The DAFT/FADA survey. I.Photometric redshifts along lines of sight to
  clusters in the z=[0.4,0.9] interval
  As a contribution to the understanding of the dark energy concept, the Dark
energy American French Team (DAFT, in French FADA) has started a large project
to characterize statistically high redshift galaxy clusters, infer cosmological
constraints from Weak Lensing Tomography, and understand biases relevant for
constraining dark energy and cluster physics in future cluster and cosmological
experiments. The purpose of this paper is to establish the basis of reference
for the photo-z determination used in all our subsequent papers, including weak
lensing tomography studies. This project is based on a sample of 91 high
redshift (z>0.4), massive clusters with existing HST imaging, for which we are
presently performing complementary multi-wavelength imaging. This allows us in
particular to estimate spectral types and determine accurate photometric
redshifts for galaxies along the lines of sight to the first ten clusters for
which all the required data are available down to a limit of I_AB=24/24.5 with
the LePhare software. The accuracy in redshift is of the order of 0.05 for the
range 0.2<z<1.5. We verified that the technique applied to obtain photometric
redshifts works well by comparing our results to with previous works. In
clusters, photoz accuracy is degraded for bright absolute magnitudes and for
the latest and earliest type galaxies. The photoz accuracy also only slightly
varies as a function of the spectral type for field galaxies. As a consequence,
we find evidence for an environmental dependence of the photoz accuracy,
interpreted as the standard used Spectral Energy Distributions being not very
well suited to cluster galaxies. Finally, we modeled the LCDCS 0504 mass with
the strong arcs detected along this line of sight.
