The photometric data collected by Gaia is of exceptionally high quality in both accuracy and homogeneity, and as such provides crucial information on the character of the sources that are being observed, such as the surface temperature and chemical composition, as well as any amount of reddening through absorption of light by interstellar dust.
The photometry comes in three different forms of flux measurements. Most accurate are the measurements obtained from the 62 CCDs that form the astrometric field in the focal plane. Here a broad pass band, referred to as G, is used to collect as much light as possible to benefit the astrometric measurements.
Two strips of CCDs in the Gaia focal plane receive light through a prism and colour filter, creating low-resolution dispersion spectra in broad blue and red pass bands. Those data are analysed in two ways, as the observed spectra and by integrating the flux for each pass band, referred to as BP and RP, often used as a so-called colour index, BP-RP in magnitude scale. This colour index can be used to derive an estimate of the surface temperature for a star. Together with the G magnitudes and distance moduli derived from the parallax measurements, these data are used to present the Hertzsprung-Russell diagram (see https://www.cosmos.esa.int/web/gaia/gaiadr2_hrd for Hertzsprung-Russell diagrams created with Gaia Data Release 2 data). The processing of the dispersion spectra data started in cycle 2, and the first release of the calibrated dispersion spectra is scheduled to be in the 3rd Gaia data release.
An important issue in the reductions of the photometric data is the way the data are created. Data come from two telescopes, 62 CCDs, and on each CCD there are 3 different window settings and 8 different gate settings. This created 1240 different photometric systems that have to be amalgamated into a single homogeneous system. Ultimately, those links should be better than the final accuracy of the photometric data, which will be at mmag level or better.
The photometric system is defined by a selection of stars and their observations over a fixed time interval. 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.
The photometric data are the input for the variability analysis, results of which have been made available in the 1st and 2nd data releases for selected variability types. In the 2nd data release also the BP and RP information has been included, resulting in colour-variation light curves in addition to the light curves for the main, G, broad band. An interesting display of what has been achieved so far can be found at: https://www.cosmos.esa.int/web/gaia/gaiadr2_cu7 .
Page last updated: 09 August 2018