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«Co-Investigators Vic Argabright1, Bill Arnold11, David Aronstein3, Paul Atcheson1, Morley Blouke1, Tom Brown4, Daniela Calzetti5, Webster Cash6, Mark ...»

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ADVANCED TECHNOLOGY

LARGE-APERTURE SPACE TELESCOPE (ATLAST):

A TECHNOLOGY ROADMAP FOR THE NEXT DECADE

A NASA Astrophysics Strategic Mission Concept Study

May 2009

Dr. Marc Postman, Principal Investigator

Space Telescope Science Institute

Co-Investigators

Vic Argabright1, Bill Arnold11, David Aronstein3, Paul Atcheson1, Morley Blouke1, Tom Brown4, Daniela Calzetti5, Webster Cash6, Mark Clampin3, Dave Content3, Dean Dailey7, Rolf Danner7, Rodger Doxsey4, Dennis Ebbets1, Peter Eisenhardt8, Lee Feinberg3, Ed Freymiller1, Andrew Fruchter4, Mauro Giavalisco5, Tiffany Glassman7, Qian Gong3, James Green6, John Grunsfeld9, Ted Gull3, Greg Hickey8, Randall Hopkins2, John Hraba2, Tupper Hyde3, Ian Jordan4, Jeremy Kasdin10, Steve Kendrick1, Steve Kilston1, Anton Koekemoer4, Bob Korechoff8, John Krist8, John Mather3, Chuck Lillie7, Amy Lo7, Rick Lyon3, Scot McArthur1, Peter McCullough4, Gary Mosier3, Matt Mountain4, Bill Oegerle3, Bert Pasquale3, Lloyd Purves3, Cecelia Penera7, Ron Polidan7, Dave Redding8, Kailash Sahu4, Babak Saif4, Ken Sembach4, Mike Shull6, Scott Smith2, George Sonneborn3, David Spergel10, Phil Stahl2, Karl Stapelfeldt8, Harley Thronson3, Gary Thronton2, Jackie Townsend3, Wesley Traub8, Steve Unwin8, Jeff Valenti4, Robert Vanderbei10, Penny Warren1, Michael Werner8, Richard Wesenberg3, Jennifer Wiseman3, Bruce Woodgate3

AFFILIATION CODES:

1 = Ball Aerospace & Technologies Corp. 2 = Marshall Space Flight Center 3 = Goddard Space Flight Center 4 = Space Telescope Science Institute 5 = Univ. Massachusetts, Amherst 6 = University of Colorado, Boulder 7 = Northrop Grumman Aerospace Systems 8 = Jet Propulsion Laboratory, California Institute of Technology 9 = Johnson Space Flight Center 10 = Princeton University 11 = Jacobs ESTS Group @ MSFC Advanced Technology Large-Aperture Space Telescope (ATLAST)

TABLE OF CONTENTS

Executive Summary

1. Scientific Motivations for ATLAST

1.1) Does Life Exist Elsewhere in the Galaxy?

1.2) Exploration of the Modern Universe

1.3) Constraining Dark Matter

2. ATLAST Technical Overview

2.1) 8-meter Monolithic Mirror Telescope

2.2) Segmented Mirror Telescope Options

3. Top Three ATLAST Technology Drivers

4. Summary of the ATLAST Technology Development Plan

4.1) ATLAST Mission Life Cycle Cost Estimates

Acknowledgements

Appendix A: Table of Acronym Definitions

Appendix B: Synergy with Other Astronomical Facilities

Appendix C. Summary of the ATLAST-8m Engineering Team Study

Appendix D. ATLAST-8m Thermal Analysis

Appendix E: Solar Torque Mitigation Systems for ATLAST-16m

Appendix F. Optical Design and Active Optics System for the ATLAST-16m Concept..............34 Basic Optical Design

Active Optics System

Active Optics Architecture

Wavefront Sensing and Control: Initialization and Updates

Wavefront Maintenance Control

Wavefront Error Budget for ATLAST-16m

Appendix G: Summary of ATLAST-9.2m Design Study

Appendix H: Actuated Hybrid Mirrors

Appendix I: ATLAST Gigapixel Camera: Focal Plane Packaging and Electronics

Appendix J: Summary of Visible Light Detector Technologies for ATLAST

Requirements of the ATLAST Focal Plane Array

Detector Architectures

Conventional CCDs

e2v L3CCD™

“Impactron” from Texas Instruments

CMOS Arrays

P-channel, Fully-depleted CCDs

Future Work

Summary of Technologies

Appendix K: Evaluation of Coronagraphic Techniques for ATLAST

Classical Lyot coronagraphs (amplitude focal plane masks)

