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).

Overall, 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.

We present a revision to the astrometric calibration of the Gemini Planet Imager (GPI), an instrument designed to achieve the high contrast at small angular separations necessary to image substellar and planetary-mass companions around nearby, young stars. We identified several issues with the GPI data reduction pipeline (DRP) that significantly affected the determination of the angle of north in reduced GPI images. As well as introducing a small error in position angle measurements for targets observed at small zenith distances, this error led to a significant error in the previous astrometric calibration that has affected all subsequent astrometric measurements. We present a detailed description of these issues and how they were corrected. We reduced GPI observations of calibration binaries taken periodically since the instrument was commissioned in 2014 using an updated version of the DRP. These measurements were compared to observations obtained with the NIRC2 instrument on Keck II, an instrument with an excellent astrometric calibration, allowing us to derive an updated plate scale and north offset angle for GPI. This revised astrometric calibration should be used to calibrate all measurements obtained with GPI for the purposes of precision astrometry.

An explanation for the origin of asymmetry along the preferential axis of the point spread function (PSF) of an AO system is developed. When phase errors from high-altitude turbulence scintillate due to Fresnel propagation, wavefront amplitude errors may be spatially offset from residual phase errors. These correlated errors appear as asymmetry in the image plane under the Fraunhofer condition. In an analytic model with an open-loop AO system, the strength of the asymmetry is calculated for a single mode of phase aberration, which generalizes to two dimensions under a Fourier decomposition of the complex illumination. Other parameters included are the spatial offset of the AO correction, which is the wind velocity in the frozen flow regime multiplied by the effective AO time delay and propagation distance or altitude of the turbulent layer. In this model, the asymmetry is strongest when the wind is slow and nearest to the coronagraphic mask when the turbulent layer is far away, such as when the telescope is pointing low toward the horizon. A great emphasis is made about the fact that the brighter asymmetric lobe of the PSF points in the opposite direction as the wind, which is consistent analytically with the clarification that the image plane electric field distribution is actually the inverse Fourier transform of the aperture plane. Validation of this understanding is made with observations taken from the Gemini Planet Imager, as well as being reproducible in end-to-end AO simulations.