures managed by institutions such as Curtin University.