Multi-stage Lyot coronagraphs

Lyot coronagraphs with focal plane phase masks

Pupil apodization

Phase modification in the pupil

Shaped Pupils

M. Postman et al.

Off-axis vs. On-axis Lyot Coronagraphy and a Comparison to the VNC Performance with a Segmented Telescope

Appendix L: ATLAST Starshade Design and Technology

Starshade Sizing

Starshade Orbits

Starshade Targeting Efficiency

Starshade Fuel Consumption Considerations

Starshade Launch Vehicle Requirements

Appendix M: Exoplanet Science with ATLAST 8-m and 16-m Concepts

Appendix N: ATLAST Communications and Telemetry Considerations

Appendix O: Servicing Benefits for ATLAST

References

Advanced Technology Large-Aperture Space Telescope (ATLAST)

Executive Summary For four centuries new technology and telescopes of increasing diameter have driven astronomical discovery for the simple reason that astronomy is a photon-limited field. The Hubble Space Telescope (HST), to date the largest UV/optical astronomical space telescope, has demonstrated the breadth of fundamental astrophysics that can be extracted from space-based observations in the UV-optical-near IR. HST’s versatility has allowed it to be used to make pioneering discoveries in fields never envisioned by its builders. The paradigm-shifting discoveries in the next two decades will be made with ever more capable instruments and facilities. Here we outline the technology developments and mission concepts required for the next step – a highly versatile UV-optical-near IR observatory in space, larger and more capable than either HST or its IR-optimized successor, the James Webb Space Telescope (JWST).





Although substantial investments are required for the next steps, the basic technologies needed either already exist or we understand the path forward, allowing us to construct schedules, budgets, and the main decision points.

The Advanced Technology Large-Aperture Space Telescope (ATLAST) is a set of mission concepts for the next generation of UVOIR space observatory with a primary aperture diameter in the 8-m to 16-m range that will allow us to perform some of the most challenging observations to answer some of our most compelling questions, including “Is there life elsewhere in the Galaxy?” We have identified two different telescope architectures, but with similar optical designs, that span the range in viable technologies. The architectures are a telescope with a monolithic primary mirror and two variations of a telescope with a large segmented primary mirror. This approach provides us with several pathways to realizing the mission, which will be narrowed to one as our technology development progresses. The concepts invoke heritage from HST and JWST design, but also take significant departures from these designs to minimize complexity, mass, or both.

Our report provides details on the mission concepts, shows the extraordinary scientific progress they would enable, and describes the most important technology development items.

These include the mirrors, the wavefront sensing and control system, the starlight suppression system (for exoplanet observations and other high-contrast imaging applications), and the detectors. Experience with JWST has shown that determined competitors, motivated by the development contracts and flight opportunities of the new observatory, are capable of achieving huge advances in technical and operational performance while keeping construction costs on the same scale as prior great observatories.

The main body of this report consists of sections 1 through 4. These 18 pages provide a concise summary the critical findings and results of our study. Complete details on many of the investigations performed during this study are provided in the appendices that follow these initial sections. Three of the sub-reports are substantial in length and are, thus, provided as separate volumes on-line. These three separate volumes are 1) our detailed technology development plan for ATLAST, 2) a summary of the ATLAST-8m engineering and design study, and 3) a summary of the ATLAST-9.2m engineering and design study. To access these additional volumes, go to http://www.stsci.edu/institute/atlast and click on “ATLAST Mission Concept Study.”

