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 Rubin Observatory Commissioning Camera (ComCam) is a scaled down (144 Megapixel) version of the 3.2 Gigapixel LSSTCam which will start the Legacy Survey of Space and Time (LSST), currently scheduled to start in 2024. The purpose of the ComCam is to verify the LSSTCam interfaces with the major subsystems of the observatory as well as evaluate the overall performance of the system prior to the start of the commissioning of the LSSTCam hardware on the telescope. With the delivery of all the telescope components to the summit site by 2020, the team has already started the high-level interface verification, exercising the system in a steady state model similar to that expected during the operations phase of the project. Notable activities include a simulated “slew and expose” sequence that includes moving the optical components, a settling time to account for the dynamical environment when on the telescope, and then taking an actual sequence of images with the ComCam. Another critical effort is to verify the performance of the camera refrigeration system, and testing the operational aspects of running such a system on a moving telescope in 2022. Here we present the status of the interface verification and the planned sequence of activities culminating with on-sky performance testing during the early-commissioning phase.
Resulting from its unusual optical configuration, the Vera C. Rubin Observatory requires precise top-end assembly (TEA) thermal control. The three-mirror system locates the large camera, the secondary mirror (M2), the secondary mirror hexapod, the camera hexapod/rotator, and associated electronics on the TEA. Escaping heat, or overcooling, crosses the optical path three times potentially significantly degrading the image quality. Most observatories follow a common thermal control strategy. A central refrigeration system, composed of chillers and pumps, supplies non-precision temperature-controlled ethylene glycol/water (EGW) coolant through long pipes, to the observatory’s subsystems including the general ones (Facility Services, Telescope machinery, etc.) and the scientific instrumentation. The refrigeration for the instrumentation is provided by EGW cooled secondary systems. The common strategy is inadequate for this application. For this application, since overcooling is just as detrimental as escaping heat, TEA thermal control is needed to levels impractical with the common strategy. Consequently, a new system was developed to provide superior thermal control. An intermediate cooling stage was added directly under the telescope. Using local chillers, recirculation pumps, and mixing valves, coolant is provided to the TEA at precise temperatures and flow rates. This system itself is cooled by EGW from the central refrigeration system. The location of the Camera, etc. on the TEA, over the main primary tertiary mirror (M1M3), produces a critical leak risk to the optical system. Many glycol/water leaks at different observatories have damaged critical electronics and optics elements. Consequently, less toxic and corrosive Dynalene was chosen, rather than the more common EGW.
The Vera C. Rubin Observatory is currently under construction on Cerro Pachón, in Chile. It was designed to conduct a 10-year multi-band survey of the southern sky with frequent re-visits (with both an intra- and extra-night cadence) to identify transient and moving objects. The mirror cell assembly was designed in Tucson, Arizona by the Rubin Observatory engineering department, and was tested twice in Tucson. The first testing campaign was performed at CAID industries, where the mirror cell was fabricated, using a steel mirror surrogate that has the same geometry and mass of the glass mirror2,4. The glass mirror is a single monolith that contains both the primary and tertiary mirrors on a single substrate. The testing results confirmed that the mirror support system was performing within the design specifications, and that it was safe to install the glass mirror on the cell. The second test campaign was performed at the Richard F. Caris Mirror Lab of the University of Arizona using the actual glass mirror16. This test campaign was performed under the test tower, which contains a vibration insensitive interferometer to measure mirror figure. This confirmed the mirror support system could achieve proper optical surface figure control for both primary and tertiary mirrors. After successful test campaigns at CAID, and the mirror Lab, the mirror cell assembly was disassembled, packed and shipped to the Rubin Observatory site at the Cerro Pachón summit in Chile. Upon arrival, the mirror cell has been integrated with the mirror surrogate once again to perform the third test campaign that confirmed the system has arrived safe and operational to the summit. This integrated system will be tested on the telescope mount assembly to verify that it still meets it requirements under the effects of variations in gravitational orientation, and dynamic (slewing) loads.
Rubin Observatory’s Commissioning Camera (ComCam) is a 9 CCD direct imager providing a testbed for the final telescope system just prior to its integration with the 3.2-Gigapixel LSSTCam. ComCam shares many of the same subsystem components with LSSTCam in order to provide a smaller-scale, but high-fidelity demonstration of the full system operation. In addition, a pathfinder version of the LSSTCam refrigeration system is also incorporated into the design. Here we present an overview of the final as-built design, plus initial results from performance testing in the laboratory. We also provide an update to the planned activities in Chile both prior to and during the initial first-light observations.
