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The mission lead is performed in a partnership between Switzerland, led by the University of Bern, and the European Space Agency with important contributions from Austria, Belgium, France, Germany, Hungary, Italy, Portugal, Spain, Sweden, and the United Kingdom. The CHEOPS mission will be the first space telescope dedicated to search for exoplanetary transits on bright stars already known to host planets by performing ultrahigh precision photometry on bright starts whose mass has been already estimated through spectroscopic surveys on ground based observations. The number of exoplanets in the mass range 1-30 MEarth for which both mass and radius are known with a good precision is extremely limited also considering the last two decades of high-precision radial velocity measurement campaigns and the highly successful space missions dedicated to exoplanets transit searches (CoRoT and Kepler). |
I.INSTRUMENT DESIGNThe CHEOPS instrument is a classical photometer measuring variations of light to a very high accuracy. The instrument is split up into four different units on the spacecraft of which two are mounted inside the spacecraft body and two on the outside. The four units are shortly summarized hereafter:
The OTA and BCA together form the CHEOPS telescope, which is mounted onto the top panel of the spacecraft. Fig. 1. shows an artist’s impression of the CHEOPS spacecraft assembly. The image looks down the tube of the BCA, which extends the telescope assembly OTA behind. The radiators that are needed for the thermal control of the CCD and proximity electronics are shown on the top. The grey disc to the left shows the BCA one-shot cover in open configuration. Fig. 2. illustrates the instrument units in more detail. On the top left of the figure, a cut through the CAM/CAD model of the OTA and BCA is shown where the primary and secondary mirror as well as the back end optics (BEO) and the focal plane module (FPM) is illustrated. The two electronics boxes used to control the instrument are shown on the right hand side of the figure. Fig. 2.CHEOPS instrument configuration overview. The four units, the OTA, BCA, BEE and SEM are shown.  The CHEOPS instrument uses a Ritchey-Chretien optical configuration with a BEO to re-imagine the light onto a CCD detector run in AIMO mode. The detector, which was selected for CHEOPS, is an e2v CCD47-20 (13-μm pixel 1k × 1k, AIMO). The CCD is nominally operated at low temperature, 233K, and stabilized to 10mK. This is achieved using a passive cooling system with a dedicated radiator and heaters close to the CCD to reach the relative stabilization. The entrance pupil of the system is formed by the primary mirror and has a diameter of 320mm. With the central obscuration of the primary mirror, the effective collecting area of the system is 76793 mm2. The size of the system have been mainly restricted by the allocated volume inside the fairing. The telescope effective focal length is 1600 mm, giving a telescope focal ratio F/5. The 0.32 degrees field of view is translating into a 1 arcsec/px plate scale on the detector. The system is using a defocused stellar point spread function (PSF) in order to enhance the photometric performance. A PSF radius of 12 ± 0.7 pixels is currently specified on the detector. The number was a trade off between the flat field performances and the AOCS performance estimations of the S/C. The spacecraft prime contractor, AIRBUS DS Spain, has been in charge of the AOCS performance estimations and the flat field of the CCD is measured at the University of Geneva. The combination of the data resulted in the optimization of the PSF size. The system was designed to have a certain degree of freedom with the alignment of the focal plane module. As the satellite is in LEO, the stray light reduction of the system had to be addressed carefully. The baffling system, comprising the BCA and the telescope itself, is therefore used to suppress stray light up to a factor of 10-12 for higher incidence angles. It is designed to limit the amount of stray light already for sources more than 35° from the optical axis. Fig. 3. illustrates the CHEOPS optical system with the baffling system on the left hand side and the telescope assembly on the right. In order to have an impression of the BCA and OTA configuration of the instrument, Fig. 4. shows a picture of the structural and thermal model (STM) which has been tested at the University of Bern. The electrical design of the instrument is based on a distributed architecture. The camera system, which consists of the Focal Plane Module (FPM) and the Sensor Electronics Module (SEM), is designed and built by DLR Berlin while the Back End Electronics (BEE) is realized by IWF Graz. The camera is controlled by the instrument main computer, the BEE, using a SpW link for high data transmission capability. In addition to the data link and software control, the Back End Electronics delivers highly stabilized voltage lines to the SEM in order to ensure the bias voltages stabilities for the camera. This is one of the key drivers to meet the very low noise and high gain stability requirements of the instrument. The BEE on the other hand interfaces the spacecraft, which provides a redundant power and communication interface. The instrument is a fully cold redundant system with the exception of the CCD and the CCD clock driver. II.DEVELOPMENT STATUSSince the mission adoption in February 2014, considerable progress has been achieved by the CHEOPS team from the consortium and the European Space Agency. The detailed design phase has been concluded, the preliminary design review (PDR) mid 2014 as well as the critical design review (CDR) end of 2015 have been successfully passed. As part of these reviews the interfaces to the spacecraft as well as the system and sub-system specifications were frozen. Form a hardware perspective, several qualification and interface verification models have been built and tested. A structural and thermal model (STM) of the instrument was realized in 2015. The STM underwent mechanical and thermal qualification before being integrated into the spacecraft mechanical qualification model. Fig. 5. shows a picture of the instrument integration at the ISO5 clean room of the University of Bern and the fully assembled spacecraft mechanical qualification model being ready for sine testing. Fig. 5.Pictures of the CHEOPS instrument on the left (University of Bern) and the instrument integrated onto the satellite on the right (Courtesy of Airbus DS Spain)  The spacecraft underwent sine vibration testing, acoustic testing and shock testing. At the end of the test campaign, the verification of the Cover Assembly of the instrument has been performed in order to verify that the launch lock mechanism performs