The largest digital camera ever built for a space mission has been painstakingly pieced together from 106 separate electronic detectors. The resulting "billion-pixel array" will serve as the supersensitive eye of the European Space Agency’s galaxy-mapping Gaia mission. While the naked human eye can see several thousand stars on a clear night, Gaia will map a billion stars within the Milky Way and its neighboring galaxies over the course of its five-year mission starting in 2013. Gaia will chart the stars’ brightness and spectral characteristics along with their three-dimensional positions and motions. Technicians from Astrium France are seen bolting and aligning onto the support structure some of the 106 CCDs that comprise Gaia’s mirror. The structure (the gray plate underneath the CCDs) weighs about 20 kg and is made of silicon carbide. The gap between adjacent CCD packages, precisely fitted together, is about 1 mm. (Image: Astrium France) To detect distant stars up to a million times fainter than the eye can see, Gaia will carry 106 CCDs developed for the mission by e2v Technologies of Chelmsford, UK. The detectors each measure 4.7 × 6 cm and are a few tens of microns thick. The 0.5 × 1.0-m mosaic has been assembled at the Toulouse facility of Gaia’s prime contractor, Astrium France. Technicians spent much of May carefully fitting together each CCD package on the support structure, leaving only a 1-mm gap between them. Working in double shifts in strict cleanroom conditions, they added an average of four CCDs per day, finally completing their task on June 1. "The mounting and precise alignment of the 106 CCDs is a key step in the assembly of the flight model focal plane assembly," said Philippe Garé, Gaia’s payload manager at the ESA. The completed mosaic is arranged in seven rows of CCDs. The main array comprises 102 detectors dedicated to star detection. Four others check the image quality of each telescope and the stability of the 106.5º angle between the two telescopes that Gaia uses to obtain stereoscopic views of stars. To increase the sensitivity of its detectors, the spacecraft will maintain their temperature of –110º C. Gaia will operate at the L2 Lagrange point, which is 1.5 million kilometers behind the Earth from the perspective of the sun; at that location, Earth's orbital motion balances out gravitational forces to form a stable point in space. As the spinning Gaia's two telescopes sweep across the sky, the images of stars in each field of view will move across the focal plane array, divided into four fields variously dedicated to star mapping, position and motion, color and intensity, and spectrometry. For more information, visit: www.esa.int