  The gravitational potential responsible for the lensing effect in SBS
1520+530 is studied over length scales from a few arc-seconds to a few
arc-minutes. For this purpose, we use sharply deconvolved Hubble Space
Telescope images in the optical and near-IR, in combination with ground based
optical data obtained over a wider field-of-view. In particular, we have
carried out a multi-color analysis in order to identify groups or clusters of
galaxies along the line of sight. Photometric redshifts are measured for 139
galaxies unveiling significant excesses of galaxies 1.0 arcmin, NW and 1.7
arcmin, SW of the main lensing galaxy. The photometric redshift inferred both
for the main lensing galaxy and for the galaxy concentrations is
z=0.9^{+0.10}_{-0.25}. This is in rough agreement with the measured
spectroscopic redshift of the main lensing galaxy, z=0.71 (Burud et al. 2002),
suggesting that it is part of a larger group or cluster. We investigate the
impact of including the galaxy cluster, first on the modelling of the lensing
system, and second on the expected time--delay between the two quasar images.

Astrophysics