Accuracy of photometric redshifts for future weak lensing surveys from
  space
  Photometric redshifts are a key tool to extract as much information as
possible from planned cosmic shear experiments. In this work we aim to test the
performances that can be achieved with observations in the near-infrared from
space and in the optical from the ground. This is done by performing realistic
simulations of multi-band observations of a patch of the sky, and submitting
these mock images to software usually applied to real images to extract the
photometry and then a redshift estimate for each galaxy. In this way we mimic
the most relevant sources of uncertainty present in real data analysis,
including blending and light pollution between galaxies. As an example we adopt
the infrared setup of the ESA-proposed Euclid mission, while we simulate
different observations in the optical, modifying filters, exposure times and
seeing values. Finally, we consider directly some future ground-based
experiments, such as LSST, Pan-Starrs and DES. The results highlight the
importance of u-band observations, especially to discriminate between low (z <
0.5) and high (z ~ 3) redshifts, and the need for good observing sites, with
seeing FWHM < 1. arcsec. The former of these indications clearly favours the
LSST experiment as a counterpart for space observations, while for the other
experiments we need to exclude at least 15 % of the galaxies to reach a
precision in the photo-zs equal to $<\frac{\sigma_z}{1+z}>$ < 0.05.
