Model parameters for a straight-line source-lens trajectory. The lens is the black dot in the centre, the source (unseen) is the orange disk 𝒮 of radius ρ, and the red and blue arcs 𝔗⁺ and 𝔗⁻ are the major and minor lensed images, respectively. The line joining the centre of the two images makes an angle α₁ with the x_E-axis, with the convention that the source centre lies at a distance u₁ > 0 from the lens, so that the major image is up for α₁ ∈ [0, π] The trajectory of the source with respect to the lens is shown as the thick black arrow. It makes an angle a with the x_E-axis, and u₀ is its signed impact parameter. As the source moves relative to the lens along the black arrow, the images rotate around the lens.