Convergence plot for the astrometric solution in Case A. The solid curves show the rms errors of the astrometric parameter (i.e., the rms differences between the calculated and true values) as functions of the iteration number; the dashed curves show the rms of the corresponding source parameter updates. The different astrometric parameters are colour coded (green: α∗, blue: δ, red: ϖ, magenta: μ_α∗, cyan: μ_δ).