Individual, Single Stars

It will become clear from the discussions that follow that availability of reliable estimates of the formal errors on the parallaxes is crucially important for their scientific exploitation. The problem is, that although precisions can be determined quite easily, accuracies are much harder to obtain. In general, errors can originate from two sources: internal (photon statistics, random noise introduced in instrument modelling), and external errors (systematic imperfections of the instrument modelling). We will first look at what the statistics of the abscissa residuals at different stages of the reductions tell us about the internal errors, and then at establishing the level of the external errors. Then we will look at how errors can affect the scientific interpretation of parallaxes. This is followed by an investigation concerning the accuracies of the proper motions and positions. The various types of disturbed solutions are described. Finally, it is shown that there is no significant transformation required to align the new catalogue with the optical realisation of the ICRS (the published Hipparcos catalogue). A comparison is made with a set of radio sources with high-accuracy astrometric data.