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To achieve its scientific objectives CHEOPS will be able to detect Earth-size planets transiting G5 dwarf stars (stellar radius of 0.9 R☉) with V-band magnitudes in the range 6 ≤ V ≤ 9 mag. Since the depth of such transits is 100 parts-per-million (ppm), this requires a photometric precision of 20 ppm (goal: 10 ppm) in 6 hours of integration time. The time interval corresponds to the transit duration of a planet with a revolution period of 50 days, and assumes an observation efficiency of 50%.
In the case of Neptune-size planets, CHEOPS will achieve a signal-to-noise ratio of 30 on such planets transiting K dwarf stars (stellar radius of 0.7 R☉) with V-band magnitudes in the range 9 ≤ V ≤ 12 (goal: 9 ≤ V ≤ 13). Since the depth of such transit is 2500 ppm, this requires a photometric precision of 85 ppm in 3 hours, assuming an observation efficiency of 80% for a planet with revolution period of 13 days.
The required photometric precision will be achieved by using a single, frame-transfer, back-illuminated CCD detector with 1024 × 1024 pixels and a pixel pitch of 13 µm which is mounted in the focal plane of a ~32 cm diameter, f/8, on-axis Ritchey-Chrétien telescope. The detector will be passively cooled to < 233 K, with a thermal stability < 10 mK.
Stray light, primarily from the Earth, will be a major source of noise and the telescope must be baffled to control its impact. Cleanliness and contamination requirements lead to the need for a door cover which is light and dust tight. The detector plus support electronics together with the telescope, back-end optics, instrument computer and thermal regulation hardware are known collectively as the CHEOPS Instrument System (CIS).
The CIS is accommodated on the upper deck of the platform and comprises a number of a number of units:
• The Optical Telescope Assembly (OTA): this includes the telescope, the optical structure (mechanical support for OTA subsystems and the two star trackers), the back-end optics, the focal plane module (including the CCD, the focal plane array and front end electronics) and the radiators which provide cooling for the focal-plane array as well as the front end electronics.
• The Baffle and Cover Assembly (BCA): this includes the external baffle (axi-symmetric design) and the cover assembly, both mounted on the platform collar. The external baffle is designed to reject stray light from angles >35 degrees from the line of sight. The cover assembly is made up of a cover lid, which is found at the entrance aperture of the external baffle and designed to minimise contamination of the CHEOPS optics up to and including launch, and a cover release mechanism to be used after launch.
• The Sensor Electronics Module (SEM): this comprises the sensor controller unit, used to control and read out the CCD, and the power conditioning unit used to condition/filter the voltages supplied to the CCD and to the CCD and front-end electronics thermal control.
• The Back-End Electronics (BEE): this comprises the digital processing unit and the power supply unit that provides the power for all elements of the CIS.