5. Detection and Observation of Galaxies with GAIA

The general purpose of the planning of a scientific undertaking must be maximizing the overall expected results under given constraints. In the case of the GAIA mission, this is a particularly delicate aspect, since the satellite will have a relatively short operational lifetime and the amount of data that it will be possible to transmit to the ground, though very large, is far from what one would hope for such a high-performance instrument.

In this framework, an observing strategy was carefully devised in order to discriminate stars from the sky background, cosmic rays and other uninteresting features and thus to bring the telemetry rate down to manageable levels. Such a strategy needs to be ``tuned'' to allow the observation of galaxies, since these are diffuse objects that the star observing strategy was devised to ignore.