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Looking to the outer space

In March 2013, the Alma (Atacama Large Millimeter/submillimeter Array) ground-based telescope starts operation. A special-purpose correlator comprising more than 134 million processors controls it. For internal communication purposes the signal blending sub-system uses more than 500 CAN networks.

The Alma radio telescope is a collection of 64 high-precision antennas (parabolic dishes that act as receivers), strewn over the Chajnantor desert plateau in northern Chile. The dry air and elevation in 5 km above sea level makes it a particularly suitable spot for capturing signals from space in the millimeter and sub-millimeter radio spectrum. At those wavelengths, the antennas can detect the so-called "cool Universe," molecular gas and dust as well as residual radiation from the Big Bang.

ONE OF THE MOST POWERFUL SUPERCOMPUTERS in the world has now been fully installed and tested in the Andes of northern Chile. In a height of 5000 m above sea level, the 134 million processors of the correlator continually combine and compare faint celestial “signals” received by as many as 50 dish-shaped antennas. They work together to make virtually a single, enormous astronomical telescope. The correlator can additionally accommodate up to 14 of the 16 antennas in the Atacama Compact Array (ACA), a separate part of ALMA provided by the National Astronomical Observatory of Japan (NAOJ), for a total of 64 antennas. In radio telescope arrays, sensitivity and image quality increase with the number of antennas.

The Alma correlator performs the first critical steps in this data processing. To make the entire system work as a single 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, there are 2016 antenna pair combinations, and as many as 17 quadrillion calculations every second. This photograph shows just some of the many thousands of cables needed to connect the electronics racks of the correlator together. The system requires 32768 rack-to-rack digital interfaces and 16384 cables to transport the signals between the racks.

A correlator technician examines the system while breathing oxygen from a backpack. Alma’s Array Operations Site, at an elevation of 5000 m in the Chilean Andes, is so high that only half as much oxygen is available as at sea level
The Alma system, which was built by the National Radio Astronomy Observatory (NRAO), uses 32767 ASIC processors to blend the signals from the antenna array. The processors, built on 0,25-micron CMOS technology, run at a modest 125-MHz frequency. As it is built for these correlation functions, the silicon is able to deliver 512 billion operations per second. The processors are arranged 64 to a board, which are connected via one CAN network running at 1 Mbit/s. In order to overcome some physical layer problems, fiber-optical transmission is used.
One of the Alma project partners is the European Southern Observatory (ESO), which uses in its projects CAN and CANopen networks since many years. The European researchers used VMEbus boards with CAN interfaces from ESD (Germany) and CANopen I/O modules by Beckhoff (Germany).