The Low-Frequency Array (LOFAR) is a large radio telescope, with an antenna network located mainly in the Netherlands, and spreading across 7 other European countries as of 2019. Originally designed and built by ASTRON, the Netherlands Institute for Radio Astronomy, it was first opened by Queen Beatrix of The Netherlands in 2010, and has since been operated by ASTRON on behalf first of the International LOFAR Telescope (ILT) partnership and now of the LOFAR ERIC by ASTRON.
LOFAR consists of a vast array of omnidirectional antennas using a modern concept, in which the signals from the separate antennas are not connected directly electrically to act as a single large antenna, as they are in most array antennas. Instead, the LOFAR dipole antennas (of two types) are distributed in stations, within which the antenna signals can be partly combined in analogue electronics, then digitised, then combined again across the full station. This step-wise approach provides great flexibility in setting and rapidly changing the directional sensitivity on the sky of an antenna station. The data from all stations are then transported over fiber to a central digital processor, and combined in software to emulate a conventional radio telescope dish with a resolving power corresponding to the greatest distance between the antenna stations across Europe.
LOFAR is thus an interferometric array, using about 20,000 small antennas concentrated in 52 stations since 2019. 38 of these stations are distributed across the Netherlands, built with regional and national funding. The six stations in Germany, three in Poland, and one each in France, Great Britain, Ireland, Latvia, and Sweden, with various national, regional, and local funding and ownership. Italy officially joined the International LOFAR Telescope (ILT) in 2018; construction at the INAF observatory site in Medicina, near Bologna, is planned as soon as upgraded (so-called LOFAR2.0) hardware becomes available. Further stations in other European countries are in various stages of planning. The total effective collecting area is approximately 300,000 square meters, depending on frequency and antenna configuration. Until 2014, data processing was performed by a Blue Gene/P supercomputer situated in the Netherlands at the University of Groningen. Since 2014 LOFAR uses a GPU-based correlator and beamformer, COBALT, for that task. LOFAR is also a technology and science pathfinder for the Square Kilometre Array.
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 or the Very Large Array), arrays of one-dimensional antennas (e.g. the Molonglo Observatory Synthesis Telescope) or two-dimensional arrays of omnidirectional antennas (e.g. Antony Hewish's Interplanetary Scintillation Array).
LOFAR combines aspects of many of these earlier telescopes; in particular, it uses omnidirectional dipole antennas as elements of a phased array at individual stations, and combines those phased arrays 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") of the stations is chosen electronically by phase delays between the antennas. LOFAR can observe in several directions simultaneously, as long as the aggregated data rate remains under its cap. This in principle allows a multi-user operation.
LOFAR makes observations in the 10 MHz to 240 MHz frequency range with two types of antennas: Low Band Antenna (LBA) and High Band Antenna (HBA), optimized for 10–80 MHz and 120–240 MHz respectively. The electric signals from the LOFAR stations are digitised, transported to a central digital processor, and combined in software in order to map the sky. Therefore, LOFAR is a "software telescope". The cost of such telescopes is dominated by the cost of electronics and will therefore mostly follow Moore's law, becoming cheaper with time and allowing increasingly large telescopes to be built. Each antenna is fairly simple- but there are about 20,000 of them in the LOFAR array.
LOFAR stations To make radio surveys of the sky with adequate resolution, the antennas are arranged in clusters that are spread out over an area of more than 1000 km in diameter. The LOFAR stations in the Netherlands reach baselines of about 100 km. LOFAR currently receives data from 24 core stations (in Exloo), 14 'remote' stations in The Netherlands, and 14 international stations. Each of the core and remote stations has 48 HBAs and 96 LBAs and a total of 48 digital Receiver Units (RCUs). International stations have 96 LBAs and 96 HBAs and a total of 96 digital Receiver Units (RCUs).
