How VLBI works Very-long-baseline interferometry

recording data @ each of telescopes in vlbi array. extremely accurate high-frequency clocks recorded alongside astronomical data in order synchronization correct

in vlbi interferometry, digitized antenna data recorded @ each of telescopes (in past done on large magnetic tapes, nowadays done on large arrays of computer disk drives). antenna signal sampled extremely precise , stable atomic clock (usually hydrogen maser) additionally locked onto gps time standard. alongside astronomical data samples, output of clock recorded on tape/disk media. recorded media transported central location. more recent experiments have been conducted electronic vlbi (e-vlbi) data sent fibre-optics (e.g., 10 gbit/s fiber-optic paths in european geant2 research network) , not recorded @ telescopes, speeding , simplifying observing process significantly. though data rates high, data can sent on normal internet connections taking advantage of fact many of international high speed networks have significant spare capacity @ present.

at location of correlator data played back. timing of playback adjusted according atomic clock signals on (tapes/disk drives/fibre optic signal), , estimated times of arrival of radio signal @ each of telescopes. range of playback timings on range of nanoseconds tested until correct timing found.

playing data each of telescopes in vlbi array. great care must taken synchronize play of data different telescopes. atomic clock signals recorded data in getting timing correct.

each antenna different distance radio source, , short baseline radio interferometer delays incurred distance 1 antenna must added artificially signals received @ each of other antennas. approximate delay required can calculated geometry of problem. tape playback synchronized using recorded signals atomic clocks time references, shown in drawing on right. if position of antennas not known sufficient accuracy or atmospheric effects significant, fine adjustments delays must made until interference fringes detected. if signal antenna taken reference, inaccuracies in delay lead errors




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b




{\displaystyle \epsilon _{b}}

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c




{\displaystyle \epsilon _{c}}

in phases of signals tapes b , c respectively (see drawing on right). result of these errors phase of complex visibility cannot measured very-long-baseline interferometer.

the phase of complex visibility depends on symmetry of source brightness distribution. brightness distribution can written sum of symmetric component , anti-symmetric component. symmetric component of brightness distribution contributes real part of complex visibility, while anti-symmetric component contributes imaginary part. phase of each complex visibility measurement cannot determined very-long-baseline interferometer symmetry of corresponding contribution source brightness distributions not known.

r. c. jennison developed novel technique obtaining information visibility phases when delay errors present, using observable called closure phase. although initial laboratory measurements of closure phase had been done @ optical wavelengths, foresaw greater potential technique in radio interferometry. in 1958 demonstrated effectiveness radio interferometer, became used long-baseline radio interferometry in 1974. @ least 3 antennas required. method used first vlbi measurements, , modified form of approach ( self-calibration ) still used today.