At a high altitude site in the remote Andes of northern Chile, where the skies are so dark you can see your shadow by the light of the Milky Way, one of the world’s most powerful supercomputers is now fully installed. It is one of the major remaining milestones to completion of the Atacama Large Millimeter/submillimeter Array (ALMA), the most sophisticated ground-based telescope ever built. The ALMA correlator has more than 134 million processors and performs up to 17 quadrillion operations per second, a speed comparable to the fastest general-purpose supercomputer in operation today.
The correlator is a crucial element of ALMA. An astronomical telescope composed of an array of 66 dish-shaped sub-millimeter interferometer antennas on the Chajnantor plateau in the Chilean Andes, ALMA uses the correlator’s 134 million processors to continually combine and compare dim celestial signals received by the antennas in the ALMA array. The array’s antennas are separated by up to 16 kilometres, allowing them to work as a massive single telescope.
Information recieved by each antenna must be combined with that from every other antenna. At the correlator’s maximum capacity of 64 antennas, as many as 17 quadrillion calculations every second must be performed (17 quadrillion written out is 17,000,000,000,000,000).The correlator was built specially for this task, but the number of calculations per second is comparable to the performance of the fastest general-purpose supercomputers in the world.
At the time of this writing, current record holder in the TOP500 list of general-purpose supercomputers is the Cray Inc. built Titan, which has clocked at 17.59 quadrillion floating point operations per second. Since the ALMA correlator is a special-purpose supercomputer it is not eligible for this list.
Enormous Technical Challenges
Original design of the correlator, in addition to its construction and installation, were led by the US National Radio Astronomy Observatory, lead North American partner in ALMA. The correlator was funded by the US National Science Foundation, with contributions from ESO.
“The completion and installation of the correlator is a huge milestone towards the fulfillment of North America’s share of the international ALMA construction project,” said Mark McKinnon, ALMA Project Director at National Radio Astronomy Observatory. “The technical challenges were enormous, and our team pulled it off.”
One of the challenges was the installation’s extreme location. The correlator is housed in the ALMA Array Operations Site Technical Building, the highest altitude high-tech facility in the world.
The air is thin at 5000 metres, so twice the regular airflow is required to cool the machine, which draws 140 kilowatts of power.
Spinning computer disk drives cannot be used in the thin air, since their read/write heads depend on a cushion of air to stop them from crashing into their platters. In addition to the altitude, seismic activity is common in the area, so the correlator had to be designed to withstand vibrations associated with earthquakes.
Digital Filtering System
The European partner in ALMA, European Southern Observatory, provided a key part of the correlator; a totally new digital filtering system designed in Europe and incorporated into the initial NRAO design. The set of 550 state-of-the-art digital filter circuit boards was designed and built for ESO by the University of Bordeaux in France. With these filters, the wavelengths of light which ALMA sees can be split up 32 times more finely than in the initial design, into ranges that can be finely tuned.
“This vastly improved flexibility is fantastic; it lets us ‘slice and dice’ the spectrum of light that ALMA sees, so we can concentrate on the precise wavelengths needed for a given observation, whether it’s mapping the gas molecules in a star-forming cloud, or searching for some of the most distant galaxies in the Universe,” said Alain Baudry, from the University of Bordeaux.
ALMA began science observations in 2011 with a partial array of antennas, but now the correlator is ready for ALMA to begin operating with more antennas. This will increase the sensitivity and image quality of the observations. The entire ALMA is reaching completion and will be starting operations in March of 2013. To read more about alma visit http://www.eso.org/public/teles-instr/alma.html