The main uncertainties come from the intrinsic shape and redshift distributions of the sources, and also from the seeing degradation and pixelisation of CCD frames. The coherent pattern shows an orthoradial distribution of ellipticity of background sources with respect to the cluster center, as expected for a gravitational shear. Unfortunately, we only observed one off-axis field. Other distant fields around the center are needed to confirm this result, and also to have a more precise idea on how the shear decreases with radius. The study of the weak shear far from cluster centers should be a real breakthrough in our knowledge of these systems.
Similar studies are now in progress in various places.
Smail et al. The figure is composed of two deep CCD images obtained at CFHT which have been rotated and co-added. The inner region of the cluster is on the right and concerns a small field. The off-centered field is on the left and covers a much larger field. The orientation of the shear is given by the orientation of the line.