The 2012 Hubble Ultra Deep Field (UDF12): Observational Overview
  We present the 2012 Hubble Ultra Deep Field campaign (UDF12), a large
128-orbit Cycle 19 \HST\ program aimed at extending previous WFC3/IR
observations of the UDF by quadrupling the exposure time in the F105W filter,
imaging in an additional F140W filter, and extending the F160W exposure time by
50%. The principal scientific goal of this project is to determine whether
galaxies reionized the universe; our observations are designed to provide a
robust determination of the star formation density at $z$$\,\gtrsim\,$8,
improve measurements of the ultraviolet continuum slope at
$z$$\,\sim\,7\,-\,$8, facilitate the construction of new samples of
$z$$\,\sim\,9\,-\,$10 candidates, and enable the detection of sources up to
$z$$\,\sim\,$12. For this project we committed to combining these and other
WFC3/IR imaging observations of the UDF area into a single homogeneous dataset,
to provide the deepest near-infrared observations of the sky currently
achievable. In this paper we present the observational overview of the project,
motivated by its scientific goals, and describe the procedures used in reducing
the data as well as the final products that are produced. We have used the most
up up-to-date methods for calibrating and combining the images, in particular
paying attention to correcting several instrumental effects. We release the
full combined mosaics, comprising a single, unified set of mosaics of the UDF,
providing the deepest near-infrared blank-field view of the universe obtained
to date, reaching magnitudes as deep as AB$\,\sim\,$30 in the near-infrared,
and yielding a legacy dataset on this field of lasting scientific value to the
community.
