How you get an image like this
A telescope's resolution depends on its size. To see an object this small in the sky, the dish would have to be the size of a planet, which is impossible to build, but widely separated antennas can be combined.
Each observatory records the signal with an ultra-precise timestamp from an atomic clock. The recordings are then combined, so the antennas work like pieces of one enormous mirror.
There was so much data that sending it over the internet was impractical. The disks were physically flown to two computing centers, and some of the cargo from the southern station had to wait until winter ended.
What the image actually shows
- Record simultaneously
- Fly disks by plane
- Combine signals
- Reconstruct image
You can't see the hole itself, but its shadow: a dark circle against the glowing matter around it. Light cannot escape from beneath the event horizon, but the hot gas nearby creates a ring whose shape could be measured.
To avoid mistaking what they hoped to see for reality, independent teams reconstructed the image using different methods without showing one another the results. The matching rings became the main argument.
The first image was taken of a distant giant galaxy, not the center of our own: the object there is larger, and the image changes more slowly. The center of the Milky Way was imaged later, and it proved more difficult because it changes rapidly.
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Sources
- The image of the shadow of the supermassive object at the center of the galaxy M87 was published in 2019 by the Event Horizon Telescope collaboration
- it used very-long-baseline interferometry, combining observatories on different continents with synchronization to atomic clocks. The data was transported on physical storage media. The image of the object at the center of the Milky Way was published in 2022