Cheops optical calibration campaign

The ESA S mission CHEOPS relies on a photometer for the precise measurement of the radius of exoplanet. This information is essential to better constrain the nature and structure of exoplanets. The CHEOPS payload is a small defocused telescope of 32-cm aperture that went through its characterization campaign in the first quarter of 2018. We report here on the techniques developed for the calibration campaign and the results of the obtained. In particular we have measured high-precision color-dependent flat fields, measured the characteristics of the detection chain, gain, full-well capacity, read-out noise, measured the instrument distortion and explored the PSF behavior across the field. The end-toend photometric stability of the instrument has also been measured and we will be reporting on the results and the difficulties that have come up while attempting this very difficult measurement, for which sophisticated design and methods were developed.