Astronomers building an Earth-size virtual telescope capable of photographing the event horizon of the black hole at the center of our Milky Way have extended their instrument to the bottom of the Earth, thanks to recent efforts by a team led by Dan Marrone of the University of Arizona.

Marrone, an assistant professor in the UA's Department of Astronomy and Steward Observatory, and several colleagues flew to the National Science Foundation's (NSF) Amundsen-Scott South Pole Station in December to bring the South Pole Telescope, or SPT, into the largest virtual telescope ever built - the Event Horizon Telescope (EHT). By combining telescopes across the Earth, the EHT will take the first detailed pictures of black holes.

The EHT is an array of radio telescopes connected using a technique known as very long baseline interferometry, (VLBI). Larger telescopes can make sharper observations, and interferometry allows multiple telescopes to act like a single telescope as large as the separation - or baseline - between them.

"Now that we've done VLBI with the SPT, the Event Horizon Telescope really does span the whole Earth, from the Submillimeter Telescope on Mount Graham in Arizona, to California, Hawaii, Chile, Mexico, Spain and the South Pole," Marrone said, according to a press release. "The baselines to SPT give us two to three times more resolution than our past arrays, which is absolutely crucial to the goals of the EHT. To verify the existence of an event horizon, the 'edge' of a black hole, and more generally to test Einstein's theory of general relativity, we need a very detailed picture of a black hole. With the full EHT, we should be able to do this."

The prime EHT target is the Milky Way's black hole, known as Sagittarius A* (pronounced "A-star"). Even though it is four million times more massive than the sun, it is tiny to the eyes of astronomers. Because it is smaller than Mercury's orbit around the sun, yet almost 26,000 light-years away, studying its event horizon in detail is equivalent to standing in California and reading the date on a penny in New York.

Weighing 280 tons and standing 75 feet tall, the SPT sits at an elevation of 9,300 feet on the polar plateau at Amundsen-Scott, which is located at the geographic south pole. The University of Chicago built SPT with funding and logistical support from the NSF's Division of Polar Programs. The division manages the U.S. Antarctic Program, which coordinates all U.S. research on the southernmost continent.

The 10-meter SPT operates at millimeter wavelengths to make high-resolution images of cosmic microwave background radiation, the light left over from the Big Bang. Because of its location at the Earth's axis and at high elevation where the polar air is largely free of water vapor, it can conduct long-term observations to explore some of the biggest questions in cosmology, such as the nature of dark energy and the process of inflation that is believed to have stretched the universe exponentially in a tiny fraction of the first second after the Big Bang.

"We are thrilled that the SPT is part of the EHT," said SPT lead John Carlstrom, according to the press release. "The science, which addresses fundamental questions of space and time, is as exciting to us as peering back to the beginning of the universe."

This work was funded through NSF grants AST-1207752 to Marrone; AST-1207704 to Doeleman at MIT's Haystack Observatory; and AST-1207730 to Carlstrom at the University of Chicago.