Astron. Astrophys. 359, 1195-1200 (2000) 7. The basic data setA set of 101352 stars with SAO reference and astrometric solution were selected from the Hipparcos catalogue (ESA 1997). Position and proper motion parameters were read, together with the corresponding error matrices. The data was transformed from the Hipparcos epoch (J1991.25) to J2000 and the corresponding error matrices were computed assuming a linear motion model: and similar equations for the declination components. In these equations, is the initial epoch, T the final epoch, and the standard deviations and correlation coefficients among position and proper motion parameters. Data for the same star set was read from the SAO catalogue (SAO 1966) and the error matrices were computed for J2000 using a linear motion model. Finally, the differences and the respective standard deviations were computed for each of the astrometric parameters. Four stars with gross differences in the position were eliminated, so the final data set consisted in 101348 stars. Each entry in the data set consisted in the following fields:
Due to the large number of items, the data was divided in 10 subsets, covering the full sky. Each subset was formed taking the stars whose index end in the digits 0-9, i.e. the first subset contains all stars whose numbers end by 0, the second subset contains all stars whose numbers end by 1, and so on. Table 1 shows the initial statistics for full data set and the 10 subsets. Fig. 1 shows the covering of the sky by the G1 group of stars. The difference position errors histogram is shown in Fig. 2. They are obviously dominated by the SAO catalogue errors.
Table 1. Initial statistics of the data set. The units for the second column is arcsec, for all the others, milliarcsec. is an abbreviation for , and the same for . © European Southern Observatory (ESO) 2000 Online publication: July 13, 2000 |