First M1 Roundels Produced by Coherent for TMT
Pasadena, CA - TMT and Coherent reached a new milestone this month with the completion of the first two roundels for TMT’s Primary Mirror (M1). Coherent precisely contoured and polished the optical surfaces of the roundels under the direction of TIO. They are the first of 231 roundels Coherent will provide for the TMT primary mirror array, nearly half of the total of 492 required to complete the telescope. 

“This milestone has been nearly three years in the making,” noted Greg Feller, Project Manager at Coherent. “We know that this project is important and is being closely monitored by our customer community as well as our own internal team.”
The two roundels underwent several final meticulous inspections to check the polishing quality. In addition to careful on-site visual and technical inspections, the data package for each roundel, including reports related to the manufacturing processes and metrology files detailing the roundels optical prescription, was reviewed.
Shipping preparation of first M1 polished roundels produced at Coherent facility in California - March 2021 - Following a detailed shipping and storage procedure plan, Coherent personnel carefully handle the large optics for TMT’s Primary Mirror -
Image credit: TMT International Observatory
On-site inspection of TMT roundels produced by Coherent Inc. in March 2021 - A highly qualified Coherent inspector verifies the roundel surface of each TMT finished roundel before it is sealed up in its packaging at the supplier’s facility -
Image credit: TMT International Observatory
TMT’s strict technical requirements and specifications have been verified and the roundels have been authorized to travel to the TMT storage facility in California.

Each roundel is 1.52m (59.8") in diameter, 45mm (1.77") thick, and weighs 0.25 metric tons (550 pounds), precisely cut and polished to an accuracy of less than 2 microns.
Precision manufacturing and quality are essential to the long-term success of these large optical components. TMT Telescope Optics Group Leader Ben Gallagher noted that the roundel polishing has been a great partnership between TMT and Coherent. The TMT and Coherent teams jointly refined a manufacturing process, called Stressed Mirror Polishing (SMP), which uses specially-designed fixtures to apply a calculated force that warps the roundels into the desired aspherical shape.

“We are happy and excited to have the production mirrors,” said Gallagher. When asked what’s next, Gallagher replied, “Coherent is gearing up for what we call the ‘Hexing’ process, where the roundels are fabricated into hexagonal segments. And each roundel converted to a hexagonal substrate will become a finished segment that will populate TMT’s Primary Mirror.”

Subsequently, Coherent will cut all the newly made roundels into the polished hexagonal mirror segments for the optic array. The TMT uses a hexagonal array of mirrors, but there are slight differences in the final perimeter of each component to ensure a precise mounted alignment with a mere 2.5mm (0.1") between each mirror. In total, 82 slightly different hexagonal shapes will need to be made, each serialized for quality tracing and alignment identification. Small pockets will be carved out behind the mirrors to allow room for mounting hardware, such as the electronic alignment sensors.
Inspection of the first TMT finished roundel at Coherent’s facility in California - March 2021 - Glen Cole, TIO Lead Optical Fabrication Engineer, visits Coherent’s production unit dedicated to TMT while observing strict safety protocols due to the Covid-19 pandemic -
Image credit: Coherent Inc.
TMT Roundels specially packed at Coherent, Inc. facility - TMT roundels manufactured by Coherent are the first TMT polished roundels produced in the U.S.                 
Image credit: Coherent Inc.
The Thirty Meter Telescope (TMT) Project has been developed as collaboration among Caltech, the University of California (UC), the Association of Canadian Universities for Research in Astronomy (ACURA), and the national institutes of Japan, China, and India with the goal to design, develop, construct, and operate a thirty-meter class telescope and observatory on Maunakea in cooperation with the University of Hawaii (TMT Project). The TMT International Observatory LLC (TIO), a non-profit organization, was established in May 2014 to carry out the construction and operation phases of the TMT Project. The Members of TIO are Caltech, UC, the National Institutes of Natural Sciences of Japan, the National Astronomical Observatories of the Chinese Academy of Sciences, the Department of Science and Technology of India, and the National Research Council (Canada); the Association of Universities for Research in Astronomy (AURA) is a TIO Associate. Major funding has been provided by the Gordon & Betty Moore Foundation.

For more information about the TMT project, visit, or follow @TMTHawaii.
Sandra Dawson
TMT Manager, Hawaii Community Affairs
 111 Nowelo Street,
Hilo, HI 96720
Phone (808) 284-9922