Gaia in the UK

Taking the Galactic Census

Focal plane and observation sequence

 Gaia's focal plane

Gaia's focal plane. The image of an object moves across the focal plane (on this diagram, from left to right) as Gaia scans the sky. View larger image (325 KB). (Image courtesy of ESA - A. D. Short.)

Gaia's focal plane, which is just over 104 cm long and just over 42 cm wide, is composed of an array of 106 charge-coupled devices (CCDs) and contains nearly 1 billion pixels. The CCDs, made in the UK by e2v, are arranged into 7 rows and 17 strips. The focal plane consists of five distinct areas:

  1. The Wave Front Sensor (2 CCDs) and the Basic Angle Monitor (2 CCDs).
  2. The Sky Mappers (2 strips, 14 CCDs).
  3. The Astrometric Field (9 strips, 62 CCDs).
  4. The Blue and Red Photometer (2 strips, 14 CCDs).
  5. The Radial Velocity Spectrometer (3 strips of 4 CCDs, 12 CCDs).

As an object enters a field of view of one of the telescopes, its image starts to move across the focal plane:

  • The image enters the Sky Mapper, where it is detected (one strip of CCDs is used for each telescope). A surrounding window, which is used in the measurements, is assigned to the object.
  • The first strip of the Astrometric Field confirms the object (this is done to exclude events, such as cosmic rays hits). The image crosses the remaining strips of the Astrometric Field, which measure star positions. (Read how the Astrometric Field CCDs are used to measure astrometric parameters and the procedure used for detecting faint companions of measured objects, Source Environment Analysis.)
  • Next, the image moves across a prism, which produces a low dispersion spectrum of the light emitted by the object. This spectrum, after passing through a blue filter, is measured by the Blue Photometer CCDs.
  • In the next step, another prism is used to produce the second low dispersion spectrum. This time, the spectrum passes through a red filter before it is measured by the Red Photometer CCDs.
  • Finally, the image enters the Radial Velocity Spectrometer, where, for the stars brighter than 17th magnitude, a diffraction grating is used to generate a narrow section of a high resolution spectrum. This spectrum is measured by the Radial Velocity Spectrometer CCDs.

Download The Gaia Focal Plane Information Sheet (pdf format).

Watch "Inside Gaia's billion-pixel camera" animation for more information about how Gaia carries out measurements.

Page last updated: 16 January 2014