April 24, 2017 Pasadena, CA - The TMT project has accepted 72 mirror blanks produced by OHARA, a Japanese optical glass manufacturer in Sagamihara near Tokyo, and polished by CANON Inc., under the direction of the National Astronomical Observatory of Japan (NAOJ). This official acceptance confirms the excellent quality of the blanks and is based on stringent verification of the technical specifications set for all 492 TMT primary segment blanks (plus an additional 82 spare units) that will ultimately make TMT’s 30-meter diameter primary mirror.
To receive this acceptance, the blanks were tested against many requirements such as the lack of flaws, cracks and non-uniformity in the glass which could be caused by machining or grinding. The chemical resistance, stability and coefficient of thermal expansion of the material were also tested under various conditions.
A set of 70 blanks will be delivered to the U.S. later this year and stored in northern California waiting for the next steps in processing. The other two blanks will be sent to Nanjing Institute of Astronomical Optics and Technology (NIAOT), where TMT Chinese partners will start their trial polishing activities to develop a robust industrial base for TMT mirror segment production in Nanjing, China.
The acceptance of the first mirror blanks produced by Japan represents a milestone for the TMT project. Japan will produce the entire stock of 574 meniscus blanks and perform surface polishing of 174 of them. The rest of the blanks will be polished within China, India and the United States, first as roundels, next hexagonal cut and further processed into the mirror segments that will comprise the TMT primary mirror.
Mirror fabrication within the partnerships
An efficient manufacturing process is required for the fabrication of TMT primary mirror segments:
Mirror production starts with casting of the CLEARCERAM®-Z material, a special glass ceramic that is an ideal material for telescope mirrors due to a nearly zero coefficient of thermal expansion and excellent polishability. OHARA has produced 213 blanks as of March 2017.
The flat blanks are first ground into a meniscus shaped blank, with a concave surface called the “segment optical surface.” To date, Okamoto, a company contracted by Canon, has manufactured 166 meniscus blanks, stored at the Canon’s Utsunomyia facility in Japan.
Using a conventional polishing machine, meniscus blanks are accurately polished into a smooth aspherical shape and are then referred to as ‘roundels’. Canon has polished 12 roundels so far in Japan. In the U.S., Coherent Inc. will be the supplier responsible for polishing blanks and transforming them into roundels.
After these first polishing steps, each roundel is cut and shaped into a hexagon; some pockets and reference features are ground into the back surface to mount the segment to the Segment Support Assembly (SSA), a mechanism that enables precise adjustments to ensure proper positioning of each mirror segment. The processes of roundel polishing, hexagonal cutting and segment support assembly mounting are developed in Japan, China, India and the United States. Harris Precision Optics in New York will receive U.S. roundels to create the polished mirror assemblies.
The polished mirror assemblies receive a final verification of their optical surface quality and ultra-fine correction is applied using precise bombardment from an ion source, also known as Ion Beam Figuring. Harris Precision Optics will gather all hexagons produced by partners and be responsible to apply the final figuring process of ion figuring.
About blanks and segments
Weighing 185 kilograms each, with a diameter of 1.52 meters and a thickness of 46.1 millimeters, each blank is identified by a unique serial number which is linked to a data package containing information such as the batch identification number, date of melt, coefficient of thermal expansion, and compliance with mechanical and quality requirements.
Each segment, when mounted to the telescope, will be separated from its neighbor by a narrow and uniform gap of 2.5mm. Once in operations, the position and orientation of the 1.44-meter wide hexagonal segments will be controlled so that the array functions as a single, highly accurate 30-m diameter mirror. Although each of the segments will have approximately the same thickness, their optical figure will be slightly different to account for the overall hyperboloidal shape of TMT’s primary mirror.
The layout of the 492 segments is made of six identical sectors, each of them composed of 82 segments with a unique hexagonal shape and optical surface configuration. An additional set of 82 spare segments will be manufactured, each segment unit being used during a two-year recoating cycle.
Molten glass ceramic poured into a mold for TMT primary mirror blank production. The first cast of blanks took place in 2013.
TMT primary mirror blanks prior to being ground to a meniscus shape.
TMT blank being polished into a smooth aspherical shape at Canon Inc. in Utsunomiya, Japan.
