3. Astrometric accuracy
The internal standard errors of the observed Tycho-2 positions in RA and Dec are given for each star in the catalogue. The errors are plotted as function of magnitude in Fig. 2a and they appear in Table 2. The errors are discussed in detail in H2000a.
For the mean positions, standard errors are derived based on the weights of the source catalogues and their respective errors. When more than two positions are available, a standard error based on the scatter was also computed (see H2000a). We include the ratio of these two error determinations in the catalogue as a measure of the goodness of fit.
External errors in astrometry have been studied by comparisons with the Hipparcos Catalogue. The ratio of external to internal standard errors of positions is about 1.0. The same ratio is found for proper motions. The Hipparcos and Tycho-2 proper motions are equally accurate for stars fainter than . For brighter stars a comparison is pending.
The systematic errors of positions and proper motions have been studied by considering the average differences between Hipparcos and Tycho-2 values for the common stars in areas of typically . Assuming the systematic errors of Hipparcos astrometry to be negligible in this context, the systematic errors of Tycho-2, on angular scales of 6 deg or more, are less than 1 mas for positions and less than about 0.5 mas/yr for proper motions.
© European Southern Observatory (ESO) 2000
Online publication: March 9, 2000