By Orbital Team
A supermassive black hole's event horizon, even for one of the largest known, subtends an angle in the sky so small that no single dish telescope built or buildable could resolve it. The angular resolution needed was roughly equivalent to reading a newspaper on the surface of the Moon from Earth.
Very long baseline interferometry solves this by linking radio telescopes scattered across different continents so they observe the same target simultaneously, then combining their signals as if they were one telescope with a dish the size of the distance between the stations, in practice, the diameter of the Earth itself.
Each station recorded raw data alongside an extremely precise atomic clock timestamp, since the signals had to be combined after the fact with nanosecond level timing accuracy. The physical linking happens entirely in software, correlating petabytes of recorded data long after the actual observation ended.
Weather was a major constraint: every participating station needed clear skies simultaneously, since radio interferometry at the frequencies used is still degraded by atmospheric water vapor, which narrowed the usable observing windows to a handful of nights per year.
The resulting image, an orange ring of glowing plasma around a dark central shadow, was not a photograph in any conventional sense. It was a mathematical reconstruction built from the interference patterns of radio waves collected by telescopes that never once pointed at each other, only at the same distant target, together.