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.
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.
Coherent also designed a two-dimensional profilometer (2DP) to precisely measure and polish the desired surface figure of roundels. Japan and India partners are also using additional 2DP instruments designed by Coherent for the manufacture of their roundels. TMT’s remaining roundels will be provided by the Nanjing Institute of Astronomical Optics and Technology in China, which is developing its own SMP concept and machining technique.
Making all of these large components takes up a lot of space. Coherent recently completed a significant factory redesign in their Richmond, CA facility to accommodate the TMT project. The new TMT manufacturing bay provides the expanded space required for producing and finishing the polished roundels. It also accommodates additional support areas for building the equipment that supports TMT colleagues in India.