distributions. There the errors could also be correlated over several degrees. The overall median of the actual parallax errors is $-0.6~\mu$as. The error maps for the other astrometric parameters have similar characteristics. The size of the actual errors is shown in panel **(b)**, displaying the RSE per pixel.

In an astrometric solution of real data the errors cannot be assessed by comparing the solution with the true values. Error estimates must instead come from the formal standard errors (uncertainties), computed as the square-roots of the diagonal elements of the covariance matrix (possibly adjusted depending on the size of the residuals in the solution). It is important that the formal standard errors (panel **c**) correctly characterize the actual errors. In the ideal case, the normalized error, i.e., the ratio of the actual error to the formal standard error, should follow a normal distribution with zero mean and unit standard deviation all over the sky. It was already shown (by the maps of the actual errors) that the mean values are close to zero. Panel **(d)** then shows the RSE values of the normalized parallax errors. These are around 1.0 everywhere[^2], with a relatively small scatter in the Galactic plane, where there 