10.4.1 Sky coverage and completeness

The overall Gaia DR2 content has been tested with respect to the Hipparcos, Tycho-2, 2MASS and UCAC4 catalogues. The tests performed here use the cross-match between Gaia and these catalogues provided by the Gaia Archive (Marrese et al. 2019). The variation over the sky of four key parameters are checked: the number of cross-matched sources, the mean number of neighbours (stars which could have been considered as cross-matched, but for which the cross-match was not as good as for the selected source; hereafter the best neighbour), the number of Gaia stars with the same best neighbour, and the number of Gaia sources without any matches. Finally, a random subset of about 5 million sources has been selected in order to check the different properties, if present, in magnitude, colour, proper motion, goodness of fit, etc., of the above four categories of stars.

The completeness has also been tested versus QSO catalogues: quasars with ground-based quasar compilations: GIQC (Andrei et al. 2014), LQAC4 (Souchay et al. 2015) and SDSS DR14 (Pâris et al. 2014) catalogues and with galaxies catalogues: SDSS DR12 sources (Alam et al. 2015) with a galaxy spectral classification.

The small scale completeness of Gaia DR2 and its variation with the sky stellar density have been tested in comparison with two catalogues: Version 1 of the Hubble Space Telescope (HST) Source Catalogue (HSC; Whitmore et al. 2016) and a selection of fields observed by OGLE (Udalski et al. 2008) with different sky densities.

The HSC (Whitmore et al. 2016) is a very non-uniform catalogue based on deep pencil-beam HST observations made using a wide variety of instruments (Wide Field Planetary Camera 2 (WFPC2), Wide Field Camera 3 (WFC3) and the Wide Field Channel of the Advanced Camera for Surveys (ACS) and observing modes. The spatial resolution of Gaia is comparable to that of Hubble and the HSC is therefore an excellent tool to test the completeness of Gaia on specific samples of stars. To check the completeness as a function of $G$, we computed an approximate $G$-band magnitude from HST F555W and F814W magnitudes ($G_{\mathrm{HST}}$) using theoretical colour-colour relations derived following the procedure of Jordi et al. (2010). The first test was made in a crowded field of one degree radius around Baade’s Window. Nearly 13 000 stars were considered, observed in both the F555W and F814W HST filters with either WFPC2 or WFC3. The second test was made on samples of stars observed with one of the three HST cameras, using the red filter F814W and either F555W or F606W. Sources were selected following the recommendations of Whitmore et al. (2016) to reduce the number of artefacts. Moreover, only stars with an absolute astrometric correction flag in HST set to yes have been selected, leading to a typical absolute astrometric accuracy of about 0.1 ${}^{\mathrm{\prime \prime }}$. The size of the resulting samples varies from 1600 stars for ACS-F555W to nearly 120 000 stars for ACS-F606W, going through 15-23 000 stars for the four other samples.

OGLE: We selected fields within the OGLE-III Disk (Szymański et al. 2010), OGLE-III Bulge (Szymański et al. 2011) and OGLE-IV LMC (Soszyński et al. 2012) surveys. A $G$-band magnitude is computed from OGLE $V$ and $I$ magnitudes ($G_{\mathrm{OGLE}}$) using an empirical relation derived from the matched Gaia/OGLE sources (two relations were derived, one for OGLE-III and one for OGLE-IV due to their different filters). The stellar densities were estimated from the OGLE data themselves at $G$ =20, therefore they are certainly slightly under-estimated.

The spatial resolution of the Gaia Catalogue is tested using the Washington Visual Double Star Catalogue (WDS; Mason et al. 2001). A selection is made of sources composed of only 2 components, with the magnitudes for both the primary and the secondary brighter than 20 mag and a separation smaller than 10 ${}^{\mathrm{\prime \prime }}$. Sources had also to have been observed at least twice with differences between the two observed separations smaller than 2 ${}^{\mathrm{\prime \prime }}$and magnitude differences had to be smaller than 3 magnitude, and must not have a note indicating an approximate position (!), a dubious double (X), uncertain identification (I) nor photometry from a blue ($B$) or near-IR band ($K$). The resulting selection contains 43 580 systems.