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The TMT Primary Mirror System Successfully Passes Final Design Review

PASADENA (July 26-27, 2018) – The Thirty Meter Telescope Optics Group recently achieved a critical milestone by passing its Primary Mirror Optics System (M1S) final design review. The review board, comprised of international reviewers and observers from various institutions including ESO, ITCC, JPL, JWST/Ball Aerospace, KECK, NAOJ, NIAOT, and the TMTPO*, met at TMT headquarters in Pasadena.

The expert review committee recognized the technical maturity of the M1S design since the preliminary design review. The panel noted the extensive and clearly documented progress made by the M1S team; the M1 system having significantly matured in all aspects of design, prototyping and systems engineering. The program will now enter the construction phase. The fabrication of M1S is an international effort with TIO members from the USA, India, Japan, and China making significant contributions to the M1S. 

TMT M1 Optics System is composed of the TMT primary mirror glass segments and the glass-mount assemblies. The TMT primary mirror is a 30-meter-diameter hyperboloid segmented mirror comprised of 492 hexagonal aspheric segments, each approximately 45mm thick and 1.44m wide across opposite corners. Each segment is separated from its neighbors by a 2.5-millimeter uniform gap, which is necessary to prevent contact due to changes of telescope elevation, temperature variations, manufacturing and alignment errors. 

The mirror has six-fold symmetry, with 82 unique segments made from low expansion glass-ceramic CLEARCERAM^®-Z. A seventh set of 82 segments will be provided as spares, which are needed to allow segments to be swapped out for recoating. In the end, a total of 574 Segments will be manufactured. The polished and coated mirror segments are each supported on a Segment Support Assembly (SSA) to form a primary segment assembly, itself supported by the steel mirror cell structure. 

Each of the 82 unique segment types has a slightly different surface shape. Axial support is provided by a 27-point whiffletree, and lateral support is provided by a central diaphragm mounted in a pocket machined into the back of the mirror segment. The moving frame structure of the mirror provides a stiff platform for the axial and lateral supports.

An automated warping harness with 21 actuators per segment will provide the ability to remotely control the surface figure of each segment to correct for effects, such as coating stresses, figuring and lateral position errors, and temperature fluctuations altering the mirror shape. The positions and tip/tilt of the segments will be controlled continuously so that the array functions as a single, continuous mirror. The Primary Mirror Control System (M1CS) will maintain the alignment and phasing of the segments and control the active support systems. As the telescope changes elevation angle, the varying gravity vector causes small, millimeter-level deformations of the mirror cell structure. M1CS will compensate for the disturbance, and other effects, by controlling each segment, thereby maintaining the optimal optical surface shape of M1 within a few nanometers. 

Given the number of segments in TMT (492 + 82 spares), cost control is a major consideration in the design process and very stringent requirements apply on fabrication costs for every subsystem. In the case of the SSA, the replication of an identical assembly gives an opportunity to consider a mass production approach, which will help control costs. TMT’s stringent design requirements were verified throughout the manufacturing process by a strong quality assurance approach and a rigorous verification test program to catch any possible defects before they propagate into the final product. 

“The team has done a good job updating the design drawings and models based on the inputs received from partners and vendors. We recognize a tremendous team effort, representing years of hard work, with lots of contribution from many other groups, particularly the System Engineering group, the Environment, Safety & Health group, the Controls group, and several international industrial partners who were also involved,” said Fengchuan Liu, TMT Deputy Project Manager.

*ESO: European Southern Observatory
ITCC: India TMT Coordination Centre
JPL: Jet Propulsion laboratory
JWST: James Webb Space Telescope
NAOJ: National Astronomical Observatory of Japan
NIAOT: Nanjing Institute of Astronomical Optics and Technology
TMTPO: Thirty Meter Telescope Project Office

TMT optics and system engineers and international review committee photographed at TMT’s headquarters in Pasadena on July 26, 2018 during M1S Final Design Review phase 1. Image Credit: 51³Ô¹ÏPro/Fred Kamphues