indicates a better image quality and centroiding performance. Red lines refer to the preceding field of view; blue lines to the following field of view. The discontinuities refer to the decontaminations (dotted vertical lines) and refocussings (dashed blue and red lines; Sect. 6.4). OBMT stands for onboard mission timeline in units of six-hour revolutions since launch (Sect. 5.3.3). Gaia Data Release 1 is based on data covering the interval OBMT = 1078.3795– 2751.3518, delimited by dashed black lines.]

6.2. Daily processing As soon as science telemetry arrives from MOC at SOC, its processing in the daily pipeline is initiated (Siddiqui et al. 2014). After the MOC interface task (MIT), which provides a first-level data reconstruction into source-packet groups, and the decompression and calibration services (DCS), which decompresses, identifies, and stores the various data packets in a relational database, the initial data treatment (IDT; Fabricius et al. 2016) is run. The IDT processing includes reconstructing all details for each observation window, for example location, shape, and TDI gating, and calculating image parameters (flux and alongscan location, i.e. observation time) and preliminary colours. Also, IDT constructs a coarse version of the spacecraft attitude. This on-ground attitude version 1 (OGA-1) has sufficient accuracy that the observed sources are either identified in the initial Gaia source list (Smart & Nicastro 2014, IGSL) or added as new sources during the cross-matching. The IDT data, which has positional accuracies below 0′′.1, forms the basis of both the asteroid and photometric science alert pipelines (Tanga et al. 2016; Wyrzykowski 2016). The IDT data is disseminated to the data processing centres in DPAC (Sect. 7) for further downstream processing through the main database (MDB; Siddiqui et al. 2014).
In addition to these nominal calibrations, two types of special operations have been performed occasionally on Gaia, namely refocussing and decontamination of the optics. During commissioning, three decontamination campaigns were conducted to sublimate contaminating water ice from the optics (Sect. 4.2). During nominal operations, throughput evolution continued (Fig. 8) and two more decontaminations were performed: the first on 23 September 2014 (OBMT ∼1317) and the second on 3 June 2015 (OBMT ∼2330). At the time of writing (mid-2016), the total build-up of throughput loss has been modest (∼0.1 mag in AF) and the current rate of transmission loss is ∼0.1 mmag per six-hour revolution. During the commissioning phase, both telescopes were aligned and focussed, first in April 2014 and then again, after reaching a more stable thermal equilibrium, in July 2014 (Mora et al. 2014b). Continuous image-quality monitoring was performed via the evolution of the full width at half maximum of the point spread function and was quantified through the Cramér-Rao centroid diagnostic (e.g. Mendez et al. 2013) applied to bright-star data; this monitoring has revealed slow image-quality degradations of both telescopes, which is strongly correlated with the build-up of contamination (Fig. 9). This can be understood because uneven, patchy mirror apodisation introduced by the contamination broadens the point spread function. Although the image quality improved with both decontaminations carried out, there has been a need for two active refocussings to improve the image quality further, once on 24 October 2014 (OBMT ∼ 1444) for the following field of view and once on 3 August 2015 (OBMT ∼2575) for the preceding field of view. At the time of writing (mid 2016), both telescopes are optimally focussed within a few percent with slow degrading trends, most likely caused by the slow build-up of contamination.
6.3. Payload health monitoring Gaia has a payload health and scientific data-quality monitoring system, called First Look (FL; Fabricius et al. 2016), that is sophisticated and close to real time. This system is deployed in the daily pipeline. First Look operates on IDT and spacecraft housekeeping data and produces thousands of summary diagnostic quantities as well as daily instrument calibrations and the second on-ground attitude reconstruction (OGA-2). These diagnostics are reported daily and inspected by qualified First-Look scientists to identify possible actions to improve the performance of the spacecraft and/or of the on-ground data processing. A group of DPAC-wide payload experts provide expertise in interpreting First-Look diagnostics, monitor the health of the satellite based Article number, page 20 of 36