Fe] at GRVS ∼12 mag. Similar errors in Teff and [M/H] are found for A-type stars, while the surface-gravity derivation is more precise (errors of 0.07 and 0.12 dex at GRVS = 12.6 and 13.4 mag, respectively). For the faintest stars, with GRVS >∼13–14 mag, the input of effective temperature derived from the BP/RP spectro-photometry will allow the final, RVS-based parametrisation to be improved.
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8.4.

Survey coverage and completeness The survey coverage of Gaia has some particular features: Dense areas: As explained in Sect. 3.3.8, the total number of samples that can be tracked simultaneously in the readout register of a CCD differs per instrument and is 20 in AF, 71 in BP and RP, and 72 in RVS.

The associated maximum object densities (for the two superimposed viewing directions combined) depend on the along-scan window size and on the number of samples needed per object (plus the proximity-electronics settings).

For the majority of (faint) objects, onboard across-scan binning of the window contents means that one object only requires one serial sample.

In the absence of bright stars requiring 12 samples per object (10 for RVS), the maximum densities are ∼1 050 000 objects deg−2 in the astrometric field (alongscan window size 12 pixels), ∼750 000 objects deg−2 for the BP and RP photometers (along-scan window size 60 pixels), and ∼35 000 objects deg−2 for the RVS spectrograph (alongscan window size 1296 pixels).

The maximum density is proportional to the number of serial samples and is inversely proportional to the along-scan window length, which varies between the different instruments and, within a given instrument, varies with magnitude.

When a bright star that needs to be windowed with full-pixel resolution enters the CCD, the above densities are temporarily reduced.

For instance, in RVS, one bright star with GRVS < 7 mag consumes 10 serial samples, leaving 62 samples for faint stars, corresponding to ∼30 000 objects deg−2 (which is exceeded in ∼20% of the sky).

In AF, bright stars (G < 13 mag) are particularly detrimental because they have a longer window (18 along-scan pixels) and because each bright star consumes 12 of the 20 available serial samples (temporarily leaving only 8 samples for other stars, corresponding to ∼420 000 objects deg−2, which is exceeded in a few hundred square degrees on the sky).

In case of a shortage of serial samples, object selection is based on object priority, with bright stars having a higher priority than faint stars (Sect. 3.3.8).

Fortunately, the fact that each area on the sky is observed several dozen times over the course of the mission under different scanning angles means that there is no significant bias for faint sources close to bright sources in the final catalogue except for such objects receiving fewer transits.

In (very) dense areas, the number of available transits for faint objects may be (greatly) reduced, up to the level of yielding a brighter completeness limit by up to several magnitudes (see also Sect. 6.6).

Bright stars: As already mentioned in Sect. 6.5, the onboard detection efficiency at the bright end drops from ∼94% at G = 3 mag to below 10% for G = 2 mag and brighter (Sahlmann et al. 2016).

Whereas special sky-mapper SIF images are acquired for the 230 very bright stars with G < 3 mag (Sect. 6.5), in principle allowing the derivation of G-band photometry and astrometric information, a non-detection implies no BP/RP photometry and no RVS spectroscopy is collected; such data can only be acquired through a virtual object-based scheme, which is currently in preparation (Sect. 6.5).

Close double stars: As a result of the rectangular pixel size (Sect. 3.3.2), the minimum separation to resolve a close, equalbrightness double star in the sky mapper is 0′′.23 in the alongscan and 0′′.70 in the across-scan direction, independent of the brightness of the primary (de Bruijne et al. 2015); larger separations are required to resolve double stars with ∆G > 0 mag.

During the course of the mission, a given, close double star is observed many times with varying scanning angles such that it can be resolved on board in some transits and can stay unresolved in others.