# 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 processing (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 $G_{\rm BP}$ and $G_{\rm RP}$ 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 and of determining a first on-ground attitude (see Section 2.4.5) of the spacecraft, using a star catalogue (see Section 2.2.3).

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 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.