# Expected performances of the Characterising Exoplanet Satellite (CHEOPS). I. Photometric performances from ground-based calibration

## TL;DR

This paper evaluates CHEOPS's photometric performance through ground calibration and simulations, demonstrating its capability to precisely measure exoplanet radii, including Earth-sized planets, confirming compliance with mission goals.

## Contribution

It provides comprehensive analysis of pre-launch calibration data and end-to-end simulations to estimate CHEOPS's in-flight photometric precision and planet characterization capabilities.

## Key findings

- Photometric stability of ~15 ppm over 5 hours
- 2% radius ratio measurement for Neptune-sized planets
- 5% radius ratio measurement for Earth-sized planets

## Abstract

The Characterising Exoplanet Satellite (CHEOPS) is a space mission designed to perform photometric observations of bright stars to obtain precise radii measurements of transiting planets. The high-precision photometry of CHEOPS relies on careful on-ground calibration of its payload. For that purpose, intensive pre-launch campaigns of measurements were carried out to calibrate the instrument and characterise its photometric performances. We report on main results of these campaigns, provide a complete analysis of data sets and estimate in-flight photometric performance by mean of end-to-end simulation. The on-ground photometric stability of the instrument is found to be of the order of 15 parts per million over 5 hours. Our end-to-end simulation shows that measurements of planet-to-star radii ratio with CHEOPS can be determined with a precision of 2% for a Neptune-size planet transiting a K-dwarf star and 5% for an Earth-size planet orbiting a Sun-like star. It corresponds to signal-to-noise ratios on the transit depths of 25 and 10 respectively, allowing the characterisation and detection of these planets. The pre-launch CHEOPS performances are shown to be compliant with the mission requirements.