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 (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.
The Large Synoptic Survey Telescope (LSST) Commissioning Camera (ComCam) is a smaller, simpler version of the full LSST camera (LSSTCam). It uses a single raft of 9 (instead of twenty-one rafts of 9) 4K x 4K LSST Science CCDs, has the same plate scale, and uses the same interfaces to the greatest extent possible. ComCam will be used during the Project’s 6-month Early Integration and Test period beginning in 2020. Its purpose is to facilitate testing and verification of system interfaces, initial on-sky testing of the telescope, and testing and validation of Data Management data transfer, infrastructure and algorithms prior to the delivery of the full science camera.
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) large field of view is achieved through a three-lens camera system and a three-mirror optical system comprised of a unique 8.4-meter diameter monolithic primary/tertiary mirror (M1M3) and a 3.4-meter diameter secondary mirror (M2)1. The M2 is a 100mm thick meniscus convex asphere. The M2 Assembly includes a welded steel cell and a support system comprised of 72 axial and 6 tangential electromechanical actuators to control the mirror figure. The M2 Assembly (including optical polishing and integrated optical testing) is being fabricated by Harris Corporation in Rochester, NY. The summary status of this system and results are presented.
Proc. SPIE. 9911, Modeling, Systems Engineering, and Project Management for Astronomy VII
KEYWORDS: Actuators, Telescopes, Mirrors, 3D modeling, Space telescopes, Finite element methods, Computer aided design, Large Synoptic Survey Telescope, Large Synoptic Survey Telescope, Systems modeling, Solid modeling
During this early stage of construction of the Large Synoptic Survey Telescope (LSST), modeling has become a crucial system engineering process to ensure that the final detailed design of all the sub-systems that compose the telescope meet requirements and interfaces. Modeling includes multiple tools and types of analyses that are performed to address specific technical issues. Three-dimensional (3D) Computeraided Design (CAD) modeling has become central for controlling interfaces between subsystems and identifying potential interferences. The LSST Telescope dynamic requirements are challenging because of the nature of the LSST survey which requires a high cadence of rapid slews and short settling times. The combination of finite element methods (FEM), coupled with control system dynamic analysis, provides a method to validate these specifications. An overview of these modeling activities is reported in this paper including specific cases that illustrate its impact.
This paper describes the status and details of the large synoptic survey telescope1,2,3 mount assembly (TMA). On June 9th, 2014 the contract for the design and build of the large synoptic survey telescope mount assembly (TMA) was awarded to GHESA Ingeniería y Tecnología, S.A. and Asturfeito, S.A. The design successfully passed the preliminary design review on October 2, 2015 and the final design review January 29, 2016. This paper describes the detailed design by subsystem, analytical model results, preparations being taken to complete the fabrication, and the transportation and installation plans to install the mount on Cerro Pachón in Chile. This large project is the culmination of work by many people and the authors would like to thank everyone that has contributed to the success of this project.
At the core of the Large Synoptic Survey Telescope (LSST) three-mirror optical design is the primary/tertiary (M1M3) mirror that combines these two large mirrors onto one monolithic substrate. The M1M3 mirror was spin cast and polished at the Steward Observatory Mirror Lab at The University of Arizona (formerly SOML, now the Richard F. Caris Mirror Lab at the University of Arizona (RFCML)). Final acceptance of the mirror occurred during the year 2015 and the mirror is now in storage while the mirror cell assembly is being fabricated. The M1M3 mirror will be tested at RFCML after integration with its mirror cell before being shipped to Chile.
The Large Synoptic Survey Telescope (LSST) primary/tertiary (M1M3) mirror cell assembly supports both on-telescope operations and off-telescope mirror coating. This assembly consists of the cast borosilicate M1M3 monolith mirror, the mirror support systems, the thermal control system, a stray light baffle ring, a laser tracker interface and the supporting steel structure. During observing the M1M3 mirror is actively supported by pneumatic figure control actuators and positioned by a hexapod. When the active system is not operating the mirror is supported by a separate passive wire rope isolator system. The center of the mirror cell supports a laser tracker which measures the relative position of the camera and secondary mirror for alignment by their hexapods. The mirror cell structure height of 2 meters provides ample internal clearance for installation and maintenance of mirror support and thermal control systems. The mirror cell also functions as the bottom of the vacuum chamber during coating. The M1M3 mirror has been completed and is in storage. The mirror cell structure is presently under construction by CAID Industries. The figure control actuators, hexapod and thermal control system are under developed and will be integrated into the mirror cell assembly by LSST personnel. The entire integrated M1M3 mirror cell assembly will the tested at the Richard F Caris Mirror Lab in Tucson, AZ (formerly Steward Observatory Mirror Lab).
