2.1 Introduction

Author(s): David Hobbs

This chapter presents the models and processing steps used for pre-processing the raw Gaia data before the basic observables (or astrometric elementaries) can be generated and passed on to the Gaia core astrometric solution which is discussed in Chapter 3. The input to the Gaia data processing is the raw satellite telemetry; the initial Gaia source and ecliptic poles catalogues based on compilations of other catalogues to aid in the initial crossmatches; an attitude star catalogue to allow a first reconstruction of the attitude of Gaia. The astrometric calibration models are outlined in Section 2.3 including PSF/LSF models and discussions of CCD calibration challenges followed by the details of the daily and cyclic processing steps in Section 2.4. The final Section 2.5 discusses both the daily and cyclic monitoring and validation that is performed before data products are passed on to the core astrometric solution described in the next chapter.

2.1.1 Overview

Author(s): Claus Fabricius

The astrometric and photometric pre-processing produces the direct observational results for the SM and AF CCDs, thus feeding the astrometric core process (AGIS) and the $G$ band photometry (PhotPipe). More specifically, it converts the raw telemetry (see Section 2.2.2) to image parameters (see Section 2.4.8), i.e. transit time, flux, and position on the CCD, for each CCD transit of each detected object. This necessarily includes the calibration of the CCD bias (see Section 2.3.5), and cosmetics (see Section 2.3.4), as well as the background (see Section 2.3.3, Section 2.4.6), and the PSF/LSF (see Section 2.3.2). For an overview of the pre-processing, see Fabricius et al. (2016).

Another functionality of the photometric pre-processing is the reconstruction of the raw photometric measurements from the BP and RP CCDs to feed the $BP$ and $RP$ bands photometry (PhotPipe), see Section 2.4.3. In the process, it also produces preliminary spectro-photometric results (see Section 2.4.7), mainly used to monitor the correct operation of those instruments (see Section 2.5.2).

The pre-processing also takes care of the identification of objects (see Section 2.4.9) in a source list (see Section 2.2.3), and of determining a first on-ground attitude (see Section 2.4.5) of the spacecraft, using a star catalogue (see Section 2.2.5).

The pre-processing runs on a daily basis, to allow a close monitoring of the status and performance of the instruments (see Section 2.5.1), and to issue alerts on interesting sources. However, the CCDs and instrument cannot be calibrated optimally in almost real time, as the models will need refinement, and as they depend on information on the source astrometry and photometry, not yet available. Calibrations, image parameter determination, and object identification, are therefore elements of a grand iteration loop, completed once in every data reduction cycle (see Section 2.4.2). The feedback from the astrometric and photometric core processes is particularly important for the PSF calibration, which must properly take chromatic effects into account for the image shape as well as for the PSF origin.