A significantly greater than expected straylight was discovered during early commissioning by the Gaia scientists (payload experts). Lots of effort from their side, together with ESA and Airbus DS, were invested in both understanding its origin and minimising its effects. A number of tests were carried out, most notably operating Gaia at non-nominal Sun aspect angles of 42◦and 0◦, while collecting stellar data. Fig. 4 shows the straylight levels for two CCDs as a function of the heliotropic rotation phase during selected spacecraft rotations.
Two distinct behaviours are found. For some CCDs, such as VPU2, PEM06, the main straylight peak always takes place when the telescope 2 aperture is closer to the Sun, which converts to a flat non-zero level for a Sun aspect angle of 0◦. This is consistent with a pure solar origin. However, other detectors show additional peaks in addition to that of purely solar origin. This is evident by the behaviour at Sun aspect angle 0◦.
The solar contribution was completely unexpected. Different tests were carried out to determine its origin. Most notably, a gentle dependence with the Sun aspect angle was found, at odds with simple diffraction models of a two layer sun-shield. Understanding of this phenomenon was one of the mandates of the ESA-Airbus DS basic angle variations and straylight working group, which carried out the investigations after commissioning. The ESA-ESTEC team within the group finally found the origin, which is the (unfortunate) combination of three effects. First, there are sticking out Nomex fibres at some edges of the sun shield (the untapered triangular sections). the fibres are needed for mechanical integrity. The ends protruding from the blanket edges were a known fact before launch, but not considered a major risk from the mechanical and thermal points of view (the main drivers behind the sun-shield development). Second, the sun-shield design allows some diffracted light from the first blanket to illuminate the upper part of the thermal tent apertures (no direct illumination, though). Third, there are some unbaffled rogue paths that can transport that scattered light from the top part of the thermal tent down to the focal plane. See Fig. 5 for an overview of the three effects.
Regarding the non-solar straylight components. A detailed straylight analysis was carried out before launch. Some paths were identified, but they were found harmless against typical bright stars. However, it was realised by Airbus DS during commissioning that the integrated light of the whole Galaxy, when convolved with the straylight paths, could produce a measurable signature, in agreement with the in-orbit measurements.

[Figure: Figure 5. Origin of the solar straylight. It is the combination of three effects: Nomex fibres sticking out of the sun-shield blankets (left), the sun-shield design allowing some diffracted light to get through the thermal tent telescope apertures (centre) and rogue paths transporting that scattered light into the focal plane (right).]

[Figure: Figure 6. Straylight generated by the Galaxy. The Milky Way brightness distribution, when convolved with the straylight paths within the payload, can produce a significant straylight signature, in agreement with the in-orbit measurements. Plot: Airbus DS.]

See Fig. 6 for an example calculation.
Once the origin of the straylight was identified, it was realised that it could not be avoided. Mitigation measures have thus been undertaken. Mostly, the data collection magnitude threshold is now adaptive for the radial velocity spectrometer, which is now straylight Poisson noise driven, as opposed to the pre-launch CCD readout noise limited expectations. This means the faint object detection limit is now shallower during the maximum solar straylight peaks. In this way, only useful data are downlinked. Additional on-board functionalities have also been implemented, but not yet used, such as adaptive AC bin size for the radial velocity spectrometer or a smaller AC bin size for the faintest stars in the astrometric field. Finally, the on-ground downstream processing has also been improved to face the additional noise in all focal planes. The end-of-mission performance has been updated accordingly.

[Figure: Figure 7. Basic Angle Monitor overview. Left and Middle: the BAM is composed of two optical benches that divide the light from a single laser source into four beams, injecting two of them into each telescope entrance pupil. Right: for each telescope, the two input laser beams create a Young interference pattern in the focal plane. They are just very high signal to noise artificial stars, whose relative movement traces changes in the basic angle. Images: Airbus DS.]