1.1.3 The spacecraft

The spacecraft consists of a payload module (PLM; Section 1.1.3) with the instrument and a service module (SVM; Section 1.1.3) with support functions. The prime contractor of Gaia is Airbus Defence and Space (DS), Toulouse, France (formerly known as Astrium). An overview of the spacecraft is provided in Gaia Collaboration et al. (2016).

The payload module houses the two telescopes and the focal-plane assembly (FPA; Section 1.1.3). In addition, the payload houses the focal-plane computers (the video-processing units, VPUs, running the video-processing algorithms, VPAs; Section 1.1.3), the detector electronics (PEMs, for proximity electronics modules; Hambly et al. 2018), an atomic master clock (CDU, for clock-distribution unit; Section 1.1.3), metrology systems (basic-angle monitor, BAM, and wave-front sensors, WFSs; Section 1.1.3 and Section 1.1.3), and the payload data-handling unit (PDHU; Section 1.1.3). An overview of the payload module can be found in de Bruijne et al. (2010a) and Gaia Collaboration et al. (2016).

Gaia houses two telescopes, sharing a common focal plane. The lines of sight of the telescopes are separated by the basic angle ($\mathrm{\Gamma }=106{.}^{\circ }5$). Both telescopes are three-mirror anastigmats with a Korsch off-axis configuration. The entrance pupils are located at the rectangular primary mirrors, and have a dimension of $1.45\times 0.50$ m${}^2$. A beam combiner at the exit pupil merges the optical paths. Two further flat mirrors in the combined beam fold the light towards the focal plane. The total number of mirrors is hence 10 for both telescopes combined. The image quality of the telescope can be adjusted in flight through (five-degrees-of-freedom) mechanisms attached to the secondary mirrors. An overview of the payload and optical layout is displayed in Figure 1.1. More details on the telescopes are given in Gaia Collaboration et al. (2016).

The focal length of both telescopes is 35 m, providing a plate scale of $58.9\times 176.8$ mas pixel${}^{-1}$ in the along- (AL) and across-scan (AC) directions, respectively.

The focal-plane assembly (FPA) is shared by both telescopes. It houses 106 charge-coupled-device (CCD) detectors. Since Gaia is continuously scanning around its axis, the CCDs are operated in time-delayed integration (TDI) mode. Gaia’s measurements are very precise in the along-scan direction through precise timing of the signals (line-spread functions, LSFs; see Bastian and Biermann 2005 and Section 1.1.3). In the across-scan direction, the CCD images of most faint stars are binned, substantially reducing the CCD readout noise and science-data volume, with minimum impact on the astrometry. Details on Gaia’s operating principle are provided in Lindegren and Bastian (2010).

The focal plane houses five functionalities:

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  metrology (Section 1.1.3 and Section 1.1.3): a basic-angle monitor (BAM) composed of two detectors (one nominal and one cold redundant) continuously measures variations in the basic angle between the two telescopes. Two wave-front sensors (WFSs), with two associated detectors, allow monitoring the optical quality of the telescopes and support refocusing;

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  sky mapping (Section 1.1.3): both telescopes have their own sky-mapper (SM) detectors (14 devices in total). These detectors allow detecting objects of interest (point sources as well as slightly extended sources such as asteroids and unresolved galaxies) and rejecting cosmic rays, Solar protons, etc.;

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  astrometry (Section 1.1.3): 62 CCDs are devoted to astrometry in the so-called astrometric field (AF);

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  spectro-photometry (Section 1.1.3): 14 CCDs are devoted to low-resolution spectro-photometry. Spectra, dispersed along the scan direction, are generated through the use of a blue and a red prism (blue and red photometers, BP and RP);

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  spectroscopy (Section 1.1.3): 12 CCDs are devoted to spectroscopy. Medium-resolution spectra ($R\approx \mathrm{11\hspace{0.17em}700}$, dispersed along the scan direction) are generated through an integral-field grating plate in the radial velocity spectrometer (RVS). The RVS instrument is described in detail in Cropper et al. (2018).

More details on the focal plane can be found in Gaia Collaboration et al. (2016). The focal-plane layout is shown in Figure 1.2.

The 106 CCD detectors come in three different types: the broad-band CCD, the blue-enhanced CCD, and the red-enhanced CCD. Each of these types has the same architecture (e.g., number of pixels, TDI gates, read-out register, etc.) but differ in their anti-reflection coating and surface passivation, their thickness, and the resistivity of their silicon wafer. The broad-band and blue CCDs are both 16 $\mu$m thick and are both manufactured from standard-resistivity silicon; they differ only in their anti-reflection coating, which is optimised for short wavelengths for the blue CCD and optimised to cover a broad bandpass for the broad-band CCD. The red CCD, in contrast, is based on high-resistivity silicon, is 40 $\mu$m thick, and has an anti-reflection coating optimised for long wavelengths. The broad-band CCD is used in the star mapper (SM), in the astrometric field (AF), and for the wavefront sensor (WFS). The blue CCD is used in the blue photometer (BP). The red CCD is used in the basic-angle monitor (BAM), in the red photometer (RP), and in the radial velocity spectrometer (RVS).

