The M2 mirror-mechanism actuations are derived using a telescope-alignment tool based on modelled sensitivities for each degree of freedom. The number of actuators to use and the weight given to each Legendre coefficient are adjustable. The corrections applied so far after each decontamination campaign (Sect. 6.4) have consisted of pure focus displacements.
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3.3.4.

Basic angle monitor As explained in Sect. 3.1, the measurement principle of Gaia relies on transforming transit-time differences between stars observed in both telescopes into angular measurements.

This requires the basic angle Γ between the two fields of view either to be stable or to be monitored continuously at µas level and observed variations corrected as part of the data processing.

Whereas low-frequency variations that are longer than, for instance two spin periods, i.e. 12 h (Sect. 5.2), are absorbed in the geometric instrument calibration (Lindegren et al. 2016), shortterm variations, on timescales of minutes to hours, are non-trivial to calibrate and can introduce systematic errors in the astrometric results.

In particular, a Sun-synchronous, periodic basic angle variation is known to be (nearly) fully degenerate with the parallax zero point (e.g.

Lindegren et al. 1992).

For this reason, the payload of Gaia was designed to be stable on these timescales to within a few µas (but see Sect. 4.2) and arguably carries the most precise interferometric metrology system ever flown, the basic angle monitor (BAM; e.g.

Meijer et al. 2009; Gielesen et al. 2013; Mora et al. 2014b).

The BAM is composed of two optical benches fed by a common laser source that introduces two parallel, collimated beams per telescope.

The BAM creates one Young-type fringe pattern per telescope in the same detector in the focal plane.

The relative along-scan displacement between the two fringe patterns allows monitoring of the changes in the line of sight of each telescope and, thus, the basic angle.

The (short-term) precision achieved in the differential measurement is 0.5 µas each 10–15 min, which corresponds to picometer displacements of the primary mirrors.

A spare laser unit is kept in cold redundancy in case the primary source were to fail.

The BAM exposures are continuously acquired with a period of 23 s (18.7 s stare-mode integration plus 4.4 s TDI-mode readout).

A forward-modelling approach, which is based on a mathematical model representing the BAM image that is fitted using a leastsquares algorithm, is applied in the daily preprocessing pipeline (Fabricius et al. 2016) to monitor basic angle variations; basic angle variations are also monitored independently on a daily basis using cross-correlation techniques.
3.3.5.

Astrometric instrument The astrometric instrument comprises the two telescopes (Sect. 3.3.1), a dedicated area of 7 + 7 CCDs in the focal plane devoted to the sky mappers of the preceding and following telescope, and a dedicated area of 62 CCDs in the focal plane where the two fields of view are combined onto the astrometric field (AF).

The wavelength coverage of the astrometric instrument, defining the unfiltered, white-light photometric G band (for Gaia), is 330–1050 nm (Carrasco et al. 2016; van Leeuwen et al. 2016).

These photometric data have a high signal-to-noise ratio and are particularly suitable for variability studies (Eyer et al. 2016).

Unlike its predecessor mission Hipparcos, which selected its targets for observation based on a predefined input catalogue loaded on board (Turon et al. 1993), Gaia performs an unbiased, flux-limited survey of the sky.

This difference is primarily motivated by the fact that an all-sky input catalogue at the spatial resolution of Gaia that is complete down to 20th mag, does not exist.

Hence, autonomous, onboard object detection has been implemented through the Sky Mapper (Sect. 3.3.8), with the advantage that transient sources such as supernovae and near-Earth asteroids are observed too.

Every object crossing the focal plane is first detected either by SM strip 1 (SM1) or SM strip 2 (SM2).

These CCDs exclusively record, respectively, the objects from the preceding or from the following telescope.

This is achieved through a physical mask that is placed in each telescope intermediate image, at the M4/M4’ beam-combiner level (Sect. 3.3.1).

The SM CCDs are read out in full-frame TDI mode, which means without windowing.