Possible caustic crossing event in system 23 at z = 2.1878. Top: Color image combining F115W, F200W, and F356W as blue, green, and red, respectively. The outer, yellow circles mark the positions of 23.4a and 23.4b, two counterimages of a source that brackets the position of the critical curve. The white, central circle marks a bright source (called “Quyllur”) that lies approximately at the midpoint. Bottom: color image with the RGB6 combination. Red, green, and blue curves show the critical curves for the spectroscopic, Full-r, and Full-a models respectively. The two panels have the same scale and orientation as indicated at the left.