Expected performances of the Characterising Exoplanet Satellite (CHEOPS)

Abstract

Context. The CHaracterising ExOPlanet Satellite (CHEOPS) is a mission dedicated to the search for exoplanetary transits through high precision photometry of bright stars already known to host planets. The telescope will provide the unique capability of determining accurate radii for planets whose masses have already been measured from ground-based spectroscopic surveys. This will allow a first-order characterisation of the planets’ internal structure through the determination of the bulk density, providing direct insight into their composition. By identifying transiting exoplanets with high potential for in-depth characterisation, CHEOPS will also provide prime targets for future instruments suited to the spectroscopic characterisation of exoplanetary atmospheres. Aims. The CHEOPS simulator has been developed to perform detailed simulations of the data which is to be received from the CHEOPS satellite. It generates accurately simulated images that can be used to explore design options and to test the on-ground data processing, in particular, the pipeline producing the photometric time series. It is, thus, a critical tool for estimating the photometric performance expected in flight and to guide photometric analysis. It can be used to prepare observations, consolidate the noise budget, and asses the performance of CHEOPS in realistic astrophysical fields that are difficult to reproduce in the laboratory. Methods. The simulator has been implemented as a highly configurable tool called CHEOPSim, with a web-based user interface. Images generated by CHEOPSim take account of many detailed effects, including variations of the incident signal flux and backgrounds, and detailed modelling of the satellite orbit, pointing jitter and telescope optics, as well as the CCD response, noise and readout. Results. The simulator results presented in this paper have been used in the context of validating the data reduction processing chain, in which image time series generated by CHEOPSim were used to generate light curves for simulated planetary transits across real and simulated targets. Independent analysts were successfully able to detect the planets and measure their radii to an accuracy within the science requirements of the mission: for an Earth-sized planet with an orbital period of 50 days orbiting a Sun-like target with magnitude V = 6, the median measured value of the planet to star radius ratio, Rp/Rs, was 0.00923 ± 0.00054(stat) ± 0.00019(syst), compared to a true input value of 0.00916. For a Neptune-sized planet with an orbital period of 13 days orbiting a target with spectral type K5V and magnitude V = 12, the median measured value of Rp/Rs was 0.05038 ± 0.00061(stat) ± 0.00031(syst), compared to a true input value of 0.05.

The hot dayside and asymmetric transit of WASP-189 b seen by CHEOPS

“…The data were processed with the CHEOPS data reduction pipeline (DRP, Hoyer et al 2020), which performs image correction and uses aperture photometry to extract target fluxes for various apertures. The CHEOPS DRP was thoroughly tested, both using the CHEOPS data simulator (Futyan et al 2020) and data obtained during commissioning. Using simulated data, we performed a series of injection and retrieval tests covering a range of planetary transit scenarios and levels of field crowding.…”

“…As explained in Hoyer et al (2020), the DRP automatically determines the level of such contamination in the target's aperture for each exposure. The contamination is estimated from simulated images (Futyan et al 2020) that are based on the CHEOPS PSF, the roll angle of each image. and the Gaia DR2 (Gaia Collaboration et al 2018) coordinates and magnitudes of all the stars with G<19.5 mag in the field of view.…”

Expected performances of the Characterising Exoplanet Satellite (CHEOPS)

“…22). This effect is not surprising since the long 60 s exposure time used in case 2 translates in a larger number of CR per image for an equivalent integrated flux (see for example Futyan et al 2020). Furthermore, in this observation no imagettes are available to help the cosmic rays detection.…”

“…Two datasets were prepared using CheopSim simulator (Futyan et al 2020) to illustrate the performance of the DRP and compare it with the CHEOPS science requirements in terms of photometric precision. The first simulation (case 1 hereafter) represents the observation of a transit of an Earth-size planet orbiting a V-mag = 6 G0V star with a period of 50 days.…”

Expected performances of the PROBA-3/ASPIICS solar coronagraph: Simulated data

“…Therefore a proper modeling of the images, or end-to-end image simulation, is essential for the project. End-to-end science data simulation has been used with great success for earlier space missions such as Kepler (Bryson et al 2010), the Transiting Exoplanet Survey Satellite (TESS, Jenkins et al 2018), and the Characterising Exoplanet Satellite (CHEOPS, Futyan et al 2020), but to our knowledge, it has not been used in solar whitelight coronagraphy. Creating simulated data, that is, artificial images resembling those to be actually registered by the coronagraph, may serve two purposes: First, it will provide the data that can be used as input to facilitate the development and testing of the on-ground processing algorithms, and second, it will to provide a more detailed understanding of the coronagraph char-acteristics and performance.…”