the performance of faint objects, in particular in the spectroscopic instrument, which operates at very low signal levels.

Therefore, a partial mitigation was implemented in the RVS windowing strategy allowing one to adaptively adjust the across-scan size of the windows to the instantaneous straylight level to increase the signal-to-noise ratio of the spectra.

In addition, the RVS object selection was modified to adapt the faint limit to the instantaneous background level, which does not directly mitigate the straylight, but optimises the overall science quality of the RVS data acquired on board and downlinked to ground.

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4.2.3. Periodic basic angle variations
Periodic fluctuations of the basic angle were measured from the switch on of the basic angle monitor (BAM), which is designed to monitor basic angle variations at the µas level with a sampling of ∼23 s and typical random noise of 12–15 µas Hz−1/2 (Sect. 3.3.4).
These fluctuations are two orders of magnitude larger than expected based on pre-launch calculations and show a strong asymmetry between both telescopes; the line of sight of the preceding telescope fluctuates within a ±1000 µas range, while the other line-of-sight range is only ±200 µas. These fluctuations show a modulation with the 6-hour spacecraft rotation as well as a smaller, 24-hour modulation. Fluctuations are also seen on longer timescales, which is consistent with the change of solar irradiance caused by the periodic change of the Gaia-Sun distance and sunshield aging, and correlated with Galactic plane crossings of the preceding telescope (30 µas amplitude). The periodic variations seen in the BAM signal are strongly coupled to the heliotropic spin phase Ωof the satellite (i.e. with respect to the Sun; Sect. 5.2). In-orbit tests and experience have furthermore shown that variations disappear with a solar aspect angle of 0◦or when the spin is stopped; in the latter case, the variations reappear within minutes after restarting the spin. A dedicated working group, involving ESA and Airbus DS, has come to the following conclusions. First, the BAM data are reliable; they measure and reflect true basic angle variations at least at the level of accuracy that is relevant for the first intermediate data release, which is a few dozen µas. Second, a purely mechanical root cause can be ruled out. Third, there is strong evidence of a thermo-elastic origin. In particular, the 24-hour variation originates from the central data management unit, transponders, and payload datahandling unit in the service module, which have varying power dissipations and temperatures as a result of the daily downlink operations (Sect. 5.3.1), while the reaction of the preceding field of view to the Galactic plane crossings shows a perfect, delayed correlation with thermal variations of the VPUs (with a coupling coefficient of ∼500 µas K−1). A detailed sensitivity analysis was carried out in flight, applying thermal pulses to many serviceand payload-module components during one decontamination campaign. When combined with the typical, cyclic temperature changes experienced during a revolution, a number of candidates were identified as possible originators of the six-hour periodic basic angle variations. These candidates are mostly located in the Sun-illuminated part of the spacecraft. However, the (probably thermo-elastic) coupling mechanism needed to efficiently and quickly translate those perturbations to the payload module is still unclear.
In addition to the periodic variations, the BAM fringe position data also show jumps. Their size distribution follows a power law with an exponent of −0.8. There is on average one jump per day (in either field of view) exceeding 30 µas, although jumps are much more frequent after perturbations such as decontamination and (re-)focussing. There are about equal numbers of jumps affecting either both fields of view at the same time, only the preceding field, or only the following field. Large jumps show up in the astrometric residuals, allowing for the possibility that most of them are real, for instance originating within the mechanical structure of the payload.
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