Astronomers don't take a picture of a black hole in the traditional sense. They can't directly see a black hole because it doesn't emit light. Instead, they capture an image of the event horizon, the point of no return around a black hole.

Here's how they do it:

1. Observe the accretion disk: Black holes are surrounded by a swirling disk of gas and dust called an accretion disk. This disk gets incredibly hot and emits a lot of radiation, especially radio waves.

2. Radio telescopes: Astronomers use a network of radio telescopes spread across the globe. These telescopes work together to create a virtual telescope with a diameter as large as the Earth itself.

3. Very Long Baseline Interferometry (VLBI): This technique allows astronomers to combine signals from multiple telescopes, essentially stitching together their observations to create an image with very high resolution.

4. Reconstruction: The combined data is then analyzed and processed to create an image of the shadow of the black hole cast against the glowing accretion disk.

The first image of a black hole, captured by the Event Horizon Telescope (EHT) in 2019, was a groundbreaking achievement in astronomy. It provided visual confirmation of Einstein's theory of general relativity and gave us a glimpse into the enigmatic nature of these cosmic objects.

In essence, astronomers don't take a picture of the black hole itself, but rather a picture of its event horizon and the surrounding accretion disk that indirectly reveals its presence.