One of the most powerful supercomputers in the world has now been fully installed and tested at its remote, high altitude site in the Andes of northern Chile. This marks one of the major remaining milestones toward completion of the Atacama Large Millimeter/submillimeter Array (ALMA), the most elaborate ground-based telescope in history.

The special-purpose ALMA correlator has over 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 critical component of ALMA, an astronomical telescope which is composed of an array of 66 dish-shaped antennas.

The correlator's 134 million processors continually combine and compare faint celestial signals received by the antennas in the ALMA array, which are separated by up to 16 kilometres, enabling the antennas to work together as a single, enormous telescope. The information collected by each antenna must be combined with that from every other antenna. At the correlator's maximum capacity of 64 antennas [1] as many as 17 quadrillion calculations every second must be performed [2].

The correlator was built specifically for this task, but the number of calculations per second is comparable to the performance of the fastest general-purpose supercomputers in the world [3].

"This unique computing challenge needed innovative design, both for the individual components and the overall architecture of the correlator," says Wolfgang Wild, the European ALMA Project Manager, from ESO.

The initial design of the correlator, as well as its construction and installation, was led by the US National Radio Astronomy Observatory (NRAO), the lead North American partner in ALMA. The correlator project 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, North American ALMA Project Director at NRAO. "The technical challenges were enormous, and our team pulled it off," he added.

As the European partner in ALMA, ESO also provided a key part of the correlator: an entirely new and versatile digital filtering system conceived in Europe was incorporated into the initial NRAO design.

A set of 550 state-of-the-art digital filter circuit boards was designed and built for ESO by the University of Bordeaux in France [4]. 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, the European ALMA correlator team leader.

Another challenge was the extreme location. The correlator is housed in the ALMA Array Operations Site (AOS) Technical Building, the highest altitude high-tech building in the world. At 5000 metres, the air is thin, so twice the normal airflow is necessary to cool the machine, which draws some 140 kilowatts of power.

In this thin air, spinning computer disk drives cannot be used, as their read/write heads rely on a cushion of air to stop them crashing into their platters. Seismic activity is common, so the correlator had to be designed to withstand the vibrations associated with earthquakes.

ALMA began science observations in 2011 with a partial array of antennas. A section of the correlator was already being used to combine the signals from the partial array, but now the full system is complete. The correlator is ready for ALMA to begin operating with a larger number of antennas, which will increase the sensitivity and image quality of the observations. ALMA is nearing completion and will be inaugurated in March 2013.

Notes
[1] The ALMA correlator is one of two such systems in the ALMA complex. ALMA's total of 66 antennas comprise a main array of 50 antennas (half provided by ESO, and half by NRAO) and an additional, complementary array of 16 antennas called the Atacama Compact Array (ACA), which is provided by the National Astronomical Observatory of Japan (NAOJ).

A second correlator, built by the Fujitsu company and delivered by NAOJ, provides independent correlation of the 16 antennas in the ACA, except for times when select ACA antennas are combined with the 50 more widely dispersed main array antennas.

[2] 17 quadrillion = 17 000 000 000 000 000.

[3] The current record holder in the TOP500 list of general-purpose supercomputers is the Titan, from Cray Inc., which has been measured at 17.59 quadrillion floating point operations per second. Note that the ALMA correlator is a special-purpose supercomputer and is not eligible for this ranking.

[4] This work followed work on new concepts for the correlator, done by the University of Bordeaux in a consortium also involving ASTRON in the Netherlands, and the INAF-Osservatorio di Arcetri in Italy.