The main objective of the astrometric instrument (ASTRO) is to obtain accurate measurements of the relative positions of all objects that cross the fields of view of Gaia's two telescopes. The two fields of view are combined onto the single focal plane.
During its five-year mission Gaia will systematically scan the whole sky and will have obtained some 70 sets of relative position measurements for each star. These permit a complete determination of each star's five basic astrometric parameters: two specifying the angular position, two specifying the proper motion, and one - the parallax - specifying the star's distance. The five-year long mission also permits the determination of additional parameters, for example those relevant to orbital binaries, extra-solar planets, and solar-system objects.
By measuring the instantaneous image centroids from the data sent to ground, Gaia measures the relative separations of the thousands of stars simultaneously present in the combined two fields. The spacecraft operates in a continuously scanning motion, such that a constant stream of relative angular measurements is built up as the fields of view sweep across the sky. High angular resolution (and hence high positional precision) in the scanning direction is provided by the large primary mirror of each telescope. The wide-angle measurements provide high rigidity of the resulting reference system.
The astrometric instrument (ASTRO) comprises the two telescopes and the dedicated area of 62 CCDs in the focal plane, where the two fields of view are combined onto the astrometic field (AF). The CCDs each are read out in TDI (time-delayed integration) mode, synchronised to the scanning motion of the satellite. In practice, stars entering the combined field of view first pass across the column of Sky Mapper (SM) CCDs, where each object is detected. Information on an object's position and brightness is processed on board in real-time to define the windowed region around the object to be read out by the following CCDs.
On-ground Data Processing
The a posteriori on-ground data processing is a highly complex task, linking all relative measurements and transforming the location (centroiding) measurements in pixel coordinates to angular field coordinates through a geometrical calibration of the focal plane, and subsequently to coordinates on the sky through calibrations of the instrument attitude and basic angle.
Further necessary corrections to be performed include those for optics effects (systematic chromatic shifts and abberation) and general-relativistic effects (light bending due to the Sun, the major planets plus some of their moons, and the most massive asteroids).
The accuracy of the measurements depends on the stellar type and relies on the stability of the basic angle of 106.5° between the two telescopes. This angle is monitored by the Basic Angle Monitoring (BAM) system.