Distribution of position errors for the three cases A, B, and C in Fig. 1. Blue dots: individual astrometric errors. Red curve: 90% confidence ellipse. Prior A is too tight and essentially gives a two-parameter solution. Prior B at the intersection of the curves in Fig. 1 (semi-major axis of the 90% confidence ellipse equals the 90th percentile of the actual errors) produces sensible error estimates. Prior C is too loose and yields a degenerate solution – although not visible in the diagram, 90% of the points are contained in the extremely elongated ellipse.