University of Hertfordshire

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From the same journal

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  • N. J. Jackson
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  • C. Groeneveld
  • A. Kappes
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  • I. M. van Bemmel
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  • A. Bonafede
  • W. N. Brouw
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  • B. Ciardi
  • A. Corstanje
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  • A. W. Gunst
  • M. P. van Haarlem
  • M. Hoeft
  • A. J. van der Horst
  • E. Jütte
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  • L. V. E. Koopmans
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  • W. Reich
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  • M. Ruiter
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  • M. Tagger
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Original languageEnglish
JournalAstronomy & Astrophysics
Publication statusAccepted/In press - 1 Apr 2021


The International LOFAR Telescope is an interferometer with stations spread across Europe. With baselines of up to ~2,000 km, LOFAR has the unique capability of achieving sub-arcsecond resolution at frequencies below 200 MHz, although this is technically and logistically challenging. Here we present a calibration strategy that builds on previous high-resolution work with LOFAR. We give an overview of the calibration strategy and discuss the special challenges inherent to enacting high-resolution imaging with LOFAR, and describe the pipeline, which is publicly available, in detail. We demonstrate the calibration strategy by using the pipeline on P205+55, a typical LOFAR Two-metre Sky Survey (LoTSS) pointing. We perform in-field delay calibration, solution referencing to other calibrators, self-calibration, and imaging of example directions of interest in the field. For this specific field and these ionospheric conditions, dispersive delay solutions can be transferred between calibrators up to ~1.5 degrees away, while phase solution transferral works well over 1 degree. We demonstrate a check of the astrometry and flux density scale. Imaging in 17 directions, the restoring beam is typically 0.3" x 0.2" although this varies slightly over the entire 5 square degree field of view. We achieve ~80 to 300 $\mu$Jy/bm image rms noise, which is dependent on the distance from the phase centre; typical values are ~90 $\mu$Jy/bm for the 8 hour observation with 48 MHz of bandwidth. Seventy percent of processed sources are detected, and from this we estimate that we should be able to image ~900 sources per LoTSS pointing. This equates to ~3 million sources in the northern sky, which LoTSS will entirely cover in the next several years. Future optimisation of the calibration strategy for efficient post-processing of LoTSS at high resolution (LoTSS-HR) makes this estimate a lower limit.


Accepted to a special issue of A&A on sub-arcsecond imaging with LOFAR. 24 pages, 16 figures

ID: 25797194