Photometric and Imaging Performance

Three topics within GAIA photometry have been selected for discussion here. (1) The sampling scheme of the CCDs has been tuned to serve the various purposes: detection of stars, astrometry, broad- and medium-band photometry, and imaging of a small area around each detected star, but the tuning may not yet be optimal. (2) The calibration of CCD sensitivity is critical since we aim for millimagnitude accuracy, the spikes of the diffraction image are suited for an accurate calibration of the magnitude scale, and for extending the range of astrometry and photometry to very bright stars. (3) Finally the various photometric filter systems are illustrated.