Statistics of PLATOs point-like camera ghosts

Kurzfassung

The main goal of the PLATO mission is to detect Earth-like planets in the habitable zone around Sunlike stars by analysing the light curves of stars (Rauer et al. 2025). For heavily saturated stars, it might be difficult to extract the photometry with the existing PLATO on-board processing because these stars create many artifacts and a significant fraction of the incident flux is lost. The goal of this analysis is to assess the feasibility of extracting the light curves in those cases from the point-like ghosts instead of from the images of the stars themselves. For the analysis, the Gaia Data Release 3 (DR3) star catalogue (European Space Agency, Gaia Collaboration, 2022) was used, and all stars up to a magnitude of 14, which lie in PLATOs field-of-view (FoV), were selected (in total: 1,272,684). The stars magnitudes and positions are known, and their pixel coordinates on the CCDs and those of their point-like ghosts can be computed using geometric models of the different cameras. The magnitudes of the ghosts have been calculated using a simplistic model based on simulations. We restricted our analysis to ghosts with an estimated magnitude brighter than 15. Of the stars considered in our investigation, between around 285,000 and 570,000 are respectively in the four different N-CAM groups FoVs. In total, between 5 and 18 ghosts are brighter than magnitude 15. Furthermore, we assume that objects within a distance of up to 1 arcmin (4 pixels) to each other can influence and disturb each others signals. Our analysis shows that between 4 and 14 ghosts per camera are undisturbed. For heavily saturated stars, it is conceivable that the photometry could be more reliably extracted from their ghosts than from their images. However, we did not find cases of such stars that are estimated to exhibit undisturbed point-like ghosts brighter than magnitude 12. For magnitude < 13, six ghosts can be identified in our analysis. Our analysis has shown that the photometry of point-like ghosts does basically never lead to more robust results than that of the stars images. This means that it will most likely not be possible to derive the light curves from the heavily saturated stars at all. There are some limitations on our study that raise a word of caution on our conclusions. For example, we have not made use of the PLATO Input Catalogue (PIC) for our analysis but relied on Gaia DR3 data. We foresee to deepen our study after the public release of the PIC.