Epoch and mean spectra for an extragalactic source from the Gaia Celestial Reference Frame (Gaia-CRF, Gaia Collaboration 2018b). The presence of an outlier observation with a very low flux level does not disturb the derivation of its mean spectra (see text). The different epochs in Gaia are measured in number of six-hour revolutions done by the satellite while surveying the sky. An approximate correspondence between pseudo-wavelength values and absolute wavelengths can be found in Fig. 4.