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 one of the first observatories to apply Model-Based Systems Engineering in all major aspects of the project. This paper describes the evolution of the processes, methodologies and tools developed and utilized by the Rubin Observatory Team. It specifically focuses on the Rubin Systems Engineering Processes for Image Quality tracking, Computerized Maintenance Management System (CMMS) selection, Failure Reporting, Analysis, Corrective Action System (FRACAS) handling, and Hazard Mitigation Analysis. Here, we share updates on each topic’s workflows, experiences, and difficulties with the community.
The Vera Rubin Observatory hosts a large (8.4 meter) wide-field (3.5 degree) survey telescope4. The Secondary Mirror (M2) Assembly6 and Camera5 utilize large hexapods3 to facilitate optical positioning relative to the Primary/Tertiary Mirror. These hexapods were designed, fabricated, assembled, tested and met all their requirements1. Unfortunately, both hexapods were damaged prior to integration. The camera hexapod was damaged from overheating induced separation of the low temperature grease into constituents. The M2 hexapod was damaged from water intrusion during shipping. In both cases the critical linear encoders/tapes interior to the hexapod actuators were affected. These encoders are used by the control system to determine the length of the actuator during hexapod operations. If these encoders require servicing while deployed on the telescope, the hexapod needs to be unloaded by removing its optical payload (camera or M2), and the hexapod disassembled. The hexapod actuator then needs to be disassembled and repaired. This procedure produces an unacceptable risk to equipment, and an excessive disruption of observing. To rectify this, the actuators were redesigned to allow on-telescope servicing of these encoders. The encoder to tape orientation was inverted, and an access cover was added. This facilitates servicing the encoder/tape while on the telescope, reducing the servicing time from days to minutes. To improve reliability, alterations were also applied to the electrical system. The limit switch wiring was rearranged, and the cabling to the hexapod legs was upgraded. Also, multiple software upgrades were incorporated to improve function, performance, and compatibility with the other observatory systems.
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