2010 kavli prize in Astrophysics
The Norwegian Academy of Science and Letters has decided to award the 2010 Kavli Prize in Astrophysics to Jerry E. Nelson, Raymond N. Wilson and James Roger Prior Angel.
"For their contributions to the development of giant telescopes."
- Oddbjørn Engvold (Chair), University of Oslo, Norway
- Andrew Fabian, University of Cambridge, UK
- Reinhard Genzel, Max Planck Institute for Extraterrestrial Physics, Germany
- Joseph Silk, University of Oxford, UK
- Scott Tremaine, Institute for Advanced Study, USA
Citation from the Committee
New telescopes and their instrumentation are at the heart of progress in astronomy. The size of the telescope primary mirror determines the light-gathering power and ability to detect and resolve the faintest and most distant objects in the universe. Jerry Nelson, Ray Wilson, and Roger Angel have pioneered the development of a new generation of large optical telescopes. The basic challenge for building a large telescope is to create a precise reflecting surface that focuses the light and is at the same time able to withstand the forces due to gravity, wind, and thermal changes. Until the work of Nelson, Wilson, and Angel, these requirements limited the primary mirrors of ground-based optical telescopes to a diameter of less than 6 meters for over four decades. With conventional technology, larger mirrors would have been too heavy, too costly, and not sufficiently stiff.
In the 1970s and 1980s, Nelson, Wilson, and Angel independently proposed and demonstrated truly innovative solutions that could overcome these hurdles. Remarkably, these advances enabled not only the construction of larger mirrors but also more sophisticated shaping of the mirror surfaces, leading to lighter telescopes and more compact telescope enclosures. All three telescope concepts have had outstanding successes, leading to a wide range of fundamental discoveries.
Jerry Nelson pioneered the use of segmentation in telescope primary mirrors, transforming the problem of mirror stiffness from the scale of the primary to the scale of the segment, and thereby reducing the required weight per unit area twenty-fold. This approach requires segments with individual complex surfaces, which are aligned and controlled precisely so as to act as a single coherent reflector. Nelson and his coworkers introduced several revolutionary technologies to accomplish these goals, including polishing optical surfaces under stress and aligning segments to an overall common shape with edge sensors and harnesses. The twin 10-m diameter Keck telescopes on Mauna Kea (Hawaii), in operation since 1992, were based on this pioneering technology. Nelson was the intellectual and technical leader of this audacious enterprise. The principles developed by Nelson have been applied in the 10.4-m Gran Telescopio Canarias and the 6.5-m James Webb Space Telescope, and form the basis for still larger telescopes planned for the future. New telescopes and their instrumentation are at the heart of progress in astronomy.
Ray Wilson pioneered the closed-loop computer-controlled telescope, a method known as active optics. This process, first implemented by Wilson and his colleagues in the 3.5-m New Technology Telescope, is a prerequisite for telescopes based on thin, flexible, “meniscus” mirrors. The key concepts of active optics involve continuous wavefront sensing, active collimation, and real-time reshaping of the primary mirror surface by actuators in the backup structure, permitting the entire telescope to be significantly lighter. Thin-meniscus active optics technology is the basis of the four 8.2-m telescopes of the European Southern Observatory’s VLT, in operation on Paranal (Chile) since 1998, as well as the two Gemini 8.1-m telescopes and the Subaru 8.3-m telescope.
Roger Angel pioneered the development of lightweight mirrors with short focal ratios. These mirrors have a thin reflector surface stiffened by a honeycomb backing, a technology which Angel extended to mirrors of large diameter by casting in a spinning furnace, warping the polishing tool, and other innovations. The 6.5-m MMT, in operation since 2003 on Mount Hopkins (Arizona), the two Magellan telescopes, and the 2x8.4-m LBT all contain Angel mirrors. This technology also enables the construction of large-aperture telescopes with a very wide field of view.
The expansion in the capabilities of observational astronomy led by Nelson, Wilson, and Angel will continue in the future through even larger and more powerful telescopes based on the concepts that they developed.
The 2010 Astrophysics Kavli Prize explained
Ever since Galileo turned his first telescope toward the heavens 400 years ago and found evidence that planets orbited the sun and not the earth, advances in telescope technology have allowed us to expand our horizons and learn our place in the universe. Using telescopes, astronomers have discovered new planets and moons in our solar system, revealed that our planetary neighborhood is just a small part of a vast galaxy, that our galaxy is just one of many billions across the universe, and that most objects in the universe are flying away from us at high speed because of its overall expansion.
By Daniel Clery
New ways of polishing lenses, or of casting and coating mirrors, have allowed astronomers to see farther and farther into space, and further back in time.
