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′ NW and 1.7′ 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 .