Summary
Very-long-baseline interferometry (VLBI) uses many radio telescopes spread across the Earth. It makes the sharpest possible astronomical images. In 2019 the Event Horizon Telescope (EHT) used VLBI to take the first picture of a black hole at the center of a galaxy called M87. A picture of the black hole in our own galaxy, the Milky Way, followed in 2022. Synchronization of telescopes on different continents needs an extremely precise atomic clock. At the high frequencies needed to make the sharpest pictures the clock must be even more precise. The EHT uses the highest radio frequencies.
