This section includes some of the updated figures initially presented in the Red Book (ESA SCI(2000)4) about the photometric accuracy expected for both, Broad and Medium Band GAIA photometry. Corrections were needed because of the change in the design of GAIA. These are the accuracies expected at the end of mission (an effective observing time of 5 years), computed considering a mean of 82 transits across Astro-1 or Astro-2 and 200 transits across for the MBP (2F system i.e. Geneva-Barcelona proposal).

The G magnitude from an individual transit will provide excellent scientific information for variable stars. Differently than the other Broad or Medium Band Photometry (BBP or MBP), the G magnitude corresponding to one passage of the source through an astrometric focal plane will be derived as a weighting mean of the transits across each of the 11 CCDs of this focal plane. The accuracy givenin the G band is showed in Figure 1.

Figure 1: Photometric accuracy in the G magnitude. The accuracy has been computed for an unreddened star using 6 samples for the derivation of the star flux and 6 samples for the measurement of the background, with a read noise of 6 e per sample.

Broad and Medium band photometry

Figure 2 shows the photometric accuracy expected for the broad band photometry resulting from the Astro-1 and Astro-2 telescopes. Lindegren (2001) has derived the requirements on the BBP filter system to account for a good chromaticity calibration. Tentatively, he pointed out that four overlapping bell-shaped or triangular passbands could be the basis for further optimization, better than rectangular bands. Further studies will be necessary to analyze the scientific exploitation of these photometric systems and the use of the fifth passband. As an example, variability inside multiple systems will be one subject that need further studies in this context: the angular resolution is better in BBP than in MBP data and small-amplitude variability of one component of a binary system is not uncommon phenomena in our Galaxy. As was done in Hipparcos, "Variability Induced Movers" category of multiple star data, that is the photocentric motion caused by the variability of one of the components, will need a delicate treatment in the GAIA context.

Figure 2: Photometric accuracy for the 82 observations of a star in the Astro telescopes (left) and per transit (right) in each of the five broad photometric bands proposed by L. Lindegren (2B system, GAIA-LL-045, April 2003) The accuracy has been computed for an unreddened star assuming the use of 6 samples for the determination of the star flux and of 6 samples for the measurement of the background, with a read noise of 8 e per sample.

Figure 3 shows the accuracies expected for the 9 filters of the Medium Band Photometric system as in the Geneva-Barcelona proposal. Accuracies expected for other proposed photometric systems can be obtained from the GAIA Photometry simulator.

Figure 3: Photometric accuracy (in mag) for 200 observations of a star in the Spectro telescope (left) and per transit (right) in each of the medium photometric bands of the 2F system (Geneva-Barcelona proposal). The accuracy has been computed for an unreddened star using 3 samples for the determination of the star flux and 3 samples for the measurement of the background, with a read noise of 8 e per sample.

A table of the photometric accuracy can be found here, provided by Carme Jordi (January 8 2004)

GAIA photometry simulator

This tool provides the number of electrons readout at the CCDs for a star of given physical parameters and GAIA magnitude. Magnitudes and errors in all the presently proposed photometric band systems are provided.

• Photometry simulator (Photometric Working Group)

Laurent Eyer, January 12 2004