The payload of the COROT satellite consists of a telescope, a wide-field two-part camera - one for each of the two mission objectives (exoplanet search and asteroseismology) operating in the visible, and the on-board computer processors, provided by ESA.
The telescope itself contains a primary mirror of 30 centimetres in diameter. It consists of two parabolic mirrors, and has a 1.1-metre focal length. The field of view is a square of 2.8 x 2.8 degrees, half for the seismology mission, the other half for the exoplanet mission. A prism, used by the telescope for certain aspects of the mission, will separate colours, enabling scientists to study stellar activity during a planetary transit.
During the mission, the telescope will collect the light from thousands of stars and feed it into a special two-part camera. One half of the camera is designed to look for planets; the other half is optimised to detect the subtle variation in a star's light, caused by sound waves rippling across the surface. These waves are the equivalent of seismic waves on the Earth.
Both mission goals require the camera to be sensitive to changes in a star's light of just one part in one hundred thousand. To help reach this sensitivity, a design team at ESA came up with a highly successful baffle, external to the telescope, that prevents stray light from entering the telescope. In practice, the baffle is a device for shielding the camera from 'light pollution' coming from sources other than those objects at which the telescope is looking.
The analysis of the 3-colour light curves that COROT will gather thanks to a prism mounted in front of the exoplanet channel camera detectors (CCD's), will make it possible to distinguish between the different families of detected events (transits, stellar activity, eclipsing binaries). Planetary transits are achromatic events, while stellar activity – even though still poorly understood – is expected to be highly chromatic due to temperature variations. More than 120 000 stars, with magnitudes between 12 and 15.5, will be surveyed.