A New Concept for the World's Most Powerful Telescope

by Jeff Kuhn

A conceptual drawing of the High Dynamic Range Telescope.

Our understanding of the universe has changed in response to recent discoveries, like the detection of planets around other stars, the peculiar motion of extremely distant galaxies, and the explosive energy sources at vast distances. These discoveries stretch our imaginations even for description. The UH Institute for Astronomy (IfA) is developing the concepts for the next generation of tools to be used for exploring these frontiers.

For more than a century, the world's largest telescopes have each doubled the size of their predecessors by incorporating the newest advances in technology. The IfA is engaged in a mission to design and build an optical/infrared telescope that takes the next step: the High Dynamic Range Telescope (HDRT). It will have revolutionary optical capabilities and many times the light-collecting power of existing telescopes.

Unlike previous large telescopes, the HDRT combines the ability to see faint and distant galaxies with a tool of exquisite sensitivity for searching for planets around other stars. The HDRT can address these problems because the path that light takes through the telescope can be rapidly changed so that large parts of the sky can be observed at the same time that very high spatial resolution, and fine detail, studies are performed.

To fully realize the HDRT's spectacular imaging powers, it should be built on Mauna Kea, the world's best site. The HDRT would use technology that reduces its visual impact from sea level, making it less visible than existing telescopes on the summit, and it would be placed on a "recycled" telescope site, so it would not have an impact on new, undis-turbed summit areas.

As it is now envisioned, the telescope would be about 100 feet in height, its mirrors would be 80 feet across, and the moving mass of the telescope and its structure would be about 350 tons. The light-collecting area of the HDRT would be over 350 square yards, and it would have optical resolution of about 0.005 arcseconds, sufficient to see a basketball on the surface of the Moon.

A Keck-style 54-segment hexagonal mirror (below left) has
many more edges than the proposed HDRT arrangement
of six 8-meter primary mirrors (right).

Single mirrors cannot be made large enough for the next generation of telescopes. The HDRT will capitalize on the advantages of using six large mirrors that have the largest possible area-to-edge ratio. This approach minimizes light scattering and the number of edge supports that are needed to actively control each mirror surface, and it will lead to images of unrivaled clarity. Since the atmosphere disturbs the incoming light wavefront and distorts the image, it is important that any new telescope be designed to correct for this distortion. The HDRT is designed with an adaptive optics (AO) system to correct for the atmosphere. Unlike telescope designs based on large numbers of smaller hexagonal elements, the HDRT design responds to the important question, "How should mirror segments in a large optical/infrared astronomical telescope be arranged to maximize the image clarity?" by allowing for the best possible adaptive optics system.

A hexagonal pattern of circular mirrors with a spacing 4 percent larger than the diameter of each mirror nearly reproduces the resolution and performance of a single large mirror of that diameter. This "magic" ratio describes the placement of 8-meter (26-foot) mirrors in the HDRT pupil plane. Since its building blocks are now "conventional" 8-meter mirrors - the size of the mirrors of the Gemini and Subaru Telescopes - it is straightforward to design an adaptive optics system. This technology solves one of the leading problems facing large telescopes: how to make an AO system work on a large telescope.

Another fundamental advantage of the HDRT is the versatility of its "open" design. Since its mirrors do not actually touch each other, it is possible to design an efficient mechanical system that both supports the mirrors and instruments, and allows for great flexibility in adding secondary optics. For example, the HDRT will be unique in its ability to operate in a wide-field mode (perhaps with a field of view as large as 3 degrees) while serving as a narrow-field imaging and coronagraphic telescope.

The IfA is now engaged in a conceptual design effort that brings together some of the best and most creative scientists and engineers with telescope experience. This effort is expected to last about one year and will culminate with a fundraising campaign to generate support for a Phase A engineering design study. The IfA has already begun exploring other scientific and financial partnership opportunities that could support the HDRT program.

For further information about the HDRT, contact IfA Director Rolf-Peter Kudritzki or Dr. Jeff Kuhn. The Web site http://www.ifa.hawaii.edu/users/kuhn/hdrt.html also describes elements of the HDRT.