The Gaia mission
Introduction and overview
Author(s): Jos de Bruijne
This is the online documentation of Gaia Data Release 1 (Gaia DR1; click here for the full pdf version). This
documentation has been prepared as background information to the papers in the special Gaia A&A volume that
accompanies Gaia DR1. Gaia Collaboration et al. (2016b) describes the Gaia mission. Gaia Collaboration et al.
(2016a) provides an overview of the contents of Gaia DR1. Arenou et al. (2017) describes the overall validation
of the data. The data itself is available from the Gaia Archive at http://archives.esac.esa.int/gaia. The
Gaia mission home page is http://www.cosmos.esa.int/gaia/.
Mission history and science case
Author(s): Jos de Bruijne
Whereas astrometry originated several millennia ago (for an overview, see Perryman 2012), the last centuries – and
the last decades in particular – have shown exponential progress in the number of objects and the accuracy with
which their positions, proper motions, and parallaxes are determined. These improvements have arisen as a result of
improved technologies and instrumentation and of the possibility to eliminate the e
ﬀects of the Earth’s atmosphere
by going to space. Astrometry from space was pioneered by ESA’s Hipparcos mission, which operated from 1989
till 1993. The Hipparcos Catalogue was published in 1997 (ESA 1997; van Leeuwen 2007a). An overview of the
science revolution that Hipparcos has brought about is presented by Perryman (2009).
Hipparcos’ successor, originally named GAIA, was proposed in the early 1990’s by Perryman and Lindegren as an
interferometric concept (Lindegren & Perryman 1995). Later, the mission design was changed to a direct-imaging
approach but the name was kept for continuity reasons yet spelt as of then with small letters, i.e., Gaia. For more
details about the history of Gaia, see Høg (2011, 2014).
Objectives of Gaia
The main science goal of Gaia is to unravel the structure, dynamics, and chemo-dynamical evolution of the Milky
Way through the observation of one billion constituent stars. The data comprises astrometry and low-resolution
spectro-photometry. For the brightest subset of targets, spectra are acquired to obtain radial velocities. The full
list of Gaia’s science objectives is deﬁned in Perryman et al. (2001) and summarised in Gaia Collaboration et al.
Author(s): Jos de Bruijne, Juanma Fleitas, Alcione Mora
The spacecraft consists of a payload module with the instrument and a service module with support functions.
The prime contractor of Gaia is Airbus Defence and Space, Toulouse, France (formerly known as Astrium). An
overview of the spacecraft is provided in Gaia Collaboration et al. (2016b).
The payload module houses the two telescopes and the focal plane. In addition, the payload houses the focal-plane
computers (the video-processing units, running the video-processing algorithms), the detector electronics (PEM),
an atomic master clock, metrology systems (basic-angle monitor and wave-front sensors), and the payload data-
handling unit. An overview of the payload module can be found in de Bruijne et al. (2010a); Gaia Collaboration
et al. (2016b).
Gaia houses two telescopes, sharing a common focal plane. The lines of sight of the telescopes are separated
by the basic angle. Both telescopes are three-mirror anastigmats with a Korsch o
ﬀ-axis conﬁguration. The input
pupils are located at the rectangular primary mirrors, and have a dimension of 1.45×0.5 m. 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. An overview of the optical layout is displayed in Figure 1.1.
More details on the telescopes are given in Gaia Collaboration et al. (2016b).
The focal length of both telescopes is 35 m, providing a plate scale of 58.9 × 176.8 mas pixel
in the along- and
across-scan directions, respectively. The rectangular aperture allows one-dimensional binning of CCD images in
the across-scan direction for faint stars, substantially reducing the CCD readout noise and down-link bandwith,
with minimum impact on the astrometry.
Figure 1.1: Gaia payload overview (right) and unfolded telescope optical design (left). Courtesy Airbus DS.
The focal plane
The focal plane 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 scan direction through precise timing of the signals (line-spread functions,
LSFs). The focal plane houses ﬁve functionalities:
• metrology: a basic-angle monitor composed of two detectors (one nominal and one redundant) con-
tinuously measures variations in the basic angle between the two telescopes. Two wave-front sensors
allow monitoring the optical quality of the telescopes;
• sky mapping: 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.;
• astrometry: 62 CCDs are devoted to astrometry in the so-called astrometric ﬁeld (AF);
• spectro-photometry: 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);
• spectroscopy: 12 CCDs are devoted to spectroscopy. Medium-resolution spectra (R ≈ 11 700, dis-
persed along the scan direction) are generated through an integral-ﬁeld grating plate.
More details on the focal plane can be found in Gaia Collaboration et al. (2016b).
Video Processing Unit and Algorithms
The CCDs 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. Each CCD row, contains in order, two SM CCDs (one for each
telescope), 9 AF CCDs, 1 BP CCD, 1 RP CCD, and 3 RVS CCDs (the latter only for four of the seven CCD