Sketch defining the notation used in this paper. The presence of a perturbing star at position ﹩\mathstrut{{\boldsymbol{\xi}}} _{p}﹩ relative to the unperturbed image at ﹩\boldsymbol{x}_{i\mathrm{BH}\,}﹩ splits this image into two or four images at positions ﹩\mathstrut{{\boldsymbol{\xi}}} _{i}﹩ relative to the unperturbed image. For clarity, only one of the multiple images, due the the star, is shown and the proportions are exaggerated.