The background signal on the Gaia CCDs collects the contribution of a few components, of different origin:
the “canonical” diffuse astrophysical background;
stray light from different sources, mostly the Sun but also bright stars, as explained below;
two components, related to the CCD electronics, i.e., the dark current signal (dealt with separately) and the charge release.
The background model takes into account only the stray light, which provides the biggest contribution to the background signal
and dominates the other components by a factor which depends not only on the CCD but also on the position inside the same CCD, as evident in Figure 5.6. The worst case is for BP ROW 7 where the stray light can reach values of 27 electron/pixel/second, i.e. more than 100 times the average sky background level of 0.2 electron/pixel/second.
The majority of the stray light is caused by the Sun and the strength of
this effect depends on the position of the Sun with respect to the
satellite. It shows a periodicity corresponding to the
satellite spin phase and a slow evolution in amplitude with
the orbital solar distance. Other contributions are in phase with the scanning law, showing contributions from the Ecliptic and Galactic planes.
As a model we used a discrete map, obtained by accumulating two days of
observations (Virtual Objects, i.e., empty windows acquired for
calibration purposes) with distance from the charge injection higher
than 50 TDI lines (to avoid the charge release component). For every
transit, the median value on all samples is computed, to avoid the
accidental contribution of cosmic rays, and also the AC coordinate and
the heliotropic spin phase (Figure 3.4) are calculated. The map is built on a grid of 360 bins in the spin direction (one bin is 1 degree) and 20 bins in the AC direction (one bin is 200 pixels, corresponding to
35 arcsec). All the transits belonging to the same bin are averaged to
obtain a median value, to remove outliers. Examples of stray light maps
are shown in Figure 5.6.
For details about the use of these maps in the processing, see