–  –  –

1. Scientific Motivations for ATLAST Conceptual breakthroughs in understanding astrophysical phenomena happen when our observatories allow us to detect and characterize faint structure and spectral features on the relevant angular scales. By virtue of its ~12 milli-arcsecond angular resolution at ~500 nm coupled with its ultra high sensitivity, superb stability and low sky background, ATLAST will make these breakthroughs – both on its own and in combination with other telescopes with different capabilities. ATLAST has the performance required to detect the potentially rare occurrence of biosignatures in the spectra of terrestrial exoplanets, to reveal the underlying physics that drives star formation, and to trace the complex interactions between dark matter, galaxies, and the intergalactic medium. Because of the large leap in observing capabilities that ATLAST will provide, we cannot fully anticipate the diversity or direction of the investigations that will dominate its use – just as the creators of HST did not foresee its pioneering roles in characterizing the atmospheres of Jupiter-mass exoplanets or measuring the acceleration of cosmic expansion using distant supernovae. It is, thus, essential to ensure ATLAST has the versatility to far outlast the vision of current-day astronomers. We discuss briefly a small subset of the key scientific motivations for ATLAST that we can conceive of today.

1.1) Does Life Exist Elsewhere in the Galaxy?

We are at the brink of answering two paradigm-changing questions: Do other planets like Earth exist? Do any of them harbor life? The tools for answering the first question already exist (e.g., Kepler); those that can address the second can be developed within the next 10-20 years [1]. ATLAST is our best option for an extrasolar life-finding facility and is consistent with the long-range strategy for space-based initiatives recommended by the Exoplanet Task Force [2]. ATLAST has the angular resolution and sensitivity to characterize the atmosphere and surface of an Earth-sized exoplanet in the Habitable Zone (HZ) at distances up to ~45 pc, including its rotation rate, climate, and habitability. These expectations are based on our simulated exoplanet observing programs using the known stars, space telescopes with aperture diameters ranging from 2 m to 16 m, and realistic sets of instrumental performance parameters and background levels.

We start by selecting spectral type F,G,K stars from the Hipparcos catalog and identify, for Figure 1. The average number of F,G,K stars each telescope aperture, D, those stars whose HZ where SNR=10 R=70 spectrum of an Earthexceeds an inner working angle (IWA) of 3!/D at twin could be obtained in 500 ksec as a 760 nm (the O2 absorption feature). As a detection function of telescope aperture, D. The growth goal, we assume that each star has an Earth-twin in in the sample size scales as D.

its HZ (with "mag = 25), realizing that super Earths will be easier targets. We include plausible instrumental efficiencies and noise properties, and assume a 3-zodi background (local plus exosolar). We assume that our starlight suppression system (either internal coronagraph or external occulter) is capable of achieving a suppression of at least 25 mags (10-10) and include residual background from the star as an additional noise source. We then compute the number of stars for which an R=70 spectrum with signal-to-noise

Advanced Technology Large-Aperture Space Telescope (ATLAST)

ratio (SNR) of 10 at 760 nm could be acquired in 500 ksec or less. The results, averaged over different simulations done using various starlight suppression options (internal coronagraphs of various kinds as well as an external occulter), are shown in Figure 1. To estimate the number of potentially inhabited worlds detected, one must multiply the numbers in Figure 1 by the fraction of the FGK stars that have an Earth-sized planet in their HZ (#$ ) and also by the fraction of those exo-Earths that have detectable biosignatures. The values of these fractions are currently not constrained but their product is not likely to be close to unity. One must conclude that to maximize the chance for a successful search for life in the solar neighborhood requires a space telescope with an aperture size of at least 8 meters.

Estimates of the SNR of habitability and biosignature features in an Earth-twin spectrum, achievable with ATLAST, are shown in Table 1. For these calculations we use a fully validated model of the Earth’s spectrum [3,4], in combination with the observed visible reflection spectrum of the present Earth. We assume that the exoplanet is at maximum elongation and that the planet is observed for a length of time sufficient to achieve an SNR of 10, at a spectral resolution R = 70, in the red continuum. The Rayleigh (air column) signal is the blue enhanced albedo from atmospheric molecules. The O3 and O2 signals are biosignatures. The cloud/surface signal around 750 nm will vary with time as the planet rotates, and is therefore a rotation signature. The vegetation signal is the enhanced albedo of the Earth, from land plants, for wavelengths longer than ~720 nm [5], with a modest SNR. The H2O signal is a prime habitability indicator. Column 3 gives the width of the spectral feature. All of these SNR values can easily be improved with re-visits. In addition, ATLAST will allow us to glean substantial information about an exo-Earth from temporal variations in its features. Such variations inform us about the nature of the dominant surface features, changes in climate, changes in cloