The missing light of the Hubble Ultra Deep Field
  The Hubble Ultra Deep field (HUDF) is the deepest region ever observed with
the Hubble Space Telescope. With the main objective of unveiling the nature of
galaxies up to $z \sim 7-8$, the observing and reduction strategy have focused
on the properties of small and unresolved objects, rather than the outskirts of
the largest objects, which are usually over-subtracted.
  We aim to create a new set of WFC3/IR mosaics of the HUDF using novel
techniques to preserve the properties of the low surface brightness regions. We
created ABYSS: a pipeline that optimises the estimate and modelling of
low-level systematic effects to obtain a robust background subtraction.
  We have improved four key points in the reduction: 1) creation of new
absolute sky flat fields, 2) extended persistence models, 3) dedicated sky
background subtraction and 4) robust co-adding. The new mosaics successfully
recover the low surface brightness structure removed on the previous HUDF
published reductions.
  The amount of light recovered with a mean surface brightness dimmer than
$\overline{\mu}=26$ mar arcsec$^{-2}$ is equivalent to a m=19 mag source when
compared to the XDF and a m=20 mag compared to the HUDF12. We present a set of
techniques to reduce ultra-deep images ($\mu>32.5$ mag arcsec$^{-2}$, $3\sigma$
in $10\times10$ arcsec boxes), that successfully allow to detect the low
surface brightness structure of extended sources on ultra deep surveys. The
developed procedures are applicable to HST, JWST, EUCLID and many other space
and ground-based observatories. We will make the final ABYSS WFC3/IR HUDF
mosaics publicly available at http://www.iac.es/proyecto/abyss/.
