The Event Horizon Telescope (EHT) is a large telescope array consisting of a global network of radio telescopes and combining data from several very-long-baseline interferometry (VLBI) stations around the Earth. The aim of the EHT project is to observe the immediate environment of supermassive black holes, with an angular resolution high enough to resolve structures on the size scale of the black hole's event horizon. Among the project's observational targets are the two black holes with the largest apparent angular size: M87* at the center of the supergiant elliptical galaxy Messier 87, an...