A very brief description of LOFAR - the Low Frequency Array

LOFAR (Low Frequency Array) is an innovative radio telescope optimized for the frequency range 30-240 MHz. The telescope is realized as a phased aperture array without any moving parts. Digital beam forming allows the telescope to point to any part of the sky within a second. Transient buffering makes retrospective imaging of explosive short-term events possible. The scientific focus of LOFAR will initially be on four key science projects (KSPs): 1) detection of the formation of the very first stars and galaxies in the universe during the so-called epoch of reionization by measuring the power spectrum of the neutral hydrogen 21-cm line (Shaver et al. 1999) on the ~5' scale; 2) low-frequency surveys of the sky with of order $10^8$ expected new sources; 3) all-sky monitoring and detection of transient radio sources such as gamma-ray bursts, x-ray binaries, and exo-planets (Farrell et al. 2004); and 4) radio detection of ultra-high energy cosmic rays and neutrinos (Falcke & Gorham 2003) allowing for the first time access to particles beyond 10^21 eV (Scholten et al. 2006). Apart from the KSPs open access for smaller projects is also planned. Here we give a brief description of the telescope.Comment: 2 pages, IAU GA 2006, Highlights of Astronomy, Volume 14, K.A. van der Hucht, e

Astronomical Real-Time Streaming Signal Processing on a Blue Gene/L Supercomputer

LOFAR is the first of a new generation of radio telescopes, that combines the signals from many thousands of simple, fixed antennas, rather than from expensive dishes. Its revolutionary design and unprecedented size enables observations in a frequency range that could hardly be observed before, and allows the study of a vast amount of new science cases. In this paper, we describe a novel approach to process realtime, streaming telescope data in software, using a supercomputer. The desire for a flexible and reconfigurable instrument demands a software solution, where traditionally customized hardware was used. This, and LOFAR’s exceptional real-time, streaming signalprocessing requirements compel the use of a supercomputer. We focus on the LOFAR CEntral Processing facility (CEP), that combines the signals of all LOFAR stations. CEP consists of a 12,288core IBM Blue Gene/L supercomputer, embedded in several conventional clusters. We describe a highly optimized implementation that will do the bulk of the central signal processing on the Blue Gene/L, namely PolyPhase Filtering, Delay Compensation, and Correlation. Measurements show that we reach exceptionally high computational performance (up to 98 % of the theoretical floating-point peak performance). We also discuss how we handle external I/O performance limitations into and out of the Blue Gene/L, to obtain sufficient bandwidth for LOFAR