Focal Plane Module for the imaging sensor and Sensor Electronics Module
The triangular-shaped Focal Plane Module (FPM) and Sensor Electronics Module (SEM) shown here are both developed under the responsibility of DLR in Berlin. The FPM houses the CHEOPS CCD detector array and front-end electronics, while the SEM contains a power supply unit and a processor used to control the front-end electronics and read out the CCD. Conductive thermal straps on the rear of the FPM form the thermal connection between it and the radiators that cool the CCD to its operating temperature of minus 40 degrees Celsius.
The CCD detector is built into the Focal Plane Module (gold-plated structure on the left). The electronics for reading out the detector and for the control system are distributed across two modules: the Focal Plane Module and the Sensor Electronics Module (right).
Optical Telescope Assembly (OTA): the telescope with star tracker and radiator; Baffle and Cover Assembly (BCA): a protective shield against stray light and a telescope cover; Focal Plane Assembly (FPA): includes the CCD detector, readout electronics and control system; Sensor Electronics Module (SEM): data processing and power supply to the CCD; Back-End Electronics (BEE): digital processor and power supply for the entire telescope.
The Berlin-based DLR Institutes of Optical Sensor Systems and Planetary Research (now merged to form the DLR Institute of Space Research) are contributing two electronic modules to ESA's CHEOPS (CHaracterising ExOPlanet Satellite) mission, including the core component of the satellite – the Focal Plane Module. This module contains the CCD (charge-coupled device) detector and its electronics. The German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) is also involved in the scientific analysis of the data collected during the mission.
The challenge in designing and building the two electronic modules was to ensure their thermo-mechanical stability. Strict adherence to operating temperature and structural stability is crucial to the success of the CHEOPS mission, as it enables the highly accurate measurements required. Achieving sufficient thermo-mechanical stability meant breaking new technological ground during the development and construction of the modules. The tight timeline posed a further challenge, as both components had to be developed in a very short period, between 2013 and 2017. The Focal Plane Module (FPM) was delivered to the University of Bern for integration into the payload in November 2017; the Sensor Electronics Module (SEM) followed in May 2018.
Because CHEOPS is designed to detect extremely small variations in starlight, the thermal requirements for these components are extremely strict: the CCD detector and associated low-noise electronics must be kept at a constant operating temperature – fluctuations must not exceed 0.01 kelvin (10 millikelvin). Only if this requirement is met can the mission’s scientific goals be achieved – namely, measuring the brightness of stars with extreme precision and determining the radius of orbiting planets with the utmost accuracy.
The Focal Plane Module houses the CCD detector – the core component of CHEOPS onto which starlight is focused. On the sides of the module are the circuit boards with the readout electronics. The sensitivity of CHEOPS’ measurements depends on the thermal stability of the detector and its electronics. The Sensor Electronics Module contains additional electronics for controlling the sensor – such as a temperature controller, processor and software – and is installed slightly offset from the FPM.
A high-precision temperature control system combines a passive cooling element (radiator) and an active heater to ensure the thermo-mechanical stability of the Focal Plane Module. In addition, the structure of the FPM is made of a special beryllium alloy – characterised by its extremely low thermal expansion and low mass.
Neither the CoRoT satellite (2006–2014) nor the 26-camera PLATO satellite – currently being readied for launch in 2026 – included such a highly stable temperature control system, relying instead on light curves to subsequently calculate temperature fluctuations in their detectors.
Another technical first in the CHEOPS mission is the CCD detector's thermally insulating mount. For the first time in an ESA satellite, a component manufactured by 3D printing, via selective laser melting in a titanium alloy, has been used in a critical location. The structurally sophisticated design – the entire component is about the size of an egg – could not have been realised using conventional manufacturing methods.
