i-arcsecond per second and milli-arcsecond level, respectively. These requirements prohibit the use of moving parts, such as conventional reaction wheels, on the spacecraft, apart from moving parts within thrusters. The attitude- and orbit-control
Article number, page 7 of 36 information provided by the photometry collected for each object (Sect.

[Figure: Fig. 3. Schematic payload overview without protective tent. Most electronic boxes, e.g. clock distribution unit, video processing units, or payload data-handling unit, are physically located in the service module and hence not visible here. Credit: ESA.]

3.3.6). The telescopes are mounted on a quasi-octagonal optical bench of ∼3 m in diameter. The optical bench (composed of 17 segments, brazed together) and all telescope mirrors are made of sintered silicon carbide. This material combines high specific strength and thermal conductivity, providing optimum passive thermo-elastic stability (but see Sect. 4.2). The (required) optical quality of Gaia is high, with a total wave-front error budget of 50 nm. To reach this number in orbit, after having experienced launch vibrations and gravity release, alignment and focussing mechanisms have been incorporated at the secondary (M2) mirrors. These devices, called M2 mirror mechanisms (M2MMs), contain a set of actuators that are capable of orienting the M2 mirrors with five degrees of freedom, which is sufficient for a rotationally symmetric surface. The inorbit telescope focussing is detailed in Mora et al. (2014b, see also Sect. 6.4) and has been inferred from a combination of the science data themselves (size and shape of the point spread function) combined with data from the two wave-front sensors (WFSs; Sect. 3.3.3).
reinforced polymer in-orbit bipods; the latter have low conductance and thermally decouple the payload from the service module. The payload is covered by a thermal tent based on a carbon-fibre-reinforced-polymer and aluminium sandwich structure with openings for the two telescope apertures and for the focal plane, warm-electronics radiator. The tent provides thermal insulation from the external environment and protects the focal plane and mirrors from micro-meteoroid impacts. The payload module furthermore contains the spacecraft master clock (Sect. 3.3.10) and all necessary electronics for managing the instrument operation and processing and storing the science data (Sects. 3.3.8 and 3.3.9); these units, however, are physically located in the service module.
3.3.1. Telescope Gaia is equipped with two identical, three-mirror anastigmatic (TMA) telescopes, with apertures of 1.45 m × 0.50 m pointing in directions separated by the basic angle (Γ = 106◦.5). These telescopes and their associated viewing directions (lines of sight) are often referred to as 1 and 2 or preceding and following, respectively, where the latter description refers to objects that are scanned first by the preceding and then by the following telescope. In order to allow both telescopes to illuminate a shared focal plane, the beams are merged into a common path at the exit pupil and then folded twice to accommodate the 35 m focal length. The total optical path hence encounters six reflectors: the first three (M1–M3 and M1’–M3’) form the TMAs, the fourth is a flat beam combiner (M4 and M4’), and the final two are flat folding mirrors for the common path (M5–M6). All mirrors have a protected silver coating ensuring high reflectivity and a broad bandpass, starting around 330 nm. Asymmetric optical aberrations in the optics cause tiny yet significant chromatic shifts of the diffraction images and thus of the measured star positions. These systematic displacements are calibrated out as part of the on-ground data processing (Lindegren et al. 2016) using colour
3.3.2. Focal plane assembly The focal plane assembly of Gaia (for a detailed description, see Kohley et al. 2012; Crowley et al. 2016b) is common to both telescopes and has five main functions: (i) metrology (wave-front sensing [WFS] and basic angle monitoring [BAM]; Sects. 3.3.3 and 3.3.4), (ii) object detection in the sky mapper (SM; Sect. 3.3.5), (iii) astrometry in the astrometric field (AF; Sect. 3.3.5), (iv) low-resolution spectro-photometry using the blue and red photometers (BP and RP; Sect. 3.3.6), and (v) spectroscopy using the radial-velocity spectrometer (RVS; Sect. 3.3.7). The focal plane is depicted in Figure 4 and carries 106 charge-coupled device (CCD) detectors, arranged in a mosaic of 7 across-scan rows and 17 along-scan strips, with a
Article number, page 8 of 36 total of 938 million pixels.