Observational campaigns in both April 2017 and April 2018 were conducted over five observing nights. These data were recorded when the major target sources were optimally observable by night and when the weather was globally best for EHT observations. Each telescope observed EHT targets and various celestial calibrators for approximately 8 hours per night.
However, the EHT collaboration effort has been evolving over more than 20 years and continues to go forward. Such a daring endeavor required developing new hardware and software throughout this period and especially in the last several years, when new telescopes joined the project. Instrument coordination, advanced processing software, hardware retrofitting, and other technological and practical issues were among the issues that needed to be solved in order to align existing telescopes into the globally coordinated observations of the EHT array.
After the first data were recorded in April 2017, they were shipped to central processing facilities—at the Max Planck Institute for Radio Astronomy and the MIT Haystack Observatory—then carefully processed, calibrated, analyzed, and ultimately, interpreted using the cutting-edge computational tools created specifically for this experiment by EHT collaboration members. While "making an image" from processed and well-calibrated data takes a mere few minutes, the development of procedures and tools that enable this required a great deal of time and effort of many dedicated experts.