Scientific American has an interesting post about how to build a telescope the size of the earth (sort of).
Looking into the galactic center is hard. So much dust and gas lies between us and the center of the Milky Way that very little of the visible light emitted there makes it to us. We can peek through that dust and gas by collecting x-rays, infrared radiation, and radio waves. Even then, however, resolving the tiny speck of sky that contains the Milky Way’s central black hole, with enough clarity to see the black hole’s shadow, is extremely difficult.
You need a telescope roughly the size of the Earth to do it. This might sound impractical. Fortunately, it’s possible to mimic the performance of an Earth-size telescope by coordinating existing radio telescopes scattered around the world.
That’s the idea behind the Event Horizon Telescope (EHT). If all goes well, by the end of next year the EHT will be a coordinated array of radio telescopes stretching from the South Pole to Hawaii to Chile to Mexico, plus many points in between. The astronomers behind the EHT have been already been observing for years using a smaller telescope array. In 2007, a three-station version of the EHT resolved Sagittarius A*, the black hole at the center of the Milky Way, with unprecedented clarity, detecting something (“structure” is the proper term) on the scale that we would expect from the black hole’s event horizon. It was a big deal, the farthest into the inner sanctum of a black hole that anyone had ever seen. The goal now is to make the EHT powerful enough to take the black hole’s picture…
The EHT will combine many of the world’s most advanced radio telescopes: the Submillimeter Array (SMA) and the James Clerk Maxwell Telescope (JCMT) in Hawaii; the Combined Array for Research in Millimeter-wave Astronomy (CARMA) in California; the Submillimeter Telescope (SMT) in Arizona; the Large Millimeter Telescope (LMT) near Puebla, Mexico; the Atacama Large Millimeter Array (ALMA) and the Atacama Pathfinder Experiment (APEX) in northern Chile; the South Pole Telescope (SPT); the Greenland Telescope (GLT); the Plateau de Bure interferometer in France; and the 30-meter dish at Pico Veleta in Spain. Once properly outfitted, these telescopes will set out on a few key nights each year to observe the same black hole simultaneously. Together, they will function as one giant telescope.