9.3.1 Filtering on angular coverage
The angular coverage of Gaia observations is an important factor to consider for the analysis of the sources. The angular coverage is defined as the ratio between the area of the polygon created from the union of all the windows (SM or AF) from all the transits obtained at a given celestial coordinate and the area of a circle with a diameter equal to the diagonal of the largest observed window.
The first action of the treatment is to filter out sources along their number of transits and their resulting surface coverage with limits empirically determined. Sources with less than 25 transits or a surface coverage are discarded from the analysis. The filtering leaves galaxies and quasars to be analysed in terms of surface brightness profiles.
We present in Figure 9.3 the distribution on the sky of the sources (galaxies and QSOs) in galactic coordinates, coloured with their number of transits and their angular coverage. The visible patterns are directly related to the scanning law of Gaia. There are almost no extragalactic sources in the galactic plane.