The cosmological lens equation and the equivalent single-plane gravitational lens

The gravitational lens equation resulting from a single (non-linear) mass concentration (the main lens) plus inhomogeneities of the large-scale structure is shown to be strictly equivalent to the single-plane gravitational lens equation without the cosmological perturbations. The deflection potential (and, by applying the Poisson equation, also the mass distribution) of the equivalent single-plane lens is derived. If the main lens is described by elliptical isopotential curves plus a shear term, the equivalent single-plane lens will be of the same form. Owing to the equivalence shown, the determination of the Hubble constant from time delay measurements is affected by the same mass-sheet invariance transformation as for the single-plane lens. If the lens strength is fixed (e.g. by measuring the velocity dispersion of stars in the main lens), the determination of H_0 is affected by inhomogeneous matter between us and the lens. The orientation of the mass distribution relative to the image positions is the same for the cosmological lens situation and the single-plane case. In particular this implies that cosmic shear cannot account for a misalignment of the observed galaxy orientation relative to the best-fitting lens model.