The Large Synoptic Survey Telescope1 (LSST) is an altitude-azimuth mounted three mirror telescope and camera. The primary (M1) and tertiary (M3) mirrors are integrated into a single, monolithic borosilicate substrate 8.42 m diameter. The annular secondary (M2) mirror is located above the M1M3 mirror and the camera is nested inside the M2. The M1M3 mirror is supported on a mirror cell by two independent systems: one system is for Active Mode and the other for Static Mode.
During observing, or Active Mode2, the M1M3 mirror is supported by an array of 156 support and figure control actuators consisting of 268 pneumatic cylinders that react to gravity and inertial loads and provide figure error correction. Load cells on the actuators measure forces that are communicated to the M1M3 control system. However, the figure actuators do not define the mirror position. This is defined with six axially stiff linear actuators called hardpoints3 arranged in a hexapod pattern to restrain rigid body motion of the mirror in a kinematic fashion. By adjusting the length of each hardpoint, the mirror can be adjusted in all six degrees of freedom with respect to the cell. Displacement sensors and load cells on the hardpoints communicate displacements and forces to the control system, which processes the telemetry and issues force corrections to the figure actuators to zero out any loads and moments on the hardpoints.
In Static Mode, the M1M3 mirror is no longer supported by figure actuators and the position sensing of the hard point hexapod is inactive. A second support system consisting of 288 wire rope isolators called Static Supports come into play. The static supports mechanically capture the mirror whether in Active or Static Mode and in the event the mirror experiences motion beyond the active motion range in any direction. The static supports also safely support the mirror during seismic events for all elevation angles. In active mode, the static supports do not contact the mirror and thus, do not affect the mirror positioning or figure.
This paper focuses on the detailed design, development, testing, integration, and current status of the M1M3 pneumatic figure actuators.
The Large Synoptic Survey Telescope is an 8.4m telescope now in construction on Cerro Pachón, in Chile. This telescope is designed to conduct a 10-year survey of the southern sky in which it will map the entire night sky every few nights. In order to achieve this goal, the telescope mount has been designed to achieve high accelerations that will allow the system to change the observing field in just 2 seconds. These rapid slews will subject the M1M3 mirror to high inertial and changing gravitational forces that has to be actively compensated for in order to keep the mirror safe, aligned, and properly figured during operations. The LSST M1M3 active support system is composed of six “hard point” actuators and 156 pneumatic actuators. The hard points define the mirror position in the mirror cell (with little or no applied force) and hold that position while observing in order to maintain the alignment of the telescope optics. The pneumatic actuators provide the force-distributed mirror support plus a known (static) figure correction as well as dynamic optical figure optimizations coming from other components of the Active Optics System. Optimizing this mirror support system required the introduction of innovative control concepts in the control loops (Inner and Outer). The Inner Loop involves an extensive pressure control loop to ensure precise force feedback for each pneumatic actuator while the Outer Loop includes telescope motion sensors to provide the real-time feedback to compensate for the changing external inertial and gravitational forces. These optimizations allow the mirror support system to maximize the hard point force-offloading while keeping the glass safe when slewing and during seismic events.
The Large Synoptic Survey Telescope (LSST) primary/tertiary (M1M3) mirror cell is a 25-ton, 9-meter x 9-meter x 2- meter steel weldment that supports the 19-ton borosilicate M1M3 monolith mirror on the telescope and acts as the lower vessel of the coating chamber when optically coating the mirror surfaces. The M1M3 telescope mirror cell contract was awarded to CAID Industries, Inc., of Tucson, Arizona in October 2015. After the mirror cell final acceptance in October 2017, the integration of the mirror support system started. The M1M3 cell assembly with the surrogate mirror will take place in a dedicated controlled-environment area at CAID Industries. All components of the mirror support system that were developed and tested by the LSST Telescope and Site M1M3 team at the NOAO offices in Tucson have been moved to CAID premises and have been integrated into the cell by a team of LSST, CAID and Richard F. Caris Mirror lab personnel. After completion of the cell integration and its assembly with the surrogate, a test phase that includes zenith and offzenith testing for the mirror support system will be carried by the LSST team. These tests aim to verify that the active support system components, mirror control, and software are performing as expected and the mirror support system is safe for the next step, the M1M3 cell to borosilicate glass assembly and tests at the RFC Mirror Lab of the University of Arizona.
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