proper motions and parallaxes for the majority of the 2.5 million Hipparcos and Tycho-2 stars.
Aims. We describe three essential elements of the initial data treatment leading to this catalogue, viz. the image analysis, the construction
of a source list, and the near real-time monitoring of the payload health.We also discuss some weak points that set limitations for
the attainable precision at the present stage of the mission.
Methods. Image parameters for point sources are derived from one dimensional scans, using a maximum likelihood method, under
the assumption of a line spread function constant in time, and a complete modelling of bias and background. These conditions are,
however, not completely fulfilled. The Gaia source list is built starting from a large ground-based catalogue, but even so a significant
number of new entries have been added, and a large number have been removed. The autonomous on-board star image detection will
pick up many spurious images, especially around bright sources, and such unwanted detections must be identified. Another key step of
the source list creation consists in arranging the more than 1010 individual detections in spatially isolated groups that can be analysed
Results. Complete software systems have been built for the Gaia initial data treatment, managing some 50 million focal plane transits
daily, giving transit times and fluxes for 500 million individual CCD images to the astrometric and photometric processing chains,
and carrying out a successful and detailed daily monitoring of Gaia health.
- astrometry – methods: data analysis – space vehicles: instruments