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The LOFAR Two Meter Sky Survey: Deep Fields, I -- Direction-dependent calibration and imaging

Tasse, C. and Shimwell, T. and Hardcastle, M. J. and O'Sullivan, S. P. and van Weeren, R. and Best, P. N. and Bester, L. and Hugo, B. and Smirnov, O. and Sabater, J. and Calistro-Rivera, G. and de Gasperin, F. and Morabito, L. K. and Röttgering, H. and Williams, W. L. and Bonato, M. and Bondi, M. and Botteon, A. and Brüggen, M. and Brunetti, G. Chyży, K. T. and Garrett, M. A. and Gürkan, G. and Jarvis, M. J. and Kondapally, R. and Mandal, S. and Prandoni, I. and Repetti, A. and Retana-Montenegro, E. and Schwarz, D. J. and Shulevski, A. and Wiaux, Y. (2021) 'The LOFAR Two Meter Sky Survey: Deep Fields, I -- Direction-dependent calibration and imaging.', Astronomy and astrophysics., 648 . A1.


The Low Frequency Array (LOFAR) is an ideal instrument to conduct deep extragalactic surveys. It has a large field of view and is sensitive to large-scale and compact emission. It is, however, very challenging to synthesize thermal noise limited maps at full resolution, mainly because of the complexity of the low-frequency sky and the direction dependent effects (phased array beams and ionosphere). In this first paper of a series, we present a new calibration and imaging pipeline that aims at producing high fidelity, high dynamic range images with LOFAR High Band Antenna data, while being computationally efficient and robust against the absorption of unmodeled radio emission. We apply this calibration and imaging strategy to synthesize deep images of the Boötes and Lockman Hole fields at ∼150 MHz, totaling ∼80 and ∼100 h of integration, respectively, and reaching unprecedented noise levels at these low frequencies of .30 and .23 µJy beam−1 in the inner ∼3 deg2 . This approach is also being used to reduce the LOTSS-wide data for the second data release.

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Publisher statement:Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Date accepted:14 November 2020
Date deposited:21 April 2021
Date of first online publication:07 April 2021
Date first made open access:21 April 2021

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