Convergence of the iterative astrometric solution for the primary data set. The solid curve is the typical size of parallax updates in each iteration; the dashed curve is the typical size of AL residuals (in μas). The letters refer to the main phases of the iterative scheme: A – iterations with 180 s attitude knot interval (Sect. 3.5) and simplified calibration model; B – iterations with full calibration model; C – using 30 s attitude knot interval; D – Hipparcos alignment active (see text); E – auxiliary quasar solution (Sect. 4.2) using a fixed attitude; F – source and calibration updates after final alignment of the attitude to the ICRS (Sect. 4.3).