Low-Frequency Array

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.…

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. ===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). , next to the 100 m radio telescope (background), both run by the Max Planck Institute for Radio Astronomy Bonn, Germany]] 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 , County Offaly]] 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. One of the most exciting, but technically most challenging, applications of LOFAR will be the search for redshifted 21 cm line emission from the Epoch of Reionization (EoR). It is thought that the 'Dark Ages', the period after recombination when the Universe turned neutral, lasted until around z=20. WMAP polarization results appear to suggest that there may have been extended, or even multiple phases of reionisation, the start possibly being around z~15–20 and ending at z~6. Using LOFAR, the redshift range from z=11.4 (115 MHz) to z=6 (200 MHz) can be probed. The expected signal is small, and disentangling it from the much stronger foreground emission is challenging. ==Timeline== In the early 1990s, the study of aperture array technology for radio astronomy was being actively studied by ASTRON – the Netherlands Institute for Radio Astronomy. At the same time, scientific interest in a low-frequency radio telescope began to emerge at ASTRON and at the Dutch Universities. A feasibility study was carried out and international partners sought during 1999. In 2000 the Netherlands LOFAR Steering Committee was set up by the ASTRON Board with representatives from all interested Dutch university departments and ASTRON. In November 2003 the Dutch Government allocated 52 million euro to fund the infrastructure of LOFAR under the Bsik programme. In accordance with Bsik guidelines, LOFAR was funded as a multidisciplinary sensor array to facilitate research in geophysics, computer sciences and agriculture as well as astronomy. In December 2003 LOFAR's Initial Test Station (ITS) became operational. The ITS system consists of 60 inverse V-shaped dipoles; each dipole is connected to a low-noise amplifier (LNA), which provides enough amplification of the incoming signals to transport them over a coaxial cable to the receiver unit (RCU). On April 26, 2005, an IBM Blue Gene/L supercomputer was installed at the University of Groningen's math centre, for LOFAR's data processing. At that time it was the second most powerful supercomputer in Europe, after the MareNostrum in Barcelona. Since 2014 an even more powerful computing cluster (correlator) called COBALT performs the correlation of signals from all individual stations. In August/September 2006 the first LOFAR station (Core Station CS001, CS1 ) was put in the field using pre-production hardware. A total of 96 dual-dipole antennas (the equivalent of a full LOFAR station) are grouped in four clusters, the central cluster with 48 dipoles and other three clusters with 16 dipoles each. Each cluster is about in size. The clusters are distributed over an area of ~500 m¹ in diameter. In November 2007 the first international LOFAR station (DE601) next to the Effelsberg 100 m radio telescope became the first operational station. The first fully complete station, (CS302) on the edge of the LOFAR core, was delivered in May 2009, with a total of 40 Dutch stations scheduled for completion in 2013. By 2014, 38 stations in the Netherlands, five stations in Germany (Effelsberg, Tautenburg, Unterweilenbach, Bornim/Potsdam, and Jülich), and one each in the UK (Chilbolton), in France (Nançay) and in Sweden (Onsala) were operational. The station at Birr in Ireland was added in 2017. LOFAR was officially opened on 12 June 2010 by Queen Beatrix of the Netherlands. Regular observations started in December 2012. Interactive map of possible station locations A guided tour of the LOFAR-UK station LOFAR as a Probe of the Sources of Cosmological Reionisation. (preprint: astro-ph/0412080) LOFAR, a new low-frequency radio telescope. (preprint: astro-ph/0309537) LOFAR: A new radio telescope for low-frequency radio observations: Science and project status. (preprint: astro-ph/0307240) Square Kilometre Array (Scholarpedia article). * Das Square Kilometre Array (in German), reprint from Sterne und Weltraum 9/2006. [http://www.mpifr-bonn.mpg.de/staff/rbeck/ska.suw.pdf mpifr-bonn.mpg.de

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