Very-long-baseline interferometry

Data received at each antenna in the array include arrival times from a local atomic clock, such as a hydrogen maser.

The VLBI technique enables the distance between telescopes to be much greater than that possible with conventional interferometry, which requires antennas to be physically connected by coaxial cable, waveguide, optical fiber, or other type of transmission line.

The antenna signal is sampled with an extremely precise and stable atomic clock (usually a hydrogen maser) that is additionally locked onto a GPS time standard.

experiments have been conducted with "electronic" VLBI (e-VLBI) where the data are sent by fibre-optics (e.g., 10 Gbit/s fiber-optic paths in the European GEANT2 research network) and not recorded at the telescopes, speeding up and simplifying the observing process significantly.

If the position of the antennas is not known to sufficient accuracy or atmospheric effects are significant, fine adjustments to the delays must be made until interference fringes are detected.

[3][4] Neglecting atmospheric pressure and hydrological loading corrections at the observation level can also contaminate the VLBI measurements by introducing annual and seasonal signals, like in the Global Navigation Satellite System time series.

As the phase of each complex visibility measurement cannot be determined with a very-long-baseline interferometer the symmetry of the corresponding contribution to the source brightness distributions is not known.

Although his initial laboratory measurements of closure phase had been done at optical wavelengths, he foresaw greater potential for his technique in radio interferometry.

When one or both of these arrays are combined with space-based VLBI antennas such as HALCA or Spektr-R, the resolution obtained is higher than any other astronomical instrument, capable of imaging the sky with a level of detail measured in microarcseconds.

VLBI generally benefits from the longer baselines afforded by international collaboration, with a notable early example in 1976, when radio telescopes in the United States, USSR and Australia were linked to observe hydroxyl-maser sources.

[12] This technique is currently being used by the Event Horizon Telescope, whose goal is to observe the supermassive black holes at the centers of the Milky Way Galaxy and Messier 87.

[15] VLBI has traditionally operated by recording the signal at each telescope on magnetic tapes or disks, and shipping those to the correlation center for replay.

[16] In the quest for even greater angular resolution, dedicated VLBI satellites have been placed in Earth orbit to provide greatly extended baselines.

[citation needed] The first dedicated SVLBI satellite was HALCA, an 8-meter radio telescope, which was launched in February 1997 and made observations until October 2003.

Some of the Atacama Large Millimeter Array radio telescopes
The eight radio telescopes of the Smithsonian Submillimeter Array , located at the Mauna Kea Observatory in Hawai'i
A blurry photo of a supermassive black hole in M87.
VLBI was used to create the first image of a black hole, imaged by the Event Horizon Telescope and published in April 2019. [ 1 ]
Recording data at each of the telescopes in a VLBI array. Extremely accurate high-frequency clocks are recorded alongside the astronomical data in order to help get the synchronization correct
Playing back the data from each of the telescopes in a VLBI array. Great care must be taken to synchronize the play back of the data from different telescopes. Atomic clock signals recorded with the data help in getting the timing correct.
Geodesist Chopo Ma explains some of the geodetic uses of VLBI.
Distance to Malargue station from the other stations of the NASA VLBI network
Image of the source IRC +10420 . The lower resolution image on the left was taken with the UK's MERLIN array and shows the shell of maser emission produced by an expanding shell of gas with a diameter about 200 times that of the Solar System . The shell of gas was ejected from a supergiant star (10 times the mass of the Sun) at the centre of the emission about 900 years ago. The corresponding EVN e-VLBI image (right) shows the much finer structure of the masers made visible with the higher resolution of the VLBI array.