Top panel: grid of source positions β covering the radial range 0 ≤|β|≤ 0.66 θ_E in the first quadrant of the source plane. Each source produces a set of lensed images θ (shown in the bottom panel) under the original deflection law α and the corresponding set of under the curl-free deflection law . The color-coding refers to the largest offset |Δθ|_max associated with each source. The solid black curves locate the caustic curves (top panel) and the critical curves (bottom panel), respectively, for the NIS plus external shear. Bottom panel: set of mock lensed images θ produced by the source positions shown in the top panel and lensed by an NIS plus external shear characterized by θ_c = 0.1 θ_E and γ_p = 0.1. The color-coding represents the offsets |Δθ| in units of θ_E between θ and the images .