The on-sky photometry (top) and astrometry (middle) for the PSPL event from Figure 3. The observed centroid position of the lensed source is shown as a solid line in the astrometric panels. Color indicates time as shown in the top panel horizontal axis. The astrometry in different reference frames with the PM removed (bottom left) or the PM and parallax removed (bottom right) shows that the astrometric microlensing signal traces an ellipse. The complete set of parameters is M_L = 1 M_⊙, β = 0.5 mas, X_S,0 = [0.00, 0.50] mas, μ_S = [10, 0] mas yr⁻¹, μ_L = [0, 0] mas yr⁻¹, d_L = 3 kpc, d_S = 8 kpc, b_SFF = 1 (see Section 3.3), mag_S = 19, t₀ = 57200 MJD, α = 262﹩\mathop{.}\limits^{\unicode{x000b0}}﹩5, and δ = −30°.