HELP: A catalogue of 170 million objects, selected at 0.36-4.5 $\mu$m,
  from 1270 deg.$^{2}$ of prime extragalactic fields
  We present an optical to near-infrared selected astronomical catalogue
covering 1270 deg.$^2$. This is the first attempt to systematically combine
data from 23 of the premier extragalactic survey fields - the product of a vast
investment of telescope time. The fields are those imaged by the Herschel Space
Observatory which form the Herschel Extragalactic Legacy Project (HELP). Our
catalogue of 170 million objects is constructed by a positional cross match of
51 public surveys. This high resolution optical, near-infrared, and
mid-infrared catalogue is designed for photometric redshift estimation,
extraction of fluxes in lower resolution far-infrared maps, and spectral energy
distribution modelling. It collates, standardises, and provides value added
derived quantities including corrected aperture magnitudes and astrometry
correction over the Herschel extragalactic wide fields for the first time.
$grizy$ fluxes are available on all fields with $g$ band data reaching
$5\sigma$ point-source depths in a 2 arcsec aperture of 23.5, 24.4, and 24.6
(AB) mag at the 25th, 50th, and 75th percentiles, by area covered, across all
HELP fields. It has $K$ or $K_s$ coverage over 1146 deg.$^2$ with depth
percentiles of 20.2, 20.4, and 21.0 mag respectively. The IRAC Ch 1 band is
available over 273 deg.$^2$ with depth percentiles of 17.7, 21.4, and 22.2 mag
respectively. This paper defines the "masterlist" objects for the first data
release (DR1) of HELP. This large sample of standardised total and corrected
aperture fluxes, uniform quality flags, and completeness measures provides
large well understood statistical samples over the full Herschel extragalactic
sky.
