Focal Plane - Gaia
|The Gaia focal-plane assembly is the largest ever developed for a space application, with 106 CCDs, a total of almost 1,000 million pixels, and a physical dimension of 1.0 m × 0.4 m.
Gaia focal plane. Image credit: ESA. Acknowledgement: Alex Short.
|The focal-plane assembly is common to both telescopes and serves five main functions:
All CCDs, except those in the sky mapper, are operated in windowing mode: only those parts of the CCD data stream which contain objects of interest are read out; remaining pixel data is flushed at high speed. The use of windowing mode reduces the readout noise to a handful of electrons while still allowing reading up to 20 objects simultaneously.
Every object crossing the focal plane is first detected either by SM1 or SM2. These CCDs record, respectively, the objects only from telescope 1 or from telescope 2. This is achieved by a physical mask that is placed in each telescope intermediate image, at M4/M41 beam-combiner level. Next, the window is allocated to the object, which is propagated through the following CCDs of the CCD row as the imaged object crosses the focal plane; the actual propagation uses input from the spacecraft's attitude control system, which provides the predicted position of each object in the focal plane versus time. After detection in SM, each object is confirmed by the CCD detectors in the first strip of the astrometric field (AF1); this step eliminates false detections such as cosmic rays. The object then progressively crosses the eight next CCD strips in AF, followed by the BP, RP, and RVS detectors (the latter are present only for four of the seven CCD rows).Gaia CCDs are fabricated in three variants, AF-, BP-, and RP-type, to optimise quantum efficiency corresponding to the different wavelength ranges of the scientific functions. The AF-type variant is built on standard silicon with broadband anti-reflection coating. It is the most abundant type in the focal plane, used for all but the photometric and spectroscopic functions. The BP-type only differs from the AF-type through the blue-enhanced backside treatment and anti-reflection coating, and it is exclusively used in BP. The RP-type is built on high-resistivity silicon with red-optimised anti-reflection coating to improve near-infrared response. It is used in RP as well as in RVS.