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Mirror Segment Polishing Shines with New Methods

One of the most important milestones in the development and construction of the Thirty Meter Telescope (TMT) is the precision polishing of TMT’s primary mirror segments. Each of the 492 hexagonal segments, which measure 1.44 meters across from corner-to-corner, must have a surface that is accurate to 100 nanometers (about 1,000 times thinner than a sheet of paper).

Much of the technology to polish mirror segments was developed and perfected during the construction of the twin Keck telescopes on Mauna Kea. This technology is now the most successful means of polishing mirror segments. TMT, however, presents new challenges to the field because of the sheer volume of segments that need to be produced. So not only does TMT need precision mirror segments, it needs them to be produced in a rapid and consistent manner. TMT, as well as the European E-ELT project, are engaged with several firms in trial polishing activities in order to demonstrate a confident and robust industrial base for mirror segment production.

At the 2010 winter meeting of the American Astronomical Society in Washington, D.C., one of the potential suppliers for TMT’s mirror segments, Tinsley Laboratories, presented the latest results from their mirror polishing campaign. Their results demonstrate the potential of faster, more efficient polishing methods. Also, working with TMT, Tinsley was able to demonstrate good surface accuracy in a manner that approaches production requirements for speed and quality.

There still remains, however, the technological challenge of having all the mirror segments fit into the mosaic of TMT’s overall primary mirror. The reason this becomes a more-involved process is that the primary mirror will have a hyperboloidal shape. To achieve this shape and maintain perfect focus for the telescope, TMT will need segments with 82 different hexagonal shapes and optical prescriptions, with the most extreme prescription located on the outer edge of the primary mirror. This is perhaps the major technological hurdle in producing these segments because conventional polishing machines work best when they polish to a very uniform spherical shape.

To achieve the necessary shape at a reasonable speed and cost, TMT’s program with Tinsley uses “Stressed Mirror Polishing” (SMP), a technique developed by TMT Project Scientist Jerry Nelson for the Keck mirror segments. Tinsley employed this technique in their production of segments for the Keck mirror. For TMT, however, the SMP process has been refined by Tinsley through the development of a new polishing fixture and stressing process. This new system will enable conventional machines to polish precise amounts of astigmatism and coma (an off-axis focal point) into each segment.

The overall concept of SMP, however, remains identical to the work done by Nelson in which the mirror blank is bent (stressed) into an aspheric shape that is the opposite of that ultimately desired. Once stressed, the blank is accurately polished into a smooth spherical shape using a conventional polishing machine. After polishing, the stress is removed, and the mirror takes the desired aspheric shape. The SMP process allows for aspheric polishing using conventional polishing machines and large tools that produce smooth accurate surfaces. The Tinsley process shows how this proven technology can be scaled up to meet the quantity needed for TMT.

These results are very encouraging and represent a major milestone in the development program with Tinsley leading to meeting the demand for high-quality, low-cost mirror segments on a schedule that will accommodate TMT’s planned on-site construction start in 2011.

By Eric Williams, TMT Telescope Optics Group Leader

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