Orateur
Description
The Euclid survey was the first Stage IV survey to be commissioned, paving the way for a new era of precision cosmology. The survey's declared scientific goal is to explore the nature of dark matter and dark energy. One of the main probes it utilises is weak gravitational lensing: the coherent distortion of galaxy shapes caused by the large-scale structure of the Universe. Over its lifespan, Euclid is expected to measure the shape of 1.5 billion galaxies. However, these measurements are only as meaningful as their systematic uncertainties can be controlled. The main source of noise here is the Point Spread Function (PSF) that smears out the observed shapes. Euclid is among the first telescopes to employ a forward-modeled PSF. In spite of the challenges that arise due to the large field of view, the model is performing remarkably well after more than a decade of development. Nevertheless, due to the long-lived nature of the Euclid survey, the PSF model must be continuously updated. We have developed an algorithm to monitor the evolution of the PSF over time by precisely estimating the area enclosed by diffraction spikes of bright stars in an image. The spikes are caused by light bending around the mirror arms in the telescope entrance pupil and their position is highly correlated with the defocus of the space telescope. So far, the Euclid PSF has been remarkably stable. With our method, changes of the PSF over the field-of-view can be monitored almost in real-time, a crucial step for achieving the actual science goals of Euclid and possibly, by generalisation, other space telescopes.