The Large Synoptic Survey Telescope (LSST) is a large (8.4 meter) wide-field (3.5 degree) survey telescope, which will be located on the Cerro Pachón summit in Chile. Both the Secondary Mirror (M2) Cell Assembly and Camera utilize hexapods to facilitate optical positioning relative to the Primary/Tertiary (M1M3) Mirror. A rotator resides between the Camera and its hexapod to facilitate tracking. The final design of the hexapods and rotator has been completed by Moog CSA, who are also providing the fabrication and integration and testing. Geometric considerations preclude the use of a conventional hexapod arrangement for the M2 Hexapod. To produce a more structurally efficient configuration the camera hexapod and camera rotator will be produced as a single unit. The requirements of the M2 Hexapod and Camera Hexapod are very similar; consequently to facilitate maintainability both hexapods will utilize identical actuators. The open loop operation of the optical system imposes strict requirements on allowable hysteresis. This requires that the hexapod actuators use flexures rather than more traditional end joints. Operation of the LSST requires high natural frequencies, consequently, to reduce the mass relative to the stiffness, a unique THK rail and carriage system is utilized rather than the more traditional slew bearing. This system utilizes two concentric tracks and 18 carriages.
The Large Synoptic Survey Telescope (LSST) has a 10 degrees square field of view which is achieved through a 3 mirror optical system comprised of an 8.4 meter primary, 3.5 meter secondary (M2) and a 5 meter tertiary mirror. The M2 is a 100mm thick meniscus convex asphere. The mirror surface is actively controlled by 72 axial electromechanical actuators (axial actuators). Transverse support is provided by 6 active tangential electromechanical actuators (tangent links). The final design has been completed by Harris Corporation. They are also providing the fabrication, integration and testing of the mirror cell assembly, as well as the figuring of the mirror. The final optical surface will be produced by ion figuring. All the actuators will experience 1 year of simulated life testing to ensure that they can withstand the rigorous demands produced by the LSST survey mission. Harris Corporation is providing optical surface metrology to demonstrate both the quality of the optical surface and the correctablility produced by the axial actuators.
The Large Synoptic Survey Telescope (LSST) utilizes active optics on its three mirrors to maintain image quality. This
paper describes the philosophy behind the design and characterization of the inner loop controllers for the LSST project.
A custom approach was selected in order to satisfy the stringent requirements of the active optics control system
resulting in a very low power, robust and compact solution. The tough metrology requirements were translated into an
analog front end capable of performing with high accuracy under the varying ambient conditions, mainly temperature.
Networking capabilities are embedded in the design to adapt to different distributed control configurations. All basic
applications and some additional uses are discussed, and test results are presented.
The Large Synoptic Survey Telescope (LSST) has recently completed its Final Design Review and the Project is preparing for a 2014 construction authorization. The telescope system design supports the LSST mission to conduct a wide, fast, deep survey via a 3-mirror wide field of view optical design, a 3.2-Gpixel camera, and an automated data processing system. The observatory will be constructed in Chile on the summit of Cerro Pachón. This paper summarizes the status of the Telescope and Site group. This group is tasked with design, analysis, and construction of the summit and base facilities and infrastructure necessary to control the survey, capture the light, and calibrate the data. Several early procurements of major telescope subsystems have been completed and awarded to vendors, including the mirror systems, telescope mount assembly, hexapod and rotator systems, and the summit facility. These early contracts provide for the final design of interfaces based upon vendor specific approaches and will enable swift transition into construction. The status of these subsystems and future LSST plans during construction are presented.
The Large Synoptic Survey Telescope (LSST) is a large (8.4 meter) wide-field (3.5 degree) survey telescope, which will be located on the Cerro Pachón summit in Chile. As a result of the wide field of view, its optical system is unusually susceptible to stray light; consequently besides protecting the telescope from the environment the rotating enclosure (Dome) also provides indispensible light baffling. All dome vents are covered with light baffles which simultaneously provide both essential dome flushing and stray light attenuation. The wind screen also (and primarily) functions as a light screen providing only a minimum clear aperture. Since the dome must operate continuously, and the drives produce significant heat, they are located on the fixed lower enclosure to facilitate glycol water cooling. To accommodate day time thermal control, a duct system channels cooling air provided by the facility when the dome is in its parked position.
