Event Horizon Telescope

The Event Horizon Telescope (EHT) is an international collaboration using Earth-sized VLBI with an array of millimeter/submillimeter telescopes around the world, to study the supermassive black hole (SMBH), Sagittarius A* at the galactic center of the Milky Way. The effort's name refers to its goal of resolving the black hole's event horizon, more specifically, resolving the observable aspects of the object at that scale. The SMBH, M87* (at the center of galaxy M87) is also a target, as well as other objects as the project progresses, used for both calibration and study. Observations are carried out roughly once a year, when the geometry is such that EHT's telescopes can simultaneously observe the targets, collecting data subject to the intended interference. The effort aims for baselines in multiple directions extending to as much as 12000 km, as large as can be done from the ground, and appropriate telescopes are recruited each year toward reaching this goal, which have included many of the large telescopes that cover the EHT's wavelength range, among them, the Atacama Large Millimeter Array (ALMA), the IRAM 30m Telescope, the Large Millimeter Telescope, and the Heinrich Hertz Submillimeter Telescope. The ELT effort has included relocating a suitable telescope to Greenland (Greenland Telescope) to allow baselines reaching near the north pole, and another to Namibia (Africa Millimetre Telescope). Identical receivers for the telescopes have been developed so as to collect compatible data, which is collected and shipped on hard drives, then analyzed by independent teams (taking months or years), because the necessary analysis must make use of experimental and probabilistic methods in order to achieve the desired angular resolution.

The first image was taken in 2006 and the first image that succeeded in detecting one of the black hole shadows (of M87*) was taken in 2018 and announced in 2019 after analysis of the image revealed the shadow. In 2022, such an image of Sagittarius A*'s black hole shadow was announced. In subsequent years, more images of the two have been captured, including some images revealing their magnetic fields through polarimetry.

ngEHT is a concept in planning for expanding and upgrading the EHT, doubling the number of sites and observing at an additional lower frequency. Its planning received some initial funding in 2019.