Figure 4.

The lensing model for CLASS B1608 + 656, showing the image positions (light solid contours), locations and ellipticities of the lens components (ellipses), lens isopotential contours (dark solid contours), and surfaces of constant time delay (dashed contours, with delays labeled). Note that as in all quad systems, the observed images correspond to minima or saddle-points in the Fermat potential, while the unobserved fifth image that appears at a maximum of the time delay is highly demagnified. The Inset at the upper right shows the source-plane configuration along with the outer critical curves (the transition from one to three total images) and the inner caustics (from three to five images). The star marks the inferred location of the background radio source, which lies just inside the inner caustic. Image courtesy of Leon Koopmans (Groningen).