Advancing the Vera C. Rubin Observatory active optics control system

Abstract

The Vera C. Rubin Observatory is poised to achieve its highly anticipated first light in early 2025, marking the start of an era of transformative observational capabilities. As the observatory nears its first light, the commissioning of the Active Optics System (AOS) becomes increasingly critical. Comprising an open-loop and a closed-loop component, the AOS delivers real-time corrections for the alignment and mirror surface perturbations, ensuring seeing-limited image quality across the 3.5-degree field of view.In this paper, we present a thorough examination of recent advancements in the AOS at the Rubin Observatory. We begin by detailing the enhancements in the open-loop system, focusing on the improvement of Look-Up Tables (LUTs) for the mirror bending modes and the alignment of optical elements. Next, we discuss the closed-loop control improvements, particularly our novel approach using double Zernike polynomials. This method addresses camera rotation by defining the sensitivity matrix and the reference wavefront with a double Zernike expansion, thereby improving the system's adaptability to varying observational conditions. Finally, we address improvements made to eliminate degeneracies within the system's degrees of freedom, and discuss the upcoming verification phases during on-sky testing with the Commissioning Camera (ComCam). 1Overall, these initial open-loop verifications and closed-loop algorithmic improvements not only mark significant progress towards full-system verification with LSST Camera, but also refine the capabilities of the AOS, which is key for maintaining long-term operational efficiency and achieving the required image quality.