The M2 secondary mirror of the Vera C. Rubin Observatory, scheduled to be commissioned on-sky in 2024, will be the first active secondary mirror of 3.5m diameter in operation. Its substantial dimensions and advanced functionalities place it in league with the secondary mirrors of the upcoming 30m class telescopes. Characterizing its performance serves as a critical step towards comprehending and controlling the optics of the next generation of Extremely Large Telescopes (ELTs). This study focuses on testing and validating the M2 cell in the Observatory’s integration hall and at the Telescope Mount Assembly (TMA). We also report on the integration steps of the M2 cell onto the TMA itself, including installing the light baffle. During the testing campaign, the M2 cell is equipped with an aluminum mirror surrogate for safety reasons regarding the glass mirror. To ensure integrity when the thin glass mirror (10cm) is installed onto the telescope, the M2 support system must be actively controlled during any M2 cell movement. This prompted the development of a dedicated control system to enable closed loop mode for transporting the M2 cell with the glass mirror from the integration hall to the telescope. The tests in the integration hall were conducted with the M2 cell mounted on a rotating cart, allowing different orientations with respect to gravity as it will experience on the telescope. Upon reaching the telescope, static and dynamic tests are conducted at progressively higher telescope performance, increasing slewing speed, acceleration, and jerk. A significant novelty introduced by Rubin to astronomical instrumentation is the Verification & Validation architecture as part of the model-based Systems Engineering approach where requirements, test procedures and executions are merged into an interlaced and dynamic flow. This report presents the experimental results from the distinct test campaigns covering a wide range of M2 cell functionalities. These include characterization of actuator behavior in terms of maximum stroke and force limits, evaluation of closed-loop (active) and open-loop (passive) support system operation for the M2, system settling time and Force Balance response to different slewing speeds of the telescope.
The Vera C. Rubin Observatory is an integrated survey system, currently under construction in Chile, to accomplish a 10-year optical survey of the southern sky. The 8.4-meter Simonyi Survey Telescope mount is nearing completion and undergoing final verification and performance testing. Since the system is optimized for etendue, the telescope mount slewing performance is particularly critical to overall survey efficiency. For example, this high performance mount is required to slew 3.5 degrees, on the sky, and settle in a 4-second period. Here an account of the mount subsystem is presented and selected dynamic performance results from on-site testing are described.
The Vera C. Rubin Observatory is reaching the final stages of its construction and integration, advancing towards its 10-year Legacy Survey of Space and Time (LSST). One of the key milestones was the installation of the M1M3 Mirror Cell Assembly onto the Simonyi Survey Telescope’s (SST) Telescope Mount Assembly (TMA). The Cell Assembly actively supports the primary/tertiary mirror (M1M3), playing a crucial role in maintaining the glass safe and ensuring image quality. However, before the mirror glass installation, the Cell Assembly was installed on the TMA while supporting a steel surrogate M1M3 mirror. This surrogate closely mimics the glass mirror’s mass, center of gravity, and geometry. The M1M3 cell and surrogate were tested under conditions that simulate rapid field changes in the sky, which are essential for the observatory’s ambitious sky mapping schedule. These tests, extending from 1-100% of designed telescope slew velocities/accelerations, assessed the M1M3 active mirror support system, including the force balance system’s performance, the hardpoint behaviors, and the efficacy of the pneumatic figure control actuators. Preliminary results suggest the system meets operational requirements, ensuring safety and effectiveness at full speed.
The Simonyi Survey Telescope (formerly known as the Large Synoptic Survey Telescope) of the Rubin Observatory is an 8.4m telescope now in construction on Cerro Pachón, in Chile. This telescope has been designed to conduct a 10 years’ survey of the sky in which it will map the entire night sky every three nights. The Mirror Cell Assembly system is a 9x9m steel structure that provides positioning, support, figure correction and temperature control to the primary and tertiary mirror. It is composed of two main systems, the Support System and the Thermal Control System. The Support System provides positioning, support and figure control of the mirror as well as dynamic forces compensation. The Thermal Control System will control the bulk temperature and temperature variations throughout the mirror. The temperature variations produce thermal distortions of the mirror which produce image degrading distortion of the optical surface. Variations between the bulk temperature and the ambient degrade local seeing and can produce condensation. The mirror cell assembly was designed and build in Tucson, Arizona by the LSST engineering team, and was tested, to confirm correct integration, at the Richard F Caris Mirror Lab to confirm the optical performance of the system using the real glass mirror. After successful testing, the mirror cell assembly was disassembled, packed and shipped to the Cerro Pachón summit in Chile where it was integrated with the surrogate mirror, and installed on the telescope mount assembly (TMA) for system performance test. Once system performance test concluded, the mirror cell was transported to the maintenance level to remove the metal surrogate mirror, install the glass and coat. After coating the mirror, the mirror cell assembly will be integrated with the telescope mount assembly to conduct final testing and verification.
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