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- van Haarlem, M. P., et al.
(författare)
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LOFAR : The LOw-Frequency ARray
- 2013
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 556, s. 1-53
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Tidskriftsartikel (refereegranskat)abstract
- LOFAR, the LOw-Frequency ARray, is a new-generation radio interferometer constructed in the north of the Netherlands and across europe. Utilizing a novel phased-array design, LOFAR covers the largely unexplored low-frequency range from 10–240 MHz and provides a number of unique observing capabilities. Spreading out from a core located near the village of Exloo in the northeast of the Netherlands, a total of 40 LOFAR stations are nearing completion. A further five stations have been deployed throughout Germany, and one station has been built in each of France, Sweden, and the UK. Digital beam-forming techniques make the LOFAR system agile and allow for rapid repointing of the telescope as well as the potential for multiple simultaneous observations. With its dense core array and long interferometric baselines, LOFAR achieves unparalleled sensitivity and angular resolution in the low-frequency radio regime. The LOFAR facilities are jointly operated by the International LOFAR Telescope (ILT) foundation, as an observatory open to the global astronomical community. LOFAR is one of the first radio observatories to feature automated processing pipelines to deliver fully calibrated science products to its user community. LOFAR’s new capabilities, techniques and modus operandi make it an important pathfinder for the Square Kilometre Array (SKA). We give an overview of the LOFAR instrument, its major hardware and software components, and the core science objectives that have driven its design. In addition, we present a selection of new results from the commissioning phase of this new radio observatory.
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| 2. |
- Offringa, A. R., et al.
(författare)
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The LOFAR radio environment
- 2012
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 549
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Tidskriftsartikel (refereegranskat)abstract
- Aims. This paper discusses the spectral occupancy for performing radio astronomy with the Low-Frequency Array (LOFAR), with a focus on imaging observations.Methods. We have analysed the radio-frequency interference (RFI) situation in two 24-h surveys with Dutch LOFAR stations, covering 30-78 MHz with low-band antennas and 115-163 MHz with high-band antennas. This is a subset of the full frequency range of LOFAR. The surveys have been observed with a 0.76 kHz/1 s resolution.Results. We measured the RFI occupancy in the low and high frequency sets to be 1.8% and 3.2% respectively. These values are found to be representative values for the LOFAR radio environment. Between day and night, there is no significant difference in the radio environment. We find that lowering the current observational time and frequency resolutions of LOFAR results in a slight loss of flagging accuracy. At LOFAR's nominal resolution of 0.76 kHz and 1 s, the false-positives rate is about 0.5%. This rate increases approximately linearly when decreasing the data frequency resolution.Conclusions. Currently, by using an automated RFI detection strategy, the LOFAR radio environment poses no perceivable problems for sensitive observing. It remains to be seen if this is still true for very deep observations that integrate over tens of nights, but the situation looks promising. Reasons for the low impact of RFI are the high spectral and time resolution of LOFAR; accurate detection methods; strong filters and high receiver linearity; and the proximity of the antennas to the ground. We discuss some strategies that can be used once low-level RFI starts to become apparent. It is important that the frequency range of LOFAR remains free of broadband interference, such as DAB stations and windmills.
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| 3. |
- Tasse, C., et al.
(författare)
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LOFAR calibration and wide-field imaging
- 2012
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Ingår i: Comptes Rendus Physique. - : Elsevier BV. - 1878-1535 .- 1631-0705. ; 13:1, s. 28-32
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- LOFAR is a revolutionary instrument, operating at low frequencies (nu less than or similar to 240 MHz). It will drive major breakthroughs in the area of observational cosmology, but its use requires the development of challenging techniques and algorithms. Since its field of view and sensitivity are increased by orders of magnitude as compared to the older generation of instruments, new technical problems have to be addressed. The LOFAR survey team is in charge of commissioning the first LOFAR data produced in the imager mode as part of building the imaging pipeline. We are developing algorithms to tackle the problems associated with calibration (ionosphere, beam, etc.) and wide-field imaging for the achievement of the deep extragalactic surveys. New types of problems arise in that context, and notions such as algorithmic complexity and parallelism become fundamental.
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