1995A&A...303..331B

We present a method for estimating the shear pattern and the mass distribution of galaxy clusters, in ultra-deep sub-arcsecond CCD images, from the analysis of the weak gravitational distortion induced on images of background sources. Shape parameters of galaxies are estimated by computing their second order momenta, with a weighting function minimizing seeing effects. Shape matrices are then scaled to unit determinant, and co-added within subfields, smaller than the typical scale of the deflecting potential. After accurate correction for atmospheric/optical flaws, this procedure gives the local shear intensity and orientation, regardless of the intrinsic ellipticity distribution of sources. Regarding the observational conditions at CFHT with FOCAM, we discuss the observational strategy, for measuring the shear at distance beyond 2h₇₅^–1Mpc. We then test the projected mass density profile of the deflector with the assumption of circular symmetry by comparing the radial evolution of the shear pattern with the prediction of analytical models. The efficiency of the method has been checked by using simulated CCD images of faint sheared galaxies. They show that the shear can be detected down to 5% intensity with 1 arcmin resolution. Two applications of this technique on the rich cluster Cl0024+1654 and on the double quasar field Q2345+007, have been already published. In both cases the conclusions of the weak shear analysis have been confirmed by other authors using independent techniques, which shows that it can be applied without further improvements to search for lower density contrasts on larger scales.