The profiles of the Gaia photometric passbands are calibrated using comparisons between ground-based measurements of some 200 spectro-photometric standard stars (see for example "The Flux Calibration of Gaia" ), that have been obtained specifically for this purpose over the past 10 years. The calibration makes use of additional information on reflectivity of the telescope mirrors and the quantum efficiencies of the CCD detectors. The available data to calibrate the pass bands is not sufficient to be fully unambiguous, but the uncertainties left have very little effect on the reconstruction of the observed spectra.
The response of the instrument is evolving, and this was in particular the case during the first year of the mission. Changes in the system are due to mirror contamination and subsequent actions to de-contaminate, to small changes in the image shape due to telescope focus changes, and due to radiation damage to the CCDs. Thus, the data presented in the first Gaia data release are on a photometric system that is different from that used in the 2nd data release, and there will again be small differences for the 3rd data release.
More details on the Gaia passbands can be found at https://www.cosmos.esa.int/web/gaia/iow_20180316 .
Page last updated: 11 November 2019