Because lenses can only be supported around the edge, when they get too big they start to sag under the effect of gravity. The lens in the Yerkes Observatory’s 40-inch (102 cm) refracting telescope, completed in 1897, was the largest used for research.
By the middle of the 20th century, reflecting telescopes ran up against the same problem. Mirrors were made of large slabs of glass with one face ground into a curved shape and coated with a thin layer of aluminium. But as the mirror was moved to point at different parts of the sky, gravity would distort its shape, ruining the optics. The 5-meter wide Hale Reflector, built in 1948, and Russia’s 6-metre BTA- 6, finished in 1965, were the last of their kind – the supporting structures needed to keep the mirrors rigid were too expensive to build and too massive to turn smoothly to follow the motion of the stars. Telescope technology progressed little for decades.
Jerry Nelson, Ray Wilson, and Roger Angel were all grappling with the same problem: how do you ensure that a mirror keeps a perfect reflecting surface while it is assaulted by gravity, wind, and changes in temperature? Each came up with a very different approach, but Angel’s was perhaps the most traditional. He strove to make mirrors much lighter, while maintaining rigidity, so that gravity could not get such a hold.
Angel started out by melting Pyrex dishes in a small potter’s kiln and testing its properties. This convinced him that cheap borosilicate glass was light, strong, and rigid enough for his purposes. He started making mirrors by casting them in a mold containing an array of hexagonal pillars to give the finished mirror a honeycomb pattern of holes. This reduces the mirror’s weight to one-fifth that of an equivalent traditional mirror while maintaining rigidity. But that was not all. While the glass was cooling and solidifying, he would spin the mold so that centrifugal force gave it a ready-made curved surface.
To get to the final desired curvature Angel developed a computer controlled polishing device whose tool could change shape while moving over the mirror to achieve complex aspherical surfaces. His work led to a renaissance in large-mirror telescopes. He cast 6.5-meter mirrors for the MMT telescope and the twin Magellan telescopes, plus two 8.4-meter ones for the Large Binocular Telescope. In 2008, for the Large Synoptic Survey Telescope, he cast a doughnut-shaped 8.4-meter mirror with a 5-meter mirror in the central hole.
Ray Wilson had a different solution to the problem. Instead of striving to make a mirror rigid against the effects of gravity, he decided to make mirrors that were thin, flexible, and lightweight, and then actively managing their shape with computer-controlled actuators attached to the back and supported by a rigid frame. A key innovation was the computer control system which, as the telescope moved, could sense errors in the wavefront moving through it and correct the mirror shape for the effects of gravity, wind, and temperature minute-by-minute--a technique known as active optics.
Wilson’s system was first tested in the European Southern Observatory’s New Technology Telescope, completed in 1989. That gave ESO the confidence to build its Very Large Telescope, made up of four 8.2 meter telescopes that could work together or independently. The twin Gemini 8.1-meter telescopes as well as the 8.3-meter Subaru Telescope all use so-called thin meniscus mirrors following Wilson’s scheme.
Jerry Nelson’s solution was more radical still. He abandoned the idea of a single, large mirror altogether and instead cast a large number of small hexagonal mirror tiles that could be put together to form a single reflecting surface. The approach was tricky because it meant that all the off-center tiles needed complex aspherical shapes. Nelson achieved this by bending the mirror blank with carefully positioned weights, grinding its surface into a spherical curve, and then letting it relax back into its natural shape producing the required aspherical curve on the polished surface. Once these roughly 1.5-meter tiles are put together to form a large mirror, their positions are actively controlled by computerized actuators to constantly maintain a perfect reflecting surface.
The twin 10-metre Keck telescopes were the first test of Nelson’s segmented mirror design—each with 36 tiles—followed by the 9.2-meter mirrors of the Hobby-Eberly Telescope and the South African Large Telescope, and the 10.4-meter Grand Canaries Telescope.
In awarding the 2010 Kavli Prize for Astrophysics to Nelson, Wilson, and Angel, the judges said they used “truly innovative solutions” to overcome the hurdles they faced, adding: “All three telescope concepts have had outstanding successes, leading to a wide range of fundamental discoveries.”
Their impact does not stop there, but will soon allow telescopes to make a leap to truly enormous sizes. Angel is already casting the seven 8.4-meter mirrors for the Giant Magellan Telescope which together will act as a single 24.5-meter mirror. Nelson is involved in the planned Thirty Meter Telescope, whose mirror will be made up of 492 segments. ESO’s planned European Extremely Large Telescope will also have a mirror of 984 segments, making it 42 meters across. And their influence even extends into space: the James Webb Space Telescope, the successor to Hubble, will have an 18-segment 6.5-meter mirror.