Weak Gravitational Lensing

Multiple images, microlensing (with appreciable magnifications) and arcs in clusters are phenomena of strong lensing. In weak gravitational lensing, the Jacobi matrix A is very close to the unit matrix, which implies weak distortions and small magnifications. Those cannot be identified in individual sources, but only in a statistical sense. Because of that, the accuracy of any weak lensing study will depend on the number of sources which can be used for the weak lensing analysis. This number can be made large either by having a large number density of sources, or to observe a large solid angle on the sky, or both. Which of these two aspects is more relevant depends on the specific application. Nearly without exception, the sources employed in weak lensing studies up to now are distant galaxies observed in the optical or near-IR passband, since they form the densest population of distant objects in the sky (which is a statement both about the source population in the Universe and the sensitivity of detectors employed in astronomical observations). To observe large number densities of sources, one needs deep observations to probe the faint (and thus more numerous) population of galaxies. Faint galaxies, however, are small, and therefore their observed shape is strongly affected by the Point Spread Function, caused by atmospheric seeing (for ground-based observations) and telescope effects. These effects need to be well understood and corrected for, which is the largest challenge of observational weak lensing studies. On the other hand, observing large regions of the sky quickly leads to large data sets, and the problems associated with handling them. We shall discuss some of the most important aspects of weak lensing observations in Sect. 3.