Upper panel: Residuals calculated as the absolute value of the mean of individual bins (solid lines) for θ_E = 1 mas (red) and θ_E = 3 mas (blue). We used 30 equally spaced bins in t_obs. Histograms are normalised to 1. We assume $\alpha ={6.49}^{\circ },\delta =-{47.6}^{\circ },\varpi =1,{\mu }_{\alpha }^{\ast }=0,{\mu }_{\delta }=0,u_0=0.5,t_0=2017.3$ (denoted by a vertical dotted green line), t_E = 30 days, and π_EE = π_EN = 0.001. Residuals are fitted with a Gaussian (dashed lines). Lower panel: ${\theta }_{\mathrm{E}}^0$ as a function of the maximum of residuals obtained from a Gaussian fit for u₀ = 0.5 and (red circles) and u₀ = 2 (blue diamonds). The dashed lines denote the best linear fit of the scatter plots.