Commissioning of the Vera C. Rubin Observatory and cosmic shear systematic

• Pierre-Francois Leget (Princeton/Harvard)

Room: 108/1-XX - Grande Salle 108 The Rubin Observatory team began commissioning with its small commissioning camera (LSSTComCam) at the end of 2024; in April 2025, it started commissioning LSSTCam, the main instrument that will serve the survey's ten-year run. The official Legacy Survey of Space and Time (LSST) is scheduled to begin in 2026. The preliminary data gathered between April and January 2026, is expected to be publicly distributed as Data Preview 2 (DP2) in mid-2026. As a wide, fast, and deep optical ground-based survey, the Rubin Observatory will furnish an unrivaled dataset for constraining Dark Energy through multiple cosmological probes: cosmic shear, galaxy–galaxy lensing, galaxy clustering, galaxy clusters, and type Ia supernovae. The most powerful of these probes is expected to be cosmic shear combined with galaxy–galaxy lensing and galaxy clustering: the so-called 3×2-point correlation-function analysis. The projected constraints on the dark energy equation of state following the first year of data collection with the Rubin Observatory are expected to be comparable in precision to the recent DESI results. These 3×2pt measurements will provide a critical independent cross-check to either confirm or refute the evidence for a time-evolving dark energy signal. This potential provides a powerful motivation to produce these cosmological constraints as rapidly as possible. However, the optimistic projections assume an analysis that is not dominated by systematic effects. The first months of the Rubin Observatory commissioning have demonstrated that achieving this level of precision is by no means a foregone conclusion. There remains a significant technical gap to bridge, and the systematic issues, specifically on the instrumental side, must be fully mitigated before dark energy constraint can be realized. This seminar will give an overview of the commissioning of the Vera C. Rubin Observatory and the success and challenged we faced.