The Vera C. Rubin Observatory Primary Tertiary Mirror (M1M3), together with the fully-assembled mirror support system, underwent two optical testing campaigns at the University of Arizona Richard F. Caris Mirror Lab. The objectives of the testing campaigns were: (1) optimizing the M1M3 surfaces with support forces, and (2) characterizing how the surfaces respond to actuator forces, including measuring the bending modes and single actuator influence functions. Both objectives were successfully achieved. The differences between the measured bending modes and the Finite Element Analysis (FEA) predicted modes were shown to be less than a few percent. The surface optimizations routinely resulted in Root-Mean-Square (RMS) surface errors below 30 nm for M1 and M3, simultaneously. The entire system was shown to be robust and repeatable. In this paper, we present the results of the optical testing and the analyses performed using the data acquired.
Large aperture telescopes require active control to maintain focus, collimation, and correct figure errors in the Primary Mirror (M1) due to gravity and thermal deformations. The Giant Magellan Telescope (GMT) M1 active optics subsystem consists of the hardware and software that controls the shape, position, and thermal state of each mirror segment. Pneumatic force actuators support the weight and control the surface figure while linear position actuators control the six solid-body degrees of freedom of each mirror segment. A forced convection system comprised of fan-heat exchanger units control the mean temperature and thermal gradient of each mirror segment. The M1 Subsystem design leverages existing technology and employs innovations driven by more demanding requirements compared to heritage systems. These differences led to the identification of three key GMT project risks: determining if the vibration environment induced by the fan-heat exchanger units and the error in the applied influence functions required to shape the mirror are within image quality budget allocations. The third risk is incorporating damping to the force actuators to meet the seismic requirements. GMT is currently mitigating these risks by integrating a fully functional off-axis M1 Test Cell at the University of Arizona’s Richard F. Caris Mirror Lab. This paper summarizes our requirements and design presented at the M1 Subsystem Preliminary Design Review in June 2019, describes our risk burn-down strategy for the M1 Subsystem, and presents our integration and test progress of the M1 Test Cell.
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 Richard F. Caris Mirror Lab is in the process of fabricating 8.4 meter mirror segments for the Giant Magellan Telescope. Seven of the segments are off-axis with 14 mm of aspheric departure. In order to successfully fabricate these mirrors we are constantly taking steps towards faster, more deterministic methods, from diamond generating to stressed lap polishing. The Large Optical Generator (LOG) is celebrating its 30-year anniversary at the University of Arizona with a complement of technological updates and enhancements. This paper shows how some of these upgrades will aid in the manufacture of the GMT segments.
The Large Optical Test and Integration Site (LOTIS) at the Lockheed Martin Space Systems Company in Sunnyvale,
CA is designed for the verification and testing of optical systems. The facility consists of a large, temperature
stabilized vacuum chamber that also functions as a class 10k cleanroom. Within this chamber and atop an advanced
vibration-isolation bench are the 6.5 meter diameter LOTIS Collimator and Scene Generator, LOTIS alignment and
support equipment. The optical payloads are also placed on the vibration bench in the chamber for testing. The Scene
Generator is attached to the Collimator forming the Scene Projection System (SPS) and this system is designed to
operate in both air and vacuum, providing test imagery in an adaptable suite of visible/near infrared (VNIR) and
midwave infrared (MWIR) point sources, and combined bandwidth visible-through-MWIR point sources, for testing
of large aperture optical payloads. The heart of the SPS is the LOTIS Collimator, a 6.5m f/15 telescope, which projects
scenes with wavefront errors <85 nm rms out to a ±0.75 mrad field of view (FOV). Using field lenses, performance
can be extended to a maximum field of view of ±3.2 mrad. The LOTIS Collimator incorporates an extensive integrated
wavefront sensing and control system to verify the performance of the system, and to optimize its actively controlled
primary mirror surface and overall alignment. Using these optical test assets allows both integrated component and
system level optical testing of electro-optical (EO) devices by providing realistic scene content. LOTIS is scheduled to
achieve initial operational capability in 2008.
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