https://phys.org/news/2018-05-apex-glimpse-heart-darkness.html

Astronomers are hunting for the ultimate proof of Einstein's theory of general relativity, which is to obtain a direct image of the shadow of a black hole. This is possible by combining radio telescopes spread over the globe using a technique which is called Very Long Baseline Interferometry (VLBI). The participating telescopes are located at high altitudes to minimize the disturbance from the atmosphere and on remote sites with clear skies, allowing to observe the compact radio source Sagittarius A (Sgr A) at the centre of the Milky Way.

The research team observed Sgr A in 2013 using VLBI telescopes at four sites. The telescopes include the APEX telescope in Chile, the CARMA array in California, the JCMT and the phased SMA in Hawaii, and the SMT telescope in Arizona. Sgr A was detected with all stations and the longest baseline length reached up to almost 10,000 kilometers, indicating an ultra-compact and asymmetric (not point-like) source structure.