Pasadena, CA –The optical metrology and test set for TMT’s primary mirror segments has arrived at its warehouse in California. It was sent by Arizona Optical Systems (AOS) in Tucson, Arizona, on May 11 and arrived safely the day after at the TMT storage facility after a 500-mile journey. All fabrication and test tooling for the complete metrology system were also included in the freight.
The custom-made metrology set will be installed at a future optical test facility constructed by TMT’s metrology and finishing vendor, which will soon be finalized. The Optical Test Set will measure the off-axis aspheric shape of TMT’s mirror segments using a full-aperture reference surface to form a remote-cavity Fizeau interferometer. These measurements will reveal the surface optical prescription with nanometer accuracy, a precision required to guide the Ion Beam Figuring (IBF) final figuring and finishing process of all M1 optical surfaces. IBF is a contactless process which scans the mirror surface with a focused ion beam at variable velocity and dwell time to precisely trim away remnant material that would otherwise produce optical aberrations. The metrology hardware from AOS will work hand in hand with the IBF process to bring M1 segments to their accurate optical specifications.
TMT’s M1 segments are made of low-expansion glass ceramic CLEARCERAM®-Z and are hexagonal, each measuring 1.44 m wide across opposite corners and about 45 mm thick.
M1 segments come in 82 types with slightly different shaped front surfaces, depending on their planned location in the primary mirror array. The primary mirror segments are cast by Ohara Inc. glassmaker in Japan.
Once completed, TMT’s 30-m diameter primary mirror will comprise 492 hexagonal segments and will have a total light-collecting area of 663 m2. A total of 574 segments will need to be fabricated, including a spare set of 82 segments.
The first of the round primary mirror segments have been completed and will be cut to their hexagonal final shape in the coming months. Ultimately, all TMT mirror segments that have been polished and cut by TIO partners will be sent to a manufacturer for their final phase of surface finishing and optical testing.
“Congratulations to TMT and AOS staff on a very successful four-year partnership designing, fabricating, demonstrating and delivering this metrology system,” said Arthur Mihill, Senior Optical System Engineer who recently joined the TMT Optics team in April. “With delivery of the optical test set for the primary mirror segments, TMT has passed another milestone on the path to fabricating the telescope's primary mirror. Extremely precise measurements of each segment are necessary to drive the IBF process to remove the minute amounts of glass from the segment surface in just the right places to create a nearly perfect shape. This test-bed not only provides the needed precision but also the flexibility and efficiency to quickly measure any of the 82 different segment shapes required.”