The LOFAR Two-metre Sky Survey - I. Survey Description and Preliminary
  Data Release
  The LOFAR Two-metre Sky Survey (LoTSS) is a deep 120-168 MHz imaging survey
that will eventually cover the entire Northern sky. Each of the 3170 pointings
will be observed for 8 hrs, which, at most declinations, is sufficient to
produce ~5arcsec resolution images with a sensitivity of ~0.1mJy/beam and
accomplish the main scientific aims of the survey which are to explore the
formation and evolution of massive black holes, galaxies, clusters of galaxies
and large-scale structure. Due to the compact core and long baselines of LOFAR,
the images provide excellent sensitivity to both highly extended and compact
emission. For legacy value, the data are archived at high spectral and time
resolution to facilitate subarcsecond imaging and spectral line studies. In
this paper we provide an overview of the LoTSS. We outline the survey strategy,
the observational status, the current calibration techniques, a preliminary
data release, and the anticipated scientific impact. The preliminary images
that we have released were created using a fully-automated but
direction-independent calibration strategy and are significantly more sensitive
than those produced by any existing large-area low-frequency survey. In excess
of 44,000 sources are detected in the images that have a resolution of
25arcsec, typical noise levels of less than 0.5 mJy/beam, and cover an area of
over 350 square degrees in the region of the HETDEX Spring Field (right
ascension 10h45m00s to 15h30m00s and declination 45d00m00s to 57d00m00s).
