3.3.1 Overview

Author(s): Claus Fabricius

Below, we present the more important models applied for the calibration of the SM and AF CCDs. Some are operated on longer timescales and others on shorter scales, but they are all operated on shorter timescales than anticipated before launch. For an overview on the detector performance during the first couple of years in space, see Crowley et al. (2016).

The CCD bias and bias non-uniformity (see Section 3.3.2) exposes the difficulties involved in reading CCDs in window mode, where most samples are merely flushed. As a consequence, the precise timing for reading a particular column within the short (less than 1 ms) time available for reading a line of pixels, varies from time to time that column is read, and so does the bias. The model must therefore take the exact readout details into account.

The CCD health, charge injection and release (see Section 3.3.3) deals with questions related to individual CCD columns, like dark signal, saturation level and abnormal response. The calibration of the injection lines and subsequent charge release is also discussed.

The astrophysical background model (see Section 3.3.4) includes in fact several elements. It must describe the complexities of the two sky areas that overlap in the focal plane, but the dominating background source is the stray light which varies strongly with the spin phase of the spacecraft with respect to the Sun, and produces a complex, intermittent pattern on the focal plane (see Gaia Collaboration et al. 2016b).

The PSF/LSF model (see Section 3.3.5) must encompass many complex effects. The optical PSF depends on colour, the field of view, and the position in the focal plane, but it also changes with time. On top come effects induced by the scanning law, by the way the CCDs are operated, and by complex inefficiencies of the charge transfer within the CCDs. A final complexity is that the chromatic image shifts are included in the PSF model as shifts of the PSF origin.

The image parameter determination (see Section 3.3.6) is the main calibration model and is dependent on the others. Its purpose is to derive the core image parameters from each window, namely the location and flux of the source.