Chapter 1

Executive Summary

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This report represents the culmination of an 18-month effort by AURA's New Initiatives Office (NIO) to conduct a study of a Giant Segmented Mirror Telescope (GSMT) - the next generation optical/infrared telescope recommended by the National Research Council (NRC) decadal survey as its highest priority ground-based program for federal investment between 2001 and 2010. The study comprises three major elements:

DEVELOPING A SCIENCE CASE AND INITIAL PERFORMANCE REQUIREMENTS

A GSMT equipped with an efficient wide-field multi-object spectrograph will enable surveys of millions of galaxies similar in luminosity to the Milky Way in the redshift range z > 3, thus enabling construction of a well-sampled three-dimensional map of the large-scale structure defined by galaxies in the early universe. A survey of the intergalactic gas distribution with similar resolution will be possible via high resolution spectroscopy of 100,000 quasars.

The science enabled by the exquisite images that can be delivered by a 30-m diameter telescope is truly remarkable. By concentrating near-infrared light into an image with a core of 0.02 arcsecond size, GSMT will be able to measure the internal motions, chemical composition, and star forming activity in the first gravitationally bound star-bearing systems to form following the big bang. This would provide the essential tool for analyzing these "building blocks" for the spiral and elliptical galaxies familiar to us today. The GSMT's superb imaging quality will also provide for the first time the key to resolving the crowded star fields into individual stars in galaxies as far away as 10 million light years. Analysis of the brightness and color for samples of millions of such stars will reveal the distribution of chemically and chronologically distinct stellar subsystems - the presumed relics of the pre-galactic building blocks that merged together to form galaxies like the Milky Way.

Its power to image fine detail in high contrast scenes and to feed concentrated light from faint sources into powerful spectrographs will enable GSMT to image and analyze planets and zodiacal dust clouds around hundreds of nearby stars. GSMT will yield direct insight into the diversity of architectures characterizing extra-solar planetary systems, the physics and chemistry of extra-solar planetary atmospheres, and the nature of weather on other worlds. For the first time, it will be possible to study a sample of thousands of newborn stars to locate planets in the process of formation, to understand where and when planets of different masses form, and to determine whether planets similar to earth are likely to survive at distances from their parent suns favorable to the development of life.

Adaptive optics and coronagraphy on GSMT will enable detection of giant planets with orbital radii of 1 astronomical unit and greater for all stars within 10 pc of the earth, and inference of their atmospheric structure and chemistry.

Mid-infrared spectroscopy with GSMT will enable detection of optically thin molecular gas in "gaps" created by planets forming in circumstellar accretion disks surrounding young stars of known age, enabling inference of where and when giant planets begin to form.

At optical wavelengths, GSMT's photon-collecting power will enable simultaneous spectroscopic measurements of thousands of galaxies, thereby providing a stereoscopic "image" of how young galaxies and intergalactic gas are distributed in the early universe - and how the large scale structures defined by these probes are linked to the inhomogeneities - manifest in the cosmic microwave background - built in during the instants following the big bang .

The groundbreaking science enabled by GSMT requires a telescope that (1) delivers extremely high quality (near-diffraction-limited) imaging in the near-IR and wide field coverage in the visible, and (2) incorporates instruments - cameras and spectrographs - that fully exploit its potential.

Initial performance requirements for the GSMT point design derive directly from the science drivers and are summarized in the Science Requirements. They provide for a telescope that delivers multi-object wide-field performance at optical wavelengths and near-diffraction-limited performance in the near-IR over modest fields of view.

A POINT DESIGN FOR A 30-M TELESCOPE

The NIO has developed and analyzed a point design to identify the most significant technical challenges, risks, and cost drivers. This design builds on the heritage of large, precise radio antennas, and on the experience gained over the last decade in building the current generation of compact, low-weight, optical/infrared telescopes. Its optical system is a fast (f/1) focal ratio Cassegrain design. The primary mirror surface comprises 618 segments, each a bit larger than a meter in size, whose relative positions are orchestrated by a computer-controlled system comprising thousands of sensors and actuators. Together, the components of this system will hold the optical figure to a precision of nanometers.

The point design also serves as a test bed for simulating and evaluating the performance of a complete telescope.

