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Thirty Meter Telescope

The Project Manager's Corner: TMT Builds Its First Mirror
Gary Sanders, TMT Project Manager

April 2006

The Thirty Meter Telescope project has finished building its first mirror—a tiny flexible mirror that fits in the palm of your hand. Building and testing this small mirror means that the giant telescope project has passed a major technology hurdle. This tiny mirror may be the most challenging mirror in TMT.

A large primary mirror is the heart of any telescope. That mirror defines the optical quality of the telescope and the resulting image quality. The diameter of the primary mirror defines the maximum amount of light collected. This determines how well the telescope can take images of faint and far objects. Its diameter also defines how sharp a point the telescope can image, and how close two objects can be in the sky and still be seen to be separate by the telescope. This is the resolution of the telescope and that limit is set by the optical theory of diffraction. For the 30 meter-diameter mirror of TMT, the limit in imaging objects sharply will be nearly 5,000 times better than the human eye!

It should not be a surprise that the building of the mirror is a great event in the building of a telescope. Read The Perfect Machine: Building the Palomar Telescope, by Ronald Florence, for an account of the building of the epic Palomar 200-inch mirror and telescope. For a segmented mirror telescope like TMT, however, building the first segment is a smaller milestone. Since TMT is founded on the technology base of the Keck 10-meter telescopes, each with 36 mirror segments very similar to TMT, building the first TMT segment may not be such a great step. Together with spares, the Keck team built 84 segments. This same team is working with TMT toward building our 860 segments, including spares. The challenge is not building the segments, but building 10 times more of them at a very low price and very fast pace. So building our first segment is a minor step (though we will drink some champagne in celebration this year when we see our first segment).

Achieving resolution nearly 5,000 times better than a human eye depends upon the big primary mirror. But it depends even more critically on another mirror. A tiny mirror, about 30 centimeters (one foot) across. A deformable mirror.

This mirror has a high-quality optical surface. The thin glassy surface is mounted on about 4,200 tiny actuators that can push or pull at a point on the back of the mirror, thus distorting the surface up or down locally. This is the mirror that can compensate for the shimmering distortion that Earth’s atmosphere imposes on light waves arriving from a distant star or galaxy. This deformable mirror is the heart of adaptive optics technology. And adaptive optics is really the very heart of TMT. Yes, our big mirror is important. But TMT makes a great leap forward because it can sweep away the disturbances made by our atmosphere, and allow TMT to achieve diffraction limited performance at infrared wavelengths.

To understand how this works, you need a primer on adaptive optics. We have one on this website that is very well done, courtesy of the Gemini Observatory.

Click here to view the adaptive optics animation and watch the film several times. You can use your media player to interrupt it at each step and slow it down after you have watched it once or twice. This animation shows you step-by-step how adaptive optics works. And it shows you the crucial role of a deformable mirror. When you are done, come back to this column and we will continue with the story of our deformable mirror.

Now that you are back and expert in adaptive optics, you know that the surface of the deformable mirror has to be very agile. How agile? Our agility goal is set by our larger goal: to correct the arriving starlight after it is disturbed by the atmosphere well enough to achieve the resolution limit of the 30-meter telescope as if there were no atmosphere at all!

Light leaves a distant star heading out in all directions. The wavefront of one wave in the light forms a spherical surface leaving the star, like the spherical surface of an expanding balloon. By the time that surface travels a very great distance to reach Earth, the sphere is very large and any individual patch of area on the wavefront is quite flat. In other words, that wavefront arrives as a very flat plane with all of the rays of light in step (in phase), having traveled essentially the same distance. All of this is true at the top of the Earth’s atmosphere.

If there were no atmosphere, the wavefront would arrive at TMT and TMT would act like a 30-meter diameter cookie cutter and slice out a 30-meter diameter disk in the wavefront and focus it with the primary mirror, and eventually our instruments would see an image that is very sharp indeed, defined by this perfect wavefront in a disc.

There is an atmosphere. High in this atmosphere there is turbulence. There are regions of the atmosphere that are warmer and colder. The warmer regions are less dense and so light sees a different index of refraction. That means that it travels at a slightly slower speed for more dense regions of the atmosphere, and faster for the lower density, warmer regions. Turbulence mixes the denser and more rarified patches of sky so that any ray of light may experience a different mixture of transit speeds compared to other rays of light.

Due to this effect, the very flat plane wavefront arrives at the surface of the Earth with some portions a bit ahead and some a bit behind. Different rays of light have experienced different optical path lengths, and so they arrive out of step.

"…Twinkle twinkle, little star. How I wonder where you are…"

The image is now blurred and squirming around in the focal plane of the telescope. Look back at the animation discussed earlier. Look at the star image on the computer monitor before the AO system is turned on and the deformable mirror performs its bit of wizardry. Squirming around.

The distorted wavefront arrives like a jiggling potato chip and the deformable mirror in the path jiggles in the opposite way by just the right amount to restore near flatness to the wavefront. Voila, a sharp image approaching the limit set by diffraction theory. The telescope behaves as if the atmosphere had been swept away.

TMT needs such a mirror with about 4,200 actuators. The actuators are about 5 millimeters apart in a regular array, and they have to be able to push and pull the mirror surface up and down by about 8 to 10 microns (a human hair is about 70 microns thick). Remember that the surface of the mirror is a glass-like material. When one actuator pushes up, the nearby actuator spots must not move much. The surface must be flexible. And the entire device must operate at -30 degrees Celsius, a very cold temperature where materials stiffen and actuators are sluggish. If such a mirror could be built, TMT can correct the 30-meter diameter plane wavefront to about one-thousandth of the thickness of a human hair.

Mirrors like this, with over 4,000 actuators packed closely together that can reliably push and pull this far at very cold temperatures, do not yet exist. The best deformable mirrors fall short of these requirements. But TMT cannot operate well without such a deformable mirror.

We have built the first mirror of this type. More correctly, our industrial partner, CILAS (a French company whose name stands for, in English, "Industrial Laser Company") designed the full 4,200-actuator version shown in Figure 1. They then built a portion of this mirror sufficient in size to demonstrate the performance (with 57 actuators) but with lower cost than the full mirror, and tested it. The mirror passed all of the required tests, even at the cold temperatures. A photograph of the mirror is shown in Figure 2.

This is a very important development. We know how to build our primary mirror segments. And now we know that we can build the crucial adaptive optics mirror to use at first light for the telescope.

— Gary Sanders

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Figure 1

Figure 2

 
 
The TMT project is a collaboration of Caltech, University of California (UC) and the Association of Canadian Universities for Research in Astronomy (ACURA). © Thirty Meter Telescope
 

 

Association of Canadian Universities for Research in Astronomy