  We describe the construction of an all-sky galaxy catalogue, using
SuperCOSMOS scans of Schmidt photographic plates from the UKST and POSS2
surveys. The photographic photometry is calibrated using SDSS data, with
results that are linear to 2% or better. All-sky photometric uniformity is
achieved by matching plate overlaps and also by requiring homogeneity in
optical-to-2MASS colours, yielding zero points that are uniform to 0.03 mag. or
better. The typical AB depths achieved are B_J<21, R_F<19.5 and I_N<18.5, with
little difference between hemispheres. In practice, the I_N plates are
shallower than the B_J & R_F plates, so for most purposes we advocate the use
of a catalogue selected in these two latter bands. At high Galactic latitudes,
this catalogue is approximately 90% complete with 5% stellar contamination; we
quantify how the quality degrades towards the Galactic plane. At low latitudes,
there are many spurious galaxy candidates resulting from stellar blends: these
approximately match the surface density of true galaxies at |b|=30 deg. Above
this latitude, the catalogue limited in B_J & R_F contains in total about 20
million galaxy candidates, of which 75% are real. This contamination can be
removed, and the sky coverage extended, by matching with additional datasets.
This SuperCOSMOS catalogue has been matched with 2MASS and with WISE, yielding
quasi-allsky samples of respectively 1.5 million and 18.5 million galaxies, to
median redshifts of 0.08 and 0.20. This legacy dataset thus continues to offer
a valuable resource for large-angle cosmological investigations.