Design of a next generation telescope is above all a systems challenge, requiring an integrated approach that takes into account: (1) site characteristics; (2) enclosure design; (3) structural design; (4) integration of active and adaptive elements with a sophisticated control system; (5) fabricating, polishing, controlling, and maintaining the segmented primary mirror surface; and (6) instrumentation. Key elements of the point design include:

  • 30-m, filled aperture
  • Fast (f/1), aspheric, segmented primary mirror
  • Hexagonal segments of size ~ 1 m
  • 2-m diameter adaptive secondary
  • Radio telescope structural design, with primary mirror above elevation axis
  • Multiple instrument ports: prime, Nasmyth and Cassegrain foci
  • Four adaptive optics (AO) modes
  • Control systems spanning bandwidths from 0.01 to 200 Hz

The scale and performance requirements of GSMT instruments will present new challenges for our astronomical communities. These instruments will rival the complexity and costs of major space- based measuring devices. New paradigms will have to be developed to produce "30-m class" instruments, involving partnerships that engage universities, national and international scientific centers, and industry to ensure that the key instruments are developed and delivered to GSMT within a defined timeline and budget.

The NIO has developed conceptual designs for five instruments:

TECHNICAL STUDIES TO ENABLE A GSMT

NIO has devoted a considerable fraction of its efforts to technology development studies critical to the design and siting of a 30-m class telescope. These include: quantifying the effects of wind on the GSMT structure; measuring atmospheric sodium layer characteristics critical to GSMT performance; developing technology and advanced control algorithms for adaptive optics; and evaluation of potential sites.

GSMT must retain its image quality while its primary mirror surface and structural elements are buffeted by wind gusts and subject to changes in temperature and the pull of gravity. Meeting these challenges will require detailed real-time analysis and control of the complex interaction of the thousands of elements that comprise the GSMT system, whose sheer size makes it far more vulnerable to external disturbances.

The nearly theoretically perfect images required to achieve many of GSMT's science goals (0.02 arcseconds at 2 microns wavelength) can only be realized using AO: the real-time correction for the continuously changing distortions of light by the earth's turbulent atmosphere. This "book" provides a detailed quantitative description of how such systems can be designed, with particular emphasis on design of an adaptive system that employs a powerful tomographic technique for reconstructing the three-dimensional turbulence.

The NIO has developed designs for and modeled the performance of four AO systems:

  • A system that corrects for the effects of ground-level turbulence and provides enhancements compared to natural seeing images over wide fields
  • A multi-conjugate adaptive optics (MCAO) system that uses laser and natural guide stars to provide 0.5 Strehl images at 2 microns wavelength over a 2 arcmin field of view
  • A Cassegrain AO system that delivers 0.9 Strehl images at 5 microns wavelength on axis via use of an adaptive secondary
  • A high performance, on-axis AO system designed to feed a coronagraph capable of imaging scenes with contrast ratios exceeding 10 million

Implementing the required suite of AO systems will necessitate the coordinated development of "kilo-actuator" systems, new and more powerful laser technologies, and fast real-time computer-control algorithms that are between 10 and 100 times more complex than AO systems in use today.

NEXT STEPS

NIO investments and ongoing activities will be guided by its principle mission: enabling a GSMT for the US community and possible partners, a central recommendation of the NRC decadal survey. During the next year, NIO will devote its primary efforts to working with the US community and potential international partners to develop a consensus regarding the highest priority scientific missions of GSMT in the JWST and ALMA era, and developing both science-driven engineering requirements and a "merit function" aimed at guiding system design choices so as to optimize GSMT's performance per dollar invested.

In parallel, the point design will be used to continue the process of evaluating the performance of a 30-m class telescope and identifying the critical areas of design that will make it possible at affordable cost.

In Spring, 2002 the NSF asked NIO to form a community-based GSMT Science Working Group charged with developing requirements and goals that represent a community consensus regarding the performance that GSMT must achieve in order to meet its scientific aspirations.

The SWG will report its findings to the NSF by Spring 2004. During this period, NIO will continue investment in technical and fabrication studies aimed at finding low-cost solutions to the challenges that face all efforts to make a 30-m telescope a reality within the next decade, and will work with the community to evolve partnerships to fund the design and construction of a GSMT.


November 2002