Nonlinear cluster lens reconstruction

We develop a method for general nonlinear cluster lens reconstruction using the observable distortion of background galaxies. The distortion measures the combination γ/(1 - κ) of shear γ and surface density κ. From this we obtain an expression for the gradient of log (1 - κ) in terms of directly measurable quantities. This allows one to reconstruct 1 - κ up to an arbitrary constant multiplier. Recent work has emphasized an ambiguity in the relation between the distortion and γ/(1 - κ). Here we show that the functional relation depends only on the parity of the images, so if one has data extending to large radii, and if the critical lines can be visually identified (as lines along which the distortion diverges), this ambiguity is resolved. Moreover, we show that for a generic two- dimensional lens it is possible to locally determine the parity from the distortion. The arbitrary multiplier, which may in fact take a different value in each region bounded by the contour κ = 1, can be determined by requiring that the mean surface excess vanish at large radii and that the gradient of κ should be continuous across κ = 1. We show how these ideas might be implemented to reconstruct the surface density, if necessary without use of the data in regions where determination of the parity is insecure, and we show how one can measure the mass contained within an aperture, again, if necessary, without using data within the aperture.