Parallax error maps for test case A0 (no observation noise) at selected points in the simple iteration scheme (SI, left column) and conjugate gradient scheme (CG, right column). The iteration number is shown in the top left corner of each map (the initial values are iteration 2 due to the “start-up” procedure described in the text). The number in the top right corner is the approximate amplitude (in μas) of the colour scale. Starting from identical initial values of the astrometric parameters, SI and CG converge to the same solution (equal to the true parallaxes to within  ± 0.001   μas), although along different paths; moreover CG converges about four times quicker.