During the last half a century our knowledge of the Universe has been revolutionized by the opening of observable windows outside the narrow visible region of the spectrum. Radio waves, infrared and ultraviolet radiation and X- and gamma rays have provided new and completely unexpected information about the nature and history of the Universe and have resulted in the discovery of a cosmic zoo of strange and exotic objects. One of the few spectral windows that still remain to be explored is at the low radio frequencies, the lowest energy extreme of the spectrum that is accessible from the Earth. LOFAR, the Low Frequency Radio Array, is a large radio telescope that will open this territory to a broad range of astrophysical studies.
The mission of LOFAR is to survey the Universe at frequencies of from ~10 - 240 MHz (corresponding to wavelengths of 1.2 - 30 m). Radio astronomy was born at these wavelengths in 1931, when Karl Jansky investigated the background noise that was plaguing transatlantic short-wave communications. Since then, low frequency radio astronomy has been neglected because of the poor resolving power of the available facilities and the disturbing effects of the ionosphere on observations.
Because the spatial resolution of a telescope is proportional to its operating frequency, radio telescopes such as the Westerbork Radio Synthesis Telescope have poor spatial resolution when operated at low frequencies. The radio images obtained by low frequency radio facilities (resolutions of arcminutes) are blurred by a factor of several thousand compared with optical pictures of the sky. The blurred images result in so-called "confusion" effects that have limited the sensitivity at low radio frequencies to the level of ~1 Jansky (1 Jansky (Jy) = 10-26 W m-2 Hz-1) and the number of objects that can be studied to the brightest few hundred.
The resolution of a radio telescope can be improved by enlarging the aperture, or in the case of a Westerbork-type array of antennas, by increasing the maximum distance between the elements of the array - i.e. the "baseline". At low frequencies, to achieve useful resolutions comparable with visible images of the sky, maximum baselines of several hundred kilometres are needed. Until now, one of the most important limitations in achieving such long baselines at low radio frequencies has been the complicated structure of the ionosphere and its variation over time. Just as the atmosphere causes stars to twinkle, the irregularities in the ionosphere produce jittering in the radio images.
There have been a number of recent technological developments that now make the idea of building a dedicated low frequency radio telescope an attractive proposition:
Both computing power and calibration algorithms have improved so much that images of very wide fields can now be created and processed on short enough timescales to monitor and correct for the ionospheric jitter.
Progress in antenna design has also made it possible to construct low-frequency antennas with several simultaneous beams that can be pointed to and used to monitor different regions of the sky simultaneously.
Such an array will for the first time probe the distant Universe at the low-energy extremity of the electromagnetic spectrum.
LOFAR will consist of an array of antennas will be distributed over 100 km within the Netherlands and reaches out to 1500 km throughout Europe, that will provide sufficient resolution to allow radio sources to be identified with visible objects, even at low frequencies. Occupying its central 2 km will be a more densely filled core, that will allow more effective calibration of the instrument and optimize its sensitivity for special experiments such as the study of transient phenomena and the reionisation phase of the Universe. The design will provide fast frequency selection and pointing, giving users the capability of rapidly imaging radio sources across the sky and spectrum. Multiple independent beams will herald a new technological approach to observing, yielding unprecedented flexibility when compared with higher cost, higher frequency ground or space-based systems.