The locations of the international LOFAR stations are:
Bulgaria – planned at the site of the National Astronomical Observatory Rozhen France Nançay – at the site of the Nançay Radio Telescope Germany Effelsberg – run by Max Planck Institute for Radio Astronomy, at the site of the Effelsberg Radio Telescope Unterweilenbach/Garching – run by Max Planck Institute for Astrophysics Tautenburg – at the site of the Thüringer Landessternwarte Tautenburg (Thuringian State Observatory) Potsdam-Bornim – run by Astrophysikalisches Institut Potsdam Jülich – run by the University of Bochum, Jacobs University Bremen, and Forschungszentrum Jülich Norderstedt – run by Hamburg Observatory and Bielefeld University Ireland Birr – run by Trinity College Dublin at the Rosse Observatory on the grounds of Birr Castle
Italy – planned at the site of the Medicina Observatory Latvia Ventspils – at the site of Ventspils International Radio Astronomy Centre in Irbene Poland Bałdy – run by the University of Warmia and Mazury in Olsztyn Borówiec – run by the Space Research Centre of Polish Academy of Sciences Łazy – run by Jagiellonian University Sweden Onsala – at the site Onsala Space Observatory United Kingdom Chilbolton – at the site of the Chilbolton Observatory
NenuFAR The New Extension in Nançay Upgrading LOFAR (NenuFAR) telescope is co-located at the Nançay radio telescope in central France. It is an extension of the Nançay LOFAR station (FR606), adding 96 low frequency tiles, each consisting of a "mini-array" of 19 crossed-dipole antennas, distributed in a circle with a diameter of approximately 400 m. The tiles are a hexagonal cluster with analogically phased antennas. The telescope can capture radio frequencies in the 10–85 MHz range, covering the LOFAR-Low Band (30–80 MHz) range as well. The NenuFAR array can work as a high-sensitivity LOFAR-compatible super-LBA station (LSS), operating together with rest of LOFAR to increase to array's global sensitivity by nearly a factor of two, and improve the array's imaging capabilities. It can also function as a second supercore to improve array availability. Due to its dedicated receiver, NenuFAR can also operate as a standalone instrument, known as NenuFAR/Standalone in this mode.
Other stations Additionally, a set of LOFAR antennas is deployed at the KAIRA (Kilpisjärvi Atmospheric Imaging Receiver Array) near Kilpisjärvi, Finland. This installation functions as a VHF receiver either in stand-alone mode or part of a bistatic radar system together with EISCAT transmitter in Tromsø.
Data transfer Data transport requirements are in the range of several gigabits per second per station and the processing power needed is several tens of TeraFLOPS. The data from LOFAR is stored in the LOFAR long-term archive. The archive is implemented as distributed storage, with data spread over the Target data centre located in the Donald Smits Center for Information Technology at the University of Groningen, centre in Amsterdam, and the Forschungszentrum Jülich in Germany.
Sensitivity The mission of LOFAR is to map the Universe at radio frequencies from ~10–240 MHz with greater resolution and greater sensitivity than previous surveys, such as the 7C and 8C surveys, and surveys by the Very Large Array (VLA) and Giant Meterwave Radio Telescope (GMRT).
LOFAR will be the most sensitive radio observatory at its low observing frequencies until the Square Kilometre Array (SKA) comes online in the late 2020s. Even then, the SKA will only observe at frequencies >50 MHz and LOFAR's angular resolution will remain far superior.
Science case
The sensitivities and spatial resolutions attainable with LOFAR make possible several fundamental new studies of the Universe as well as facilitating unique practical investigations of the Earth's environment. In the following list the term is a dimensionless quantity indicating the redshift of the radio sources seen by LOFAR. In the very distant Universe (), LOFAR can search for the signature produced by the reionization of neutral hydrogen. This crucial phase change is predicted to occur at the epoch of the formation of the first stars and galaxies, marking the end of the so-called "dark ages". The redshift at which reionization is thought to occur will shift the 21 cm line of neutral hydrogen at 1420.40575 MHz into the LOFAR observing window. The frequency observed today is lower by a factor of 1/(z+1). In the distant "formative" Universe (), LOFAR is capable of detecting the most distant massive galaxies and will study the processes by which the earliest structures in the Universe (galaxies, clusters and active nuclei) form and probe the intergalactic gas. In the magnetic Universe, LOFAR is mapping the distribution of cosmic rays and global magnetic fields in our own and nearby galaxies, in galaxy clusters and in the intergalactic medium. The high-energy Universe, LOFAR detects the ultra high energy cosmic rays as they pierce the Earth's atmosphere. A dedicated test station for this purpose, LOPES, has been in operation since 2003. Within the Milky Way galaxy, LOFAR has detected many new pulsars within a few kpc from the Sun, has searched for short-lived transient events produced by stellar mergers or black hole accretion, and will search for bursts from Jupiter-like extrasolar planets. Within the Solar System, LOFAR detects coronal mass ejections from the Sun and provide continuous large-scale maps of the solar wind. This crucial information about solar weather and its effect on Earth facilitates predictions of costly and damaging geomagnetic storms. Within the Earth's immediate environment, LOFAR will map irregularities in the ionosphere continuously, detect the ionizing effects of distant gamma-ray bursts and the flashes predicted to arise from the highest energy cosmic rays, the origins of which are unclear. By exploring a new spectral window LOFAR is likely to make serendipitous discoveries. Detection of new classes of objects or new astrophysical phenomena have resulted from almost all previous facilities that open new regions of the spectrum, or pushed instrumental parameters, such as sensitivity by more than an order of magnitude.
Key projects
The epoch of reionization