LOFAR - Technical Information

Technical Information

LOFAR was conceived as an innovative effort to force a breakthrough in sensitivity for astronomical observations at radio-frequencies below 250 MHz. Astronomical radio interferometers usually consist either of arrays of parabolic dishes (e.g. the One-Mile Telescope), arrays of one-dimensional antennas (e.g. the Molonglo Observatory Synthesis Telescope) or two-dimensional arrays of omnidirectional antennas (e.g. Antony Hewish' Interplanetary Scintillation Array).

LOFAR combines aspects of many of these earlier telescopes—in particular it uses omni-directional dipole antennas as a phased array using the aperture synthesis technique developed in the 1950s. Like the earlier Cambridge Low Frequency Synthesis Telescope (CLFST) low-frequency radio telescope, the design of LOFAR has concentrated on the use of large numbers of relatively cheap antennas without any moving parts, concentrated in stations, with the mapping performed using aperture synthesis software. The direction of observation ("beam") is chosen electronically by phase delays between the antennas. LOFAR can observe in several directions simultaneously which allows a multi-user operation.

The electric signals from the LOFAR antennas are digitised, transported to a central digital processor, and combined in software in order to map the sky. The cost is dominated by the cost of electronics and will follow Moore's law, becoming cheaper with time and allowing increasingly large telescopes to be built. So LOFAR is a "software telescope". The antennas are simple enough but there are a lot of them — about 20,000 in the full LOFAR design. To make radio pictures of the sky with adequate sharpness, these antennas are to be arranged in clusters (stations) that are spread out over an area of ultimately more than 1000 km in diameter. The 40 stations in the Netherlands reach baselines of about 100 km. 32 stations are presently in operation. In Germany five stations are operating: Bonn/Effelsberg, Garching/Unterweilenbach, Tautenburg, Potsdam/Bornim and Jülich. The Effelsberg station has been operating since November 2007, the German stations in Garching/Unterweilenbach, Tautenburg and Bornim/Potsdam since 2010, and the Jülich station since 2011. One station each has been completed in Great Britain (Chilbolton) and in France (Nançay); the Swedish station (Onsala) is under construction. Data transport requirements are in the range of several gigabits per second per station and the processing power needed is tens of TeraFLOPS.