1996A&A...310..705F

We present an attempt to reveal the existence of large foreground mass condensations directly responsible for the gravitational magnification of distant luminous QSO/radiosources in five selected fields: PKS0135-247, PKS1508-05, PKS1741-03, 3C446, and Q1622+238. The technique uses a weak lensing analysis of the distant galaxies in the field of each source. We find a coherent shear map on the line of sight to Q1622+238 and centered on a distant compact cluster. The shear amplitude in the field of the four orther bright radiosources is often correlated with the presence of groups or clusters of galaxies. For one of them, PKS0135-247, the shear is tangential to a foreground group which is a serious candidate for the lens responsible for the shear pattern. The result suggests that a substantial amount of invisible mass may be condensed in groups and clusters of galaxies. If confirmed it may explain the origin of a large angular correlation between the distribution of distant radiosources (z>1) and the distribution of low redshift galaxies (z<0.3) (Bartelmann & Schneider 1993). In this paper we discuss the feasibility and consequences of a future systematic survey with ground-based telescopes to investigate the problem of magnification bias in the fields of luminous distant objects and to probe the mass distributions of galaxy groups at intermediate redshifts.