after the environmental loads. Additionally to the STM tests, a dedicated test campaign has been conducted in order to verify the thermo-elastic performance of the instrument. The dedicated model of the OTA, which consists mainly of CFRP and a honeycomb sandwich optical bench, has been used for the tests. The performance tests were conducted at TNO in Delft [4] and showed that the tight thermo-elastic requirements can be fulfilled. The structure has consequently been refurbished and is used as flight model. In order to verify the electrical and software interfaces between the SEM/FPM and BEE as well as the interfaces towards the spacecraft, an instrument Electrical Model (EM) was built and tested. The tests were again performed at various stages (unit, instrument and spacecraft). The software used for the camera system, the SEM, is close very close to the flight software as the system is a reuse of the BepiColombo MERTIS camera from DLR [2]. The software on the BEE which is the main interface to the spacecraft provides all the expected telemetry as a heartbeat report, HK data, AOCS centroid report, other asynchronous reports and CCD images allowing testing and validating SW interfaces. Internal algorithms which include centroiding, compression as well as various other engineering algorithms have not been tested at spacecraft level because of time constraints. The EM model however is currently used for further software development at the University of Vienna in order to provide a representative hardware environment. In order to verify and de-risk the AIV procedures for the telescope optics, a dedicated Demonstration Model (DM) has been realized which was a collaboration between the University of Bern and the Italian industrial Prime LEONARDO. The DM model verification process took place at INAF in Padova and is currently being concluded successfully [5]. For the BEE, SEM and FPM qualification models are realized in advance of the flight models. The BEE box has undergone the full qualification (mechanical, thermal, performance, EMC) already successfully at the Ruag Space in Vienna and IWF in Graz. The SEM and FPM qualification models are currently being integrated and qualified. A very important milestone for the camera system is the performance verification in thermal vacuum, which will be concluded this year. In view of the flight model or proto flight models substantial progress was achieved as well. The CCD has been procured by ESA, built and tested by e2v. The device has been extensively tested in view of bias level, read out noise, dark signal, gain, non-linearity, PRNU, flat field and quantum efficiency by the Observatory of Geneva. These measurements serve as input to the performance verifications as well as calibration data for operations [6]. The flight models of the Baffle and Cover assembly as well as the telescope structure have been built and qualified for flight already as well. The optics, which are manufactured and integrated by LEONARDO in Florence, are currently under manufacturing and the integration, verification and test of the telescope, is starting still this year. The manufacturing of the flight model electronics, the SEM, FPM and BEE as well is progressing while the testing is scheduled in 2017. As soon as all individual sub-systems have been successfully manufactured and tested, they are shipped to the University of Bern where the instrument integration and verification will take place. Prior to the integration at spacecraft level, the instrument will be calibrated as well in Bern with the use of the CHEOPS calibration bench which has been designed, manufactured and tested by the Observatory of Geneva [7]. III.TECHNICAL AND PROGRAMMATIC CHALLENGESA brief overview is given hereafter concerning the major technical and programmatic challenges, which such an S-Class ESA mission faces in view of the instrument. The budget and schedule constraints for the ESA S-Class mission have been set very tight. The costs to ESA, who is providing the spacecraft, launcher and parts of the instrument, are cost capped at 50 MEuro while the schedule constraint has been set to 4 years for the development and launch of the mission. Under these constraints, CHEOPS is supposed to deliver top rated science in any area of space science. The programmatic challenges in view of the tight schedule constraints are obvious. The CHEOPS instrument as well as the spacecraft therefore needed to be a reuse to the largest extend possible. In view of the instrument, this is achieved combining several technologies, which have had flight heritage already. The major drawback is that the instrument consortium and different contributions have been growing as consequence and are more difficult to manage. In order to mitigate the schedule constraints, clearly defined and stable interfaces needed to be established early in the project. This of course is conceptually doable but still special attention needed to be paid in order to avoid changes. As well, special attention had to be paid in order to specify and maintain the stability of the requirements in order to avoid modifications. On the technical side, several topics need attention in order to meet the very demanding photometric accuracy requirements. The major topics are briefly summarized hereafter:
IV.CONCLUSIONSCHEOPS, the first small class mission (S-class) in ESA’s Science Program, is currently in phase C/D. About half time through the development time, the instrument design, development status as well as the major programmatic and technical challenges are detailed in this paper. After completing the PRR and SRR in 2013, PDR in July 2014, a complete instrument STM was built and successfully tested at instrument and spacecraft level, including a thermo-elastic stability test using the flight model of the telescope structure. The instrument EM has been tested and provided to Airbus DS Spain beginning of April 2016 and is being tested with the spacecraft EFM. An instrument Demonstration Model was built and tested as well in order to verify the telescope AIV. The instrument CDR as well as the system has been passed successfully 2016. Several flight model units and sub-units have already been manufactured and successfully tested while initial measurements provide confidence of that the mission meets the science performances. V.ACKNOWLEDGEMENTThe authors would like to gratefully acknowledge the contributions of all members of the instrument consortium and thank them for their valuable contribution and effort to make the CHEOPS instrument and mission a success. The support of the Prodex programme for the development of the CHEOPS payload is gratefully acknowledged. REFERENCESCHEOPS – Characterising ExOPlanet Satellite Definition Study Report,
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