The composite photo, the first ever to be made, was released on April 10, 2019. It was taken by the Event Horizon Telescope (EHT), a very long baseline interferometry (VLBI) array that comprises millimeter- and submillimeter-wavelength telescopes separated by distances comparable to the diameter of the Earth.
The observation was made in April 2017, and involved collecting petabytes of raw VLBI voltage signal data (around 5 PB). It took nearly 2 years to turn this massive amount of data into this image. Because the image was stitched together from multiple angles, it looks smooth, but is a low resolution image. The algorithm that made it possible to reconstruct the image from the data was developed by Katie Bouman, a PhD student at MIT, currently an assistant professor of computing and mathematical sciences at the California Institute of Technology.