Astronomers are searching for answers to fundamental questions about our universe.
Their next tool? A giant telescope thirty meters in diameter.
 The TMT will have a primary mirror with nearly 40x the area of the Hale Telescope. (Image: CalTech & Todd Mason) |
The image of an astronomer, eye to a giant telescope, is an icon of scientific investigation. Part of the romance of that symbol is the isolation of that lone figure, but more significant is its portrayal of someone removed from ordinary cares and focused on answering questions that almost define our humanity. Where did this world come from? What are the stars and how did they get there? Is anybody else out there? Although the iconic image of the lone astronomer at the telescope isn’t accurate, the search for answers to those fundamental questions is still at the heart of observational astronomy.
Every ten years the National Academy of Sciences (NAS) enlists a group of astronomers and astrophysicists to evaluate progress in astronomy and outline a course for future development. The 2001 report, Astronomy and Astrophysics in the New Millennium, highlighted five fundamental shaping questions:
• How did the universe begin and how is it evolving?
• How do galaxies first arise and mature?
• How are stars born and how do they live and die?
• How do planets form and change as they age?
• Does life exist elsewhere in the universe?
To gather data in support of answering those questions, the report suggested that the Hubble Space Telescope (HST) successor, now known as the James Webb Space Telescope (JWST), should be the top priority. Second on the list, and the top priority for ground-based astronomy: a 30-m-class telescope.
Eyes on the big questions
Nearly 14,000 ft above the ocean surrounding the big island of Hawaii, two 300-ton precision machines, also known as the Keck Telescopes, probe the secrets of the universe.
The Keck Telescopes are the world’s largest optical telescopes, with the primary mirror of each of the 25-meter-tall telescopes measuring 10m in dia. The mirrors are composed of 36 individual segments, each weighing 400 kg. To date, the Keck telescopes have measured the distance to the farthest galaxy, discovered a new class of stellar object, and found the smallest extrasolar planet. With an impressive list like that, it’s easy to imagine that the astronomical community could be satisfied with these observing capabilities. But throughout the history of astronomy each question answered leads to deeper questions.
That’s the motivation for the NAS decadal survey, and also why three other astronomical institutions were looking to the future at the same time. The Associated Universities for Research in Astronomy was investigating plans for what they called the Giant Segmented Mirror Telescope. The Association of Canadian Universities for Research in Astronomy was working on their Very Large Optical Telescope. And the Univ. of California and CalTech had partnered to design the California Extremely Large Telescope. Recognizing the priority of the NAS report and the common efforts they were engaged upon, the organizations combined to create the Thirty-Meter Telescope (TMT) project.
The TMT project combines elements of all the individual design strategies. Although the project is in its early stages, some key design decisions have already been made. First, and perhaps most surprisingly is that the primary mirror will be composed not of a single sheet of glass, but from individual segments.
One piece at a time
In general, the larger a mirror’s diameter, the thicker the mirror must be. The world’s largest monolithic mirror telescope, the 8.4 m diameter Large Binocular Telescope, has a primary mirror cast from 21 tons of glass. The reference design of the TMT mirror consists of 738 segments, each mounted on movable pivots to adjust its angle and position. In addition to making the weight manageable, the smaller segments also will come to thermal equilibrium faster than would a thicker substrate. Smaller segments also have the simple advantage that they are manufacturable. That doesn’t mean segment manufacturing doesn’t have its own challenges, but designers are incorporating manufacturability into the design process.
Correct seeing
Another critical technical evaluation currently underway is for the reference design of the adaptive optics system. Adaptive optics refers to the suite of sensors and actuators that detect and control optical aberrations induced by atmospheric distortion. The larger the telescope aperture, the greater the variation in atmospheric properties.
To take full advantage of the aperture size, nine times larger than the Keck Telescope, the adaptive optics system must be more capable than any that currently exist.
For example, laser guide stars are now being used to provide reference sources for adaptive optics systems. A laser tuned to sodium absorption lines illuminates a patch of sky. Sodium atoms, prevalent in a 10-km-thick layer of atmosphere about 90 km above the Earth, absorb and re-emit the light. The emitted light is detected by the adaptive optics and used to correct for the imperfections introduced by the atmosphere. But the TMT will detect not only aberrations of the path below the sodium layer, but also distortions introduced by variations of and within the sodium layer itself. So options for detecting multiple laser spots or separating the effects of different atmospheric layers are being evaluated.
Synergy question
But why go to all that trouble to correct for atmospheric aberrations when the JWST will be above those problems? There are several reasons why ground-based telescopes will continue to be important, but one critical aspect: synergy. The last decade offers a great example.
Supernovae of a specific kind, type Ia, explode with an intensity that is well understood, and constant. That is, one type Ia is as bright as another. So variations in their measured intensity is only a function of how far away they are. Ground-based telescopes survey the sky, identifying supernovae in distant galaxies. The HST then accurately measures their intensity, subtracting out light from their home galaxies. Meanwhile the Keck telescope, with its aperture area about 20 times larger than HST, can collect enough light to split it spectroscopically and separately detect different wavelengths. Together, these observations resulted in one of the most intriguing astronomical discoveries of the last decade: The rate at which the universe is expanding is changing. That, in turn, is evidence for “dark energy,” with far-reaching implications for the evolution of the universe.
That’s the kind of collaboration that astronomers would like to extend into the next decades. And the need for a ground-based partner of JWST is one of the primary arguments for building TMT.
—Richard Gaughan
