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Very Long Baseline Interferometry

What if we could build a telescope the size of Earth? That’s obviously not practical, but combining multiple telescopes into one giant observatory brings us as close as we can. Using a  technique known as very long baseline interferometry (VLBI), researchers link observations from radio telescopes separated by whole continents into one virtual observatory. VLBI allows high-resolution radio light observations of the centers of galaxies and other regions of space difficult to observe with single telescopes.

Our Work

Center for Astrophysics | Harvard & Smithsonian astronomers develop and contribute to VLBI observations, including:

  • Collaborating with other institutions to operate the Event Horizon Telescope, to capture an image of the ring of light surrounding the Milky Way’s supermassive black hole in the galaxy M87. Many observatories, including the CfA’s Submillimeter Array (SMA) Atacama Large Millimeter Array (ALMA) and the South Pole Telescope (SPT), are part of the EHT. The Greenland Telescope was constructed in part to contribute to the expanded EHT, which continues to observe M87 and the black hole at the center of the Milky Way. 
    CfA Plays Central Role In Capturing Landmark Black Hole Image

  • Mapping the size and structure of the Milky Way to great precision. Since we live inside the galaxy, astronomers have to observe through increasingly more clouds of gas and dust to map its farther regions. To perform this task, researchers often rely on VLBI methods, since radio light penetrates the dust. Comparing the position of an intense light source known as a maser to that of a distant quasar, astronomers were able to measure the distance across the galaxy to high precision.
    Measuring the Distance to the Far Side of the Galaxy

  • Studying the central regions of galaxies to understand what drives quasars and other energetic astronomical events. Observations of the galaxy 3C 273 using the Very Long Baseline Array (VLBA) showed that the black hole heats gas up to as much as a trillion degrees, making this one of the hottest places in the entire universe.
    Challenging the Brightness Limits of Quasars

From Many Telescopes, One Observatory

One of the biggest challenges in telescope design is “resolution”: how well can a telescope distinguish details on an astronomical source? Can that telescope distinguish — resolve, in astronomer’s terms — fine details in distant sources?

Resolution depends primarily on two things: the size of the telescope and the wavelength of the light the telescope is designed to see. Generally speaking, bigger telescopes can achieve higher resolution, but there’s a practical limit to how large we can make them.

That’s where interferometry comes in. When acting together as an interferometer, two or more telescopes observing the same object at the same time have the same resolution as one telescope as big as the separation between the real instruments. Comparing the differences in the distances the light travels to get to each telescope produces an interference pattern, enabling very precise astronomical observations.

VLBI combines telescopes separated by huge distances, creating continent-sized virtual observatories. To coordinate observations across thousands of miles, astronomers must precisely synchronize data collection using atomic clocks.

With VLBI virtual observatories that span the globe, astronomers can achieve extremely high resolution, allowing observation of hard-to-study places such as the centers of galaxies where supermassive black holes live. The most ambitious VLBI project to date is the Event Horizon Telescope (EHT), a coalition of observatories ranging from Greenland to the South Pole, which captured the first image of a black hole in the nearby galaxy M87.


A map of observatories making up the Event Horizon Telescope (EHT), which provided the first image of a black hole ever taken. The green icons mark the sites used in that observation, while purple icons represent additional observatories being added to the array, and blue icons mark telescopes no longer part of EHT.

Credit: EHT