May 14, 2020

Very Long Baseline Interferometry (VLBI)

Telescopes must be large in order to resolve fine detail in celestial objects. The sizes of telescopes are, however, in practice limited due to constraints in engineering and budgets. Very long baseline interferometry (VLBI) allows to partially overcome these constraints by forming “virtual” telescopes that can have a size larger than the diameter of Earth.

To do this, VLBI connects arrays of several telescopes of limited size. The resulting synthesized telescope achieves an angular resolution driven by the largest separation between the telescopes within the array. Provided the array is Earth–bound, separations equal to Earth’s diameter can be achieved. Some VLBI experiments also include telescopes in space, thus allowing to achieve telescope separations larger than Earth’s diameter.

The connections within a VLBI network are formed “offline,” that is, by processing recorded telescope data well after the end of observations. To do this, the stations within a VLBI array write the telescope data onto very fast hard disks, with incredibly precise timestamps provided by maser clocks. These disks are then physically shipped to a research institute hosting a “correlator,” which is a fast computer that can jointly process the data from several VLBI stations in order to synthesize a virtual telescope. The correlator at Haystack Observatory, for example, is used to process data from the Event Horizon Telescope project and the VLBI Global Observing System (VGOS).

VLBI is a well-established technology with a long history. It was developed in the 1960s, with Haystack Observatory playing a central role in creating this technique. VLBI technology continues to evolve. Current engineering projects focus, for example, on recording VLBI telescope data at ever-increasing rates, and on making VLBI techniques work at frequencies well above 100 GHz. Haystack continues to play a world-leading role in these activities.

Common terms

A baseline is the distance between two VLBI antennas. The pair of antennas are often very far apart—in some cases, thousands of kilometers. Longer baselines increase the sensitivity of the measurements.

Interferometry is a technique that uses pairs of radio antennas. The radio antennas, or telescopes, observe the same distant source at the exact same time. Their signals are superimposed (correlated) in a mathematical process that makes use of interference. In astronomy, interferometry is used to image distant objects. In geodesy, interferometry is used to measure the earth.

At correlator, the signals from many radio telescopes around the globe are combined, or correlated, using interferometry. The result of using signals from a pair of telescopes gives more information than the signal from either single telescope.

radio waves
Radio waves are a type of electromagnetic (EM) radiation. Everything in the universe emits EM radiation, including celestial objects. Measuring the radio signal or radio waves from objects in space allows us to obtain an enormous amount of information about these objects.