The CCD detectors in the focal plane are commanded by video-processing units (VPUs). Gaia has seven identical VPUs, each one dealing with a dedicated row of CCDs (see Figure 1.2). Each CCD row contains, in order: two SM CCDs (one for each telescope), 9 AF CCDs (except on row 4), 1 BP CCD, 1 RP CCD, and 3 RVS CCDs (the latter only for four of the seven CCD rows). The VPUs run seven identical instances of the video-processing algorithms (VPAs) albeit not necessarily with exactly the same parameter settings. This mix of some hardware and mostly software is responsible for object detection (after local background subtraction), object windowing (see below; see also Section 1.1.3), window-conflict resolution, data binning and prioritisation, science-packet generation, data compression, etc. More details on these functions can be found in Gaia Collaboration et al. (2016).

Science / star packets generated on board (Section 1.1.3) are stored in the payload data-handling unit (PDHU) before being down-linked to ground. In order to support user-defined prioritisation of the science telemetry, the PDHU can handle up to 256 different files to store data, each labelled with a unique File_ID. In general, bright stars have a higher down-link priority than faint stars. Under average sky conditions, the PDHU can contain several days worth of science data. When the scanning law makes Gaia scan (roughly) along the Galactic plane for several consecutive days (known as a Galactic-plane scan, GPS; Section 1.1.5), however, the PDHU saturates and a user-defined prioritisation scheme comes in play to govern on-board data deletion (see also Section 1.3.3). This happens a few times per year (Figure 1.4). In general, bright stars have a higher deletion protection than faint stars. More details on the PDHU are provided in Gaia Collaboration et al. (2016).

The clocking of all CCD detectors as well as the time tagging of the science data is based on a high-accuracy atomic rubidium clock, embedded in the clock distribution unit (CDU). The CDU contains a rubidium atomic frequency standard (RAFS). This clock is stable to within a few ns over a six-hour spacecraft revolution and has very small temperature, magnetic field, and input voltage sensitivities. The free running onboard time (OBT) counter, which is the time tag of all science data, is generated from the master clock. The spacecraft-elapsed time (SCET) counter in the central data management unit (CDMU), which is used for time tagging housekeeping data and for spacecraft operations, is continuously synchronised to OBT using a pulse-per-second (PPS) mechanism. More details on the CDU are contained in Gaia Collaboration et al. (2016).

Two CCDs in the focal plane are equipped with wave-front sensors (WFSs) to enable monitoring the optical quality of the telescopes. The WFSs also allow deriving information to actuate the M2 mirror mechanisms to (re-)align and (re-)focus the telescopes in orbit. The WFSs are of Shack-Hartmann type and sample the output pupil of each telescope with an array of microlenses. The latter focus the light of bright stars transiting the focal plane on the WFS CCDs. Comparison of the stellar spot pattern with the pattern of a built-in calibration source (used during initial tests after launch) and with the pattern of stars acquired after achieving best focus (used afterwards) allows reconstruction of the wave front. See Gaia Collaboration et al. (2016) for details.

Two CCDs in the focal plane are part of the basic-angle monitor (BAM). The BAM allows monitoring variations of the basic angle of the telescopes to $\mu$as-levels every 15 minutes or shorter. More details are provided in Gaia Collaboration et al. (2016) and in Section 2.4.4.

The astrometric field (AF) contains 62 CCDs in 9 strips and 7 rows, minus one CCD for the WFS (Section 1.1.3; Figure 1.2). The bandpass of the AF detectors, defining the Gaia $G$ band, covers 330–1050 nm; this is mainly set by the telescope transmission in the blue (resulting from the protected-silver coating on the mirrors) and the CCD response in the red. The AF CCDs are not read out in full but only small ’windows’ around objects of interest are read out (Table 1.1; see also Section 2.2.2). The typical window for faint stars is $12\times 12$ pixels (along-scan $\times$ across-scan), with the 12 pixels in the across-scan direction typically binned on chip into one single number (the intensity of the along-scan line-spread function, LSF). For bright stars, single-pixel-resolution windows are used, of size $18\times 12$ pixels (along-scan $\times$ across-scan; Table 1.1). For more details on the astrometric instrument, see Gaia Collaboration et al. (2016).

The photometric instrument contains 2 CCD strips and 7 CCD rows. Half of the CCDs are devoted to the blue photometer (BP, covering 330–680 nm) and the other half are devoted to the red photometer (RP, covering 630–1050 nm). Dispersion of light takes place in the along-scan direction and windows measure $60\times 12$ pixels (along-scan $\times$ across-scan; Table 1.1; see also Section 2.2.2). For more details on the photometric instrument, see Gaia Collaboration et al. (2016) and Riello et al. (2018).

The spectroscopic instrument contains 12 CCDs in 3 strips and 4 rows. Dispersion of light takes place in the along-scan direction and windows measure $1296\times 10$ pixels (along-scan $\times$ across-scan; Table 1.1; see also Section 2.2.2). For more details on the spectroscopic instrument, see Gaia Collaboration et al. (2016) and Cropper et al. (2018).

The service module (SVM) supports the payload, both mechanically / structurally and electronically / functionally. A description of the service module can be found in Gaia Collaboration et al. (2016). An exploded view is shown in Figure 1.5.