Behaviour of the position error and uncertainty with varying priors, for stars in the direction and magnitude bin specified in Table 1. The rows display the behaviour for different numbers of distinct transits: one, two, two (diluted), four, and eight distinct transits. The diluted case uses the first and last transit from Table 1 instead of the first two transits. Left column: size of actual position errors (blue dashed) and their formal uncertainties (red solid), for stars in the same direction and magnitude bin, as a function of the prior uncertainty. Right column: fraction of actual errors contained by the formal error ellipse. The optimum prior σ_ϖ,F90 is chosen based on the observation interval containing two distinct consecutive transits (second row). This prior is replicated in all other panels.