The LOFAR Two-metre Sky Survey - II. First data release
  The LOFAR Two-metre Sky Survey (LoTSS) is an ongoing sensitive,
high-resolution 120-168MHz survey of the entire northern sky for which
observations are now 20% complete. We present our first full-quality public
data release. For this data release 424 square degrees, or 2% of the eventual
coverage, in the region of the HETDEX Spring Field (right ascension 10h45m00s
to 15h30m00s and declination 45$^\circ$00$'$00$''$ to 57$^\circ$00$'$00$''$)
were mapped using a fully automated direction-dependent calibration and imaging
pipeline that we developed. A total of 325,694 sources are detected with a
signal of at least five times the noise, and the source density is a factor of
$\sim 10$ higher than the most sensitive existing very wide-area
radio-continuum surveys. The median sensitivity is S$_{\rm 144 MHz} =
71\,\mu$Jy beam$^{-1}$ and the point-source completeness is 90% at an
integrated flux density of 0.45mJy. The resolution of the images is 6$''$ and
the positional accuracy is within 0.2$''$. This data release consists of a
catalogue containing location, flux, and shape estimates together with 58
mosaic images that cover the catalogued area. In this paper we provide an
overview of the data release with a focus on the processing of the LOFAR data
and the characteristics of the resulting images. In two accompanying papers we
provide the radio source associations and deblending and, where possible, the
optical identifications of the radio sources together with the photometric
redshifts and properties of the host galaxies. These data release papers are
published together with a further $\sim$20 articles that highlight the
scientific potential of LoTSS.
