Adaptive optics component—key to "world's most advanced optical telescope"—passes critical test

The Thirty Meter Telescope (TMT) has marked "a major milestone on its way to becoming the world's most advanced and capable optical telescope," according to the TMT project team. An essential part of the telescope's adaptive optics (AO) system has been successfully tested and is ready to become actual hardware.

The AO component, known as the Tip-Tilt Stage, will work in tandem with a deformable mirror to correct for the blurring of Earth's atmosphere. "TMT is quickly developing the technologies that will enable it to see distant objects in the Universe as clearly as if the telescope were in space," said TMT Adaptive Optics Group Leader Brent Ellerbroek. "The Tip-Tilt Stage is essential for achieving the highest possible image quality, as needed to optimize the resolution and sensitivity of scientific observations with TMT."

Adaptive optics systems sense atmospheric turbulence in real-time. They then adjust the optics of the telescope many hundreds-of-times each second to erase the distortion caused by light passing through Earth's atmosphere.

The deformable mirror being designed for TMT will be connected to 3,000 actuators that push and pull the mirror's surface up to 800 times each second. This flowing and rippling of the mirror's surface reshapes the light-waves entering the telescope. Multiple wave-front sensors will ensure the mirror precisely changes shape to counteract the blurring of Earth's turbulent atmosphere.

Though extremely precise, the deformable mirror actuators do not have the range available to correct for the overall image motion that is one important part of the distortion. To correct for this motion (referred to as "tip-tilt"), the mirror itself—which weights 40 kilograms (88 pounds)—must change its orientation at least 20 times each second. The accuracy of these adjustments must be very precise, corresponding roughly to the angle that a star appears to move across the sky in about one-ten-thousandth of a second.

"Although tip-tilt mounts for deformable mirrors have been successfully demonstrated before," said Ellerbroek, "the TMT design is exceptional in terms of the size of the mirror, the mass of the mirror, and the 3,000 electrical connections that are attached to the back of the mirror structure."

The Tip-Tilt Stage was developed by TMT's industrial partner CILAS (Orleans, France). CILAS began work on the stage in April 2007 and the completed unit was ready for testing in September of 2008. "All critical performance characteristics of the stage have now been successfully demonstrated," said Ellerbroek. "The stage has actually exceeded our specifications, and will be able to correct for image motion at rates of up to 100 times per second. These results are one of the most exciting examples of the continuing progress being made by the TMT AO program."

The stage is now at the Canadian National Research Council's Herzberg Institute of Astrophysics in Victoria, Canada, the TMT associate responsible for integrating the Tip-Tilt Stage into the overall AO system

Controlling the overall shifting and warping of the mirror is a significant technological feat. Fortunately, scientists and engineers at TMT and its suppliers have also successfully developed and tested the specialized programs and electronics (field-programmable gate arrays) that will derive the correct commands from the measurements provided from the adaptive optics wavefront sensors. Roughly 40,000 measurements will need to processed into 7,000 commands up to 800 times each second, requiring a total of up to 200 billion calculations for each tick of the second hand.

As of April 2009, the TMT will have moved from the design into its early construction phase. TMT plans to initiate on-site construction as early as 2010, with 'first light' in early 2018.

The TMT project is a partnership among the California Institute of Technology, the University of California, and ACURA, an organization of Canadian universities. The Gordon and Betty Moore Foundation provided $50 million for the design phase of the project and pledged an additional $200 million for the construction of the telescope. ACURA committed an additional $17.5 million for the design and development of TMT. The National Astronomical Observatory of Japan has also joined TMT as a Collaborating Institution.

For more information see the Thirty Meter Telescope project's website.