PLATO camera ghosts: characterization of geometry and intensity from TVAC measurements of the flight models and the engineering model

Kurzfassung

The upcoming PLAnetary Transits and Oscillations of Stars (PLATO) mission will detect and characterize exoplanets around bright stars using high-precision, long-duration photometry and stellar seismology. We analyze optical ghosts in the PLATO cameras that can contaminate photometry, using thermal-vacuum measurements on the Engineering Model and several Flight Models and compare these to optical-design simulations. The two photometrically most relevant ghosts were studied: the main extended ghost generated by reflections between CCD and back surface of L6 (the lens closest to it), and the point-like ghost resulting from reflections between CCD and entrance window. The measured extended ghost geometries well agree with the model with small camera-specific differences that can be explained by variations in the actual CCD-L6 distance. Small deviations of the point-like ghost positions from the modeled ones can be explained by small tip/tilts of the entrance windows within the design specifications. We found the extended ghost to exhibit intensities well within the design requirements and therefore sufficiently small to be treated as noise in the photometric measurements. Point-like ghosts, which are not covered by that requirement, exhibited intensities higher than the extended-ghost limit for cases near the optical axis. However, because they are spatially confined to a few pixels, they affect only few stars, which will have to be identified and mitigated. The measured irradiance ratios were found well compatible with the simulations. These geometric and photometric characterizations validated the optical model, allowing to carry over the conclusions to the not measured cameras.