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
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
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.