HST RGB-colour composite image of the A521 arc (red filter: F160W, green: F105W, and blue: F606W). Magnifications are highlighted using coloured regions, from transparent for regions amplified 1–10 times, to red for regions magnified more than 500 times, as defined in the colour-bar (on the right side). The galaxy marked with a plus is a foreground member of the lensing cluster that overlaps the A521 arc, and the galaxy marked with a cross is a foreground member that contaminates A521.