The HST eXtreme Deep Field XDF: Combining all ACS and WFC3/IR Data on
  the HUDF Region into the Deepest Field Ever
  The eXtreme Deep Field (XDF) combines data from ten years of observations
with the HST Advanced Camera for Surveys (ACS) and the Wide-Field Camera 3
Infra-Red (WFC3/IR) into the deepest image of the sky ever in the
optical/near-IR. Since the initial observations on the Hubble Ultra-Deep Field
(HUDF) in 2003, numerous surveys and programs, including supernova followup,
HUDF09, CANDELS, and HUDF12 have contributed additional imaging data across the
HUDF region. Yet these have never been combined and made available as one
complete ultra-deep optical and near-infrared image dataset. We do so now for
the eXtreme Deep Field (XDF) program. Our new and improved processing
techniques provide higher quality reductions of the total dataset. All WFC3
near-IR and optical ACS data sets have been fully combined and accurately
matched, resulting in the deepest imaging ever taken at these wavelengths
ranging from 29.1 to 30.3 AB mag (5sigma in a 0.35" diameter aperture) in 9
filters. The gains in the optical for the four filters done in the original ACS
HUDF correspond to a typical improvement of 0.15 mag, with gains of 0.25 mag in
the deepest areas. Such gains are equivalent to adding ~130 to ~240 orbits of
ACS data to the HUDF. Improved processing alone results in a typical gain of
~0.1 mag. Our 5sigma (optical+near-IR) SExtractor catalogs reveal about 14140
sources in the full field and about 7121 galaxies in the deepest part of the
XDF (the HUDF09 region). The XDF is the deepest image of the universe ever
taken, reaching, in the combined image for a flat f_nu source, to 31.2 AB mag
5sigma (32.9 at 1sigma) in a 0.35" diameter aperture.
