ect. 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.

In the on-ground processing, however, the full resolution Article number, page 28 of 36
of the astrometric instrument, combined with the window size (at least 12 along-scan pixels of 0′′.06 each), allows one to systematically resolve double stars down to separations around 0′′.1.

A special deblending treatment of close binaries will be performed in the BP/RP data processing.

Moving objects: The majority of main-belt and near-Earth
asteroids, at least up to speeds of 100 mas s−1, are properly detected (de Bruijne et al. 2015).

Moving objects may, after successful detection, leave the window (and even the CCD) at any moment during the focal plane transit because the window propagation assumes every object is a fixed star.

In BP/RP, an additional window is assigned to bright objects (G = 13–18 mag) that have a large across-scan motion (Sect. 3.3.8).

Extended objects: Unresolved, early-type elliptical galaxies and galaxy bulges will be mostly detected by Gaia, even with effective radii of several arcseconds, while late-type spiral galaxies, even those with weak bulges, will mostly remain undetected (de Bruijne et al. 2015).

Faint stars: The sky-mapper detection method is described in de Bruijne et al. (2015).

In essence, the detection algorithm finds peaks in flat-fielded, local background-subtracted sky-mapper sample data and then accepts these as detections if their shape is consistent with that of a point source and their flux exceeds a certain, user-configurable threshold.

This threshold has been set to G = 20.7 mag.

The onboard magnitude estimation underlying the selection decision, however, has an error of ∼0.1 mag.

Hence, the faint-end completeness in AF, BP, and RP is not sharp.

In practice, the power-law slope of the objectdetection-count histogram (in log space) as a function of G magnitude already shows signs (as of mid 2016) of incompleteness starting just fainter than 20 mag, whereas object detections as faint as 21 mag are present as well.

For RVS, the faint-end selection of targets is based on RP flux measurements.

These measurements are essentially the sum of the flux of the RP samples corresponding to the RVS wavelength range, and are used as proxies for GRVS.

The RVS faint completeness limit is hence not sharp either.

In addition, the onboard software (Sect. 3.3.8) adapts the RVS threshold, through user-configurable look-up tables, to the instantaneous, straylight-dominated background level (Sect. 4.2), which means in practice that the faint limit varies between ∼15.5 and ∼16.2 mag over a spin period.

At the end of the mission, however, taking the evolving scanning law into account (Sect. 5.2), the effective faint limit will still be GRVS ≈16.2 mag (or even a bit fainter, taking onboard RP fluxmeasurement errors into account).