TMT [NFIRAOS] Narrow-Field Infrared Adaptive Optics System

NFIRAOS for the Thirty Meter Telescope (TMT) will give ground-based astronomers a clearer view of the heavens than even the famous Hubble Space Telescope, and its successor the James Webb Space Telescope, provides. With this new capability, astronomers will learn more about the origins of the universe, galaxies, black holes and stars - and perhaps even discover new solar systems.

Conceptual layout of NFIRAOS on TMT's platform

NFIRAOS (Narrow-Field Infrared Adaptive Optics System) is being designed and built at the National Research Council of Canada's research facility on Observatory Hill near Victoria. Canada is supplying NFIRAOS as part of its contribution to the international TMT partnership. A plateau near the summit of Mauna Kea in Hawai'i was chosen for the TMT in summer 2009.

Work Package Manager & Systems Engineer: Glen Herriot

NFIRAOS Scientist: Paul Hickson, UBC

AO Science: Jean-Pierre Véran, Dave Andersen

Lead Mechanical Engineer: Peter Byrnes

Mechanical Designer: Paul Welle

Lead Optical Engineer: Jenny Atwood

Lead Software Engineer: Malcolm Smith

Project Overview

Although many of the instruments planned for the TMT have their own closely-coupled adaptive optics systems, TMT's facility adaptive optics (AO) system, NFIRAOS, feeds three instruments on the Nasmyth platform. NFIRAOS will compensate for natural distortions of the incoming light by Earth's atmosphere, collecting and correcting it before sending it to the instruments that take scientific data. NFIRAOS will also correct for wavefront aberrations caused by imperfections in the science instrument and telescope optics, thereby improving overall image quality.

To see more, astronomers build larger telescopes, but that only works up to a point. Once a telescope is built wider than about 25 centimetres in diameter, the images don't get any sharper, no matter how much bigger the telescope becomes. The problem is that pockets of warm and cold air blowing in front of the telescope act like lenses and prisms, spreading out the light and making it "dance" around. In a time exposure, a fuzzy blob is more apparent than sharp details - much like looking through glass shower doors.

Taking the "twinkle" out of the stars

In shower doors, the glass surface is wrinkled with thicker and thinner spots. In the atmosphere, light passing through warmer air speeds up compared with light that has gone through colder, denser air. A wavefront from a distant object is flat when it reaches the atmosphere, but gets wrinkled by the time it reaches Earth - just like the light through the shower doors gets distorted by the bumps on the glass. The difference is that atmospheric distortion is constantly changing as the wind blows, so starlight twinkles.

To fix the twinkle, NFIRAOS takes the beam from the telescope, rapidly diagnoses what's wrong with it, and corrects the light. The key component is a pair of flexible mirrors, bent to new shapes a thousand times a second. These mirrors are comprised of a thin sheet of glass mounted on a "bed of nails" - three to four thousand computer-controlled actuators that push and pull them from the back to match the shape of the distorted wavefront. After the light reflects from the mirror, its wavefront becomes flat.

A flat wavefront means that the light rays are all traveling in the same direction, and can be focused to a sharp point. The pictures are one hundred times sharper than without NFIRAOS. Thus astronomers can see tinier objects in more crowded regions of the sky, and detect much fainter, more distant things.

How does NFIRAOS know what to do? It diverts light from distant dots, produced by six lasers aimed high into the sky, into six wavefront sensors that measure the distortions. The measurements go to a specialized, high-speed computer that figures out the prescription and adjusts the flexible mirrors to get the best possible image of those laser dots. The rest of the light is sent onwards to a camera or spectrograph, where scientists observe the other, more interesting, astronomical objects in the image with unprecedented detail.

All of this magic happens in a 17-tonne machine costing $28 million. At the National Research Council of Canada, a diverse team of engineers from many disciplines - including mechanical, electrical, software, optical and systems design - collaborate with the TMT project staff in Pasadena, California, to make it all work.

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Related Information

NFIRAOS

Institutes:

• NRC Herzberg Institute of Astrophysics