# 7.4.3 Photometric precision and accuracy

We test the Gaia provided passbands by comparing the observed photometry to the the photometry computed using the Hubble stellar standards CALSPEC database (Bohlin et al. 2014, ,2020 April Update) and the Spectrophotometric Landolt Standard Stars of Stritzinger et al. (2005). Correlations with magnitude and colours are looked for.

We compare Gaia photometry to the Hipparcos, Tycho-2, 2MASS (Skrutskie et al. 2006), Landolt standards (Landolt 1983, 1992, 2007, 2009, 2013; Clem and Landolt 2013, 2016), the SDSS tertiary standard stars of (Betoule et al. 2013) and Pan-STARRS1 (Chambers et al. 2016) photometry, selecting low extinction stars only ($A_{0}<0.05$) using the 3D extinction map of Lallement et al. (2019). We check the distribution of a mixed colour index Gaia magnitude minus the external catalogue magnitude versus an external catalogue colour. An empirical robust spline regression is derived which models the global colour-colour relation. The residuals from this model are then analysed as a function of magnitude, colour and sky position.