The payload main sub-systems are the following:
The telescope developed under LAM responsibility. It is made up of:
- The afocal part, composed of two off axis parabolic mirrors (pupil of 27 cm). The afocal collector offers exceptional stability by using technologies developed for the most advanced observation programmes.
- The external baffle (cylindrical), designed to have a rejection coefficient of 10-¹³. Thus, the maximum stray light flux coming from the Earth is of a few photons/pixel/second, if the Earth limb is at 20° from the telescope axis.
- The one-shot cover, designed to protect the telescope during launch and early attitude acquisition phase. It is released in the commissioning phase, just after dark calibrations. Afterward, it remains open.
The camera, developed under LESIA responsibility. It is made up of:
- The focal unit, hosting the 4 frame transfer CCD matrices of 2048 x 4096 pixels. The technology used (EEV, 13.5-µm thinned, back illuminated) works in the visible in the MPP mode. This mode, associated with a temperature regulated at -40° C, reduces the dark currents to a very low level. The detectors have a high quantum efficiency in the spectral range [370 nm , 950 nm].
- The dioptric objective, focusing the light (f=1200 mm) and correcting the geometric aberrations of the telescope. Its 6 lens-cylinders are made of titanium.
- Two proximity electronics units (EP)
- A shielding
- The focal unit thermal control, which guarantees a CCD temperature stability better than 0.015 degrees (0-peak) over the orbit. Such a performance is compliant with mission requirements on random noise sources. For periodic noise sources, it is necessary to calibrate the thermal sensitivity of the focal unit. Ground corrections are possible if the curve of CCD temperature is known with a precision of 0.005°C.
The image spot of a star on the seismology detectors is spread out over about 400 pixels, with exposure time of 1 second.
A prism, allowing to obtain a colour image of the stars, is installed in front of the exoplanet detectors. It makes a distinction between the stellar activity from a planetary transit, for the brightest stars. Exposure time is 32 s. Image spots are smaller (80 pixels).
The equipment bay, developed under LESIA responsibility. It is made up of:
- A mechanical structure with 4 sidewalls
- A set of fine thermal control components. The most sensitive equipment (video electronics, analogical housekeeping telemetry) is mounted on the upper part of the bay, on thick heat distributors connected to radiators. These units are thus regulated (in a passive way) with a stability better then ± 0.15°C over the orbit. The price is a significant increase of mass: 25 kg (aluminium). In the lower part of the bay, regulation is of ± 4°C.
- The scientific processing units. For each photometric chain, one finds a camera controller unit (BCC), an extraction unit (BEX) and a digital processing unit (DPU).
- The housekeeping units. The BS1 is the unit in charge of acquisition of housekeeping analogical telemetry, whereas BS2 is in charge of distributing the synchronization signals, commanding the LEDs and the fine thermal regulation system.
- Two converters for instrument power supply (BCVCAM, BCVETN).
The data coming from the camera are processed by two independent photometric chains, each including a seismology channel and a planet finding channel. To each channel corresponds one CCD. In this way, one chain covers the half of the stellar field observed by the telescope.
The Proteus star trackers are hosted by the equipment bay too (on a sidewall). They functionally belong to the AOCS sub-system of the bus and are procured by the satellite contractor.
The flight software developed under LESIA responsibility.
The flight software is in charge of the on-board data processing for both channels (production of scientific telemetry containing light curves). The application software is in memory of the two DPUs. On each seismology channel, the photocenters of two target stars are used to feed the AOCS with angle error measurements (in fine pointing mode). The scientific data are recorded in Proteus mass memory through the MIL-STD-1553 communication bus, whereas the rotation angles are made available every second through a serial link (OBDH). At any moment, in case of data invalidity, the platform can switch from one DPU to the other (hot redundancy), improving the operational duty cycle of the mission, crucial for CoRoT.