The Large Synoptic Survey Telescope (LSST) is an 8.4 meter, 3.5 degree, wide-field survey telescope. The survey mission requires a short slew, settling time of 5 seconds for a 3.5 degree slew. Since it does not include a fast steering mirror, the telescope has stringent vibration limitations during observation. Meeting these requirements will be facilitated by a stiff compact Telescope Mount Assembly (TMA) riding on a robust pier and by added damping. The TMA must also be designed to facilitate maintenance. The design is an altitude over azimuth welded and bolted assembly fabricated from mild steel.
The 3.5-meter diameter Large Synoptic Survey Telescope (LSST) secondary (M2) mirror utilizes a 100mm thick
meniscus ULE™ blank completed by Corning Incorporated in 2009. Sub-aperture interferometry will guide the
polishing process to meet mirror structure function requirements. The convex asphere is actively supported by 72
axial actuators and 6 tangential links. These tangent links utilize an embedded lever system to meet the
requirements. The axial actuators have force limiting devices. The control system utilizes a higher level "outer loop
controller" for monitoring and commanding the tangent links and axial actuators. Numerous sensors determine the
assembly status. To prevent thermally induced image degradation, the interior air of the M2 cell is conditioned.
Results from determining the optical turbulence profile (OTP) on the LSST site, El
Peñon, located on Cerro Pachón (Chile) are presented. El Peñón appears to be an
excellent observatory site with a surface layer seeing contribution on the order of 0.15”
with most of this seeing being produced below 20m. These measurements also helped to
confirm that the telescope is elevated high enough above ground. As part of the LSST site
characterization campaign, microthermal measurements were taken in order to determine
the contribution of the surface layer turbulence to the atmospheric seeing. Such
measurements are commonly used for this purpose where pairs of microthermal sensors
mounted on a tower measure atmospheric temperature differences. In addition, the lunar
scintillometer LuSci was installed on El Peñon for short campaigns near full moon for the
same purpose. LuSci is a turbulence profiler based on measuring spatial correlation of
moonlight scintillations. The comparison of the results from both instruments during
simultaneous data acquisition showed a remarkable temporal correlation and very similar
The very short slew times and resulting high inertial loads imposed upon the Large Synoptic Survey Telescope (LSST) create new challenges to the primary mirror support actuators. Traditionally large borosilicate mirrors are supported by pneumatic systems, which is also the case for the LSST. These force based actuators bear the weight of the mirror and provide active figure correction, but do not define the mirror position. A set of six locating actuators (hardpoints) arranged in a hexapod fashion serve to locate the mirror. The stringent dynamic requirements demand that the force actuators must be able to counteract in real time for dynamic forces on the hardpoints during slewing to prevent excessive hardpoint loads. The support actuators must also maintain the prescribed forces accurately during tracking to maintain acceptable mirror figure. To meet these requirements, candidate pneumatic cylinders incorporating force feedback control and high speed servo valves are being tested using custom instrumentation with automatic data recording. Comparative charts are produced showing details of friction, hysteresis cycles, operating bandwidth, and temperature dependency. Extremely low power actuator controllers are being developed to avoid heat dissipation in critical portions of the mirror and also to allow for increased control capabilities at the actuator level, thus improving safety, performance, and the flexibility of the support system.
The Discovery Channel Telescope (DCT) is a 4.2-m telescope being built at a new site near Happy Jack, in northern Arizona. The DCT features a 2-degree-diameter field of view at prime focus and a Ritchey-Chretien (RC) configuration with Cassegrain and Nasmyth focus capability for optical/IR imaging and spectroscopy. Formal groundbreaking at the Happy Jack site for the DCT occurred on 12 July 2005, with construction of major facility elements underway.
The Discovery Channel Telescope control system incorporates very demanding requirements regarding fast serviceability and remote operation of the telescope itself as well as facility management tools and security systems. All system capabilities are accessible from a central user interface anywhere, anytime. Although the mature stage of telescope control technology allows focusing more on science rather than on telescope operation, the time and effort needed to integrate a large suite of software modules still impose a challenge to which reusing existing software is one of the answers, especially for advanced subsystems with distributed collaborative development teams. DCT's large CCD camera presents enormous computational problems due to the overwhelming amount of generated data. Properly implemented preventive maintenance and reliability aspects of telescope operation call for historical and real time data in order to determine behavioral trends and permit early detection of failure factors. In this new approach utility monitoring and power conditioning and management are integral parts of the control system. Proposed real time spectral analysis system of sound and vibration of key mount components allows tracking mechanical component deterioration that could lead to performance degradation. Survival control cells and unmanned operation systems are other options being explored for operation in harsh climatic conditions.