VLBI

VLBI
Name	Very-long-baseline interferometry
Caption	Array example
Type	Astronomical technique
Invented	1960s
Inventors	Antony Hewish, Martin Ryle, John Bolton
Institutions	National Radio Astronomy Observatory, MIT Haystack Observatory, European Southern Observatory
Notable users	Event Horizon Telescope, Very Long Baseline Array, Max Planck Institute for Radio Astronomy

VLBI is an astronomical observational technique that achieves the highest angular resolution in radio astronomy by combining signals from widely separated radio telescopes. It enables imaging and astrometry at milliarcsecond and sub-milliarcsecond scales, supporting studies of compact sources across the sky. The method underpins major projects and observatories and has been crucial for tests of fundamental physics and geodesy.

Introduction

Very-long-baseline interferometry operates by synchronously recording radio-frequency wavefronts at multiple facilities such as Karl G. Jansky Very Large Array, Atacama Large Millimeter/submillimeter Array, Arecibo Observatory, Green Bank Observatory, and international dishes in arrays like the Very Long Baseline Array and the European VLBI Network. Its reach extends to collaborations involving institutions such as National Radio Astronomy Observatory, Institute for Radio Astronomy in the Millimeter Range, Max Planck Institute for Radio Astronomy, and missions supported by agencies including NASA, European Space Agency, and Japan Aerospace Exploration Agency. VLBI observations contribute to projects like the Event Horizon Telescope, the RadioAstron mission, and multiwavelength campaigns with telescopes such as Hubble Space Telescope and Chandra X-ray Observatory.

Principles and Technique

The technique relies on precise timekeeping using standards like hydrogen masers and comparisons to timing systems maintained by International Bureau of Weights and Measures, enabling synchronization across baselines spanning continents and space to baselines like those involving Spektr-R hardware. Signals are timestamped and recorded at stations such as Westerbork Synthesis Radio Telescope and Effelsberg 100-m Radio Telescope, preserving phase and amplitude information for later combination. Interferometric observables—fringe phases, amplitudes, and delays—are derived from cross-correlation of recorded data, allowing reconstruction of brightness distributions via Fourier inversion methods employed in software developed at centers like MIT Haystack Observatory and JIVE (Joint Institute for VLBI ERIC). Calibration uses references such as quasars cataloged by programs connected to International Celestial Reference Frame efforts.

Instrumentation and Networks

Key hardware includes large apertures like Lovell Telescope, cryogenically cooled receivers developed at facilities including National Radio Astronomy Observatory, digital backends using FPGA platforms from labs such as NRAO Electronics Division, and recording systems evolved from magnetic tapes to disk arrays and high-speed networks exemplified by e-VLBI links. Networks encompass regional and global consortia: the European VLBI Network, the Long Baseline Array (Australia), the East-Asian VLBI Network, and space-ground combinations like the RadioAstron project. Supporting institutions include Caltech, University of Cambridge, Leiden University, and national observatories in China, India, Russia, South Africa, and Chile.

Data Processing and Correlation

Raw data are correlated in supercomputing facilities at correlator centers such as DiFX clusters operated by MIT Haystack Observatory, the JIVE correlator, and hardware correlators from collaborations with Arecibo Observatory and NRAO. Correlators solve for geometric delays using Earth orientation parameters provided by International Earth Rotation and Reference Systems Service, applying models including tropospheric delays referenced to products from European Centre for Medium-Range Weather Forecasts and ionospheric corrections using data from global navigation satellite systems like Global Positioning System and GLONASS. Post-correlation imaging and model-fitting employ packages developed by teams associated with Bell Labs, Harvard-Smithsonian Center for Astrophysics, and Jodrell Bank Centre for Astrophysics.

Scientific Applications

VLBI enables imaging of accretion flows and jets in active galactic nuclei studied by groups at Harvard University and Max Planck Institute for Radio Astronomy, direct imaging of black hole shadows in campaigns led by the Event Horizon Telescope collaboration and institutions including MIT, CfA, and University of Arizona, and measurement of parallaxes and proper motions for masers and pulsars by projects involving NRAO and JAXA. Geodetic VLBI supports terrestrial reference frame maintenance by International GNSS Service partners and monitors plate motions critical to work at California Institute of Technology and University of Cambridge. VLBI has contributed to tests of general relativity exemplified by experiments connected to European Space Agency missions and constrains on cosmological parameters via angular-size measurements of compact radio cores studied at University of Bonn and Princeton University.

Limitations and Challenges

Practical limits arise from atmospheric phase stability over arrays including stations in Hawaii, Chile, South Africa, and Australia, requiring water-vapor radiometer data and site characterization like that performed by Atacama Large Millimeter/submillimeter Array teams. Sensitivity constraints depend on collecting area and bandwidth requiring coordination among facilities such as Effelsberg, GBT, and Phased ALMA efforts. Data volumes and real-time correlation impose demands on networks like European Research and Education Network and computing centers at National Institute for Supercomputing Applications. Legal, funding, and international coordination challenges involve agencies such as National Science Foundation, European Commission, and national ministries of science.

History and Development

Origins trace to early interferometry experiments at institutions like Cambridge University and contributions from scientists associated with Cavendish Laboratory and Radiophysics Laboratory (Australia). Pioneering VLBI demonstrations in the 1960s involved collaborations among groups at Haystack Observatory, Jodrell Bank Observatory, and MIT, evolving through tape-based campaigns with participants from US Naval Research Laboratory and Max Planck Institute toward digital and e-VLBI eras. Major milestones include space-VLBI missions such as RadioAstron and the development of the Event Horizon Telescope synthesis, driven by consortia of universities and observatories including Caltech, Smithsonian Institution, and international partners.

Category:Radio astronomy