LOFAR FACET CALIBRATION

R. Weeren (1) , W. Williams (2, 3, 4) , M. Hardcastle (4) , T. Shimwell (2) , D. Rafferty (5) , J. Sabater (6) , G. Heald (3, 7) , S. Sridhar (3, 7) , T. Dijkema (3) , G. Brunetti (8) , M. Brüggen (5) , F. Andrade-Santos (1) , G. Ogrean (1) , H. Röttgering (9) , W. Dawson (10) , W. Forman (1) , F. De Gasperin (2, 5) , C. Jones (1) , G. Miley (9) , L. Rudnick (11) , C. Sarazin (12) , A. Bonafede (5) , P. Best (13) , L. Bîrzan (5) , R. Cassano (8) , K. Chyży (14) , J. Croston (15) , T. Ensslin (16) , C. Ferrari (17, 18) , M. Hoeft (19) , C. Horellou (20) , M. Jarvis (21, 22) , R. Kraft (1) , M. Mevius (3) , H. Intema (2, 23) , S. Murray (1, 24) , E. Orrú (3) , R. Pizzo (3) , A. Simionescu (25) , A. Stroe (2) , S. van Der Tol (3) , G. White (26, 27)

1 CfA - Harvard-Smithsonian Center for Astrophysics
2 Universiteit Leiden = Leiden University
3 ASTRON - Netherlands Institute for Radio Astronomy
4 UH - University of Hertfordshire [Hatfield]
5 Hamburger Sternwarte/Hamburg Observatory
6 IfA - Institute for Astronomy [Edinburgh]
7 Kapteyn Astronomical Institute [Groningen]
8 IRA - Istituto di Radioastronomia [Bologna]
9 Leiden Observatory [Leiden]
10 LLNL - Lawrence Livermore National Laboratory
11 University of Minnesota [MN, USA]
12 University of Virginia
13 Edin. - University of Edinburgh
14 UJ - Uniwersytet Jagielloński w Krakowie = Jagiellonian University
15 University of Southampton
16 Max Planck Institute for Astrophysics
17 LAGRANGE - Joseph Louis LAGRANGE
18 Observatoire de la Côte d'Azur
19 TLS - Thüringer Landessternwarte Tautenburg
20 Chalmers University of Technology [Gothenburg, Sweden]
21 Oxford Astrophysics
22 UWC - University of the Western Cape
23 NRAO - National Radio Astronomy Observatory [Socorro]
24 JHU - Johns Hopkins University
25 ISAS - Institute of Space and Astronautical Science
26 OU - The Open University [Milton Keynes]
27 RAL - STFC Rutherford Appleton Laboratory

LOFAR, the Low-Frequency Array, is a powerful new radio telescope operating between 10 and 240 MHz. LOFAR allows detailed sensitive high-resolution studies of the low-frequency radio sky. At the same time LOFAR also provides excellent short baseline coverage to map diffuse extended emission. However, producing high-quality deep images is challenging due to the presence of direction dependent calibration errors, caused by imperfect knowledge of the station beam shapes and the ionosphere. Furthermore, the large data volume and presence of station clock errors present additional difficulties. In this paper we present a new calibration scheme, which we name facet calibration, to obtain deep high-resolution LOFAR High Band Antenna images using the Dutch part of the array. This scheme solves and corrects the direction dependent errors in a number of facets that cover the observed field of view. Facet calibration provides close to thermal noise limited images for a typical 8 hr observing run at ∼ 5′′ resolution, meeting the specifications of the LOFAR Tier-1 northern survey.

Domaines: Cosmologie et astrophysique extra-galactique [astro-ph.CO]; Instrumentation et méthodes pour l'astrophysique [astro-ph.IM]

R. Weeren, W. Williams, M. Hardcastle, T. Shimwell, D. Rafferty, et al.. LOFAR FACET CALIBRATION. The Astrophysical Journal Supplement Series, 2016, 223 (1), pp.2. ⟨10.3847/0067-0049/223/1/2⟩. ⟨hal-03648687⟩