SkyMapper Southern Survey: Second Data Release (DR2)
  We present the second data release (DR2) of the SkyMapper Southern Survey, a
hemispheric survey carried out with the SkyMapper Telescope at Siding Spring
Observatory in Australia, using six optical filters: $u,v,g,r,i,z$. DR2 is the
first release to go beyond the $\sim$18mag (10${\sigma}$) limit of the Shallow
Survey released in DR1, and includes portions of the sky at full survey depth
that reach >21mag in $g$ and $r$ filters. The DR2 photometry has a precision as
measured by internal reproducibility of 1% in $u$ and $v$, and 0.7% in $griz$.
More than 21 000 deg$^2$ have data in some filters (at either Shallow or Main
Survey depth) and over 7 000 deg$^2$ have deep Main Survey coverage in all six
filters. Finally, about 18 000 deg$^2$ have Main Survey data in $i$ and $z$
filters, albeit not yet at full depth. The release contains over 120 000
images, as well as catalogues with over 500 million unique astrophysical
objects and nearly 5 billion individual detections. It also contains
cross-matches with a range of external catalogues such as Gaia DR2, Pan-STARRS1
DR1, GALEX GUVcat, 2MASS, and AllWISE, as well as spectroscopic surveys such as
2MRS, GALAH, 6dFGS, and 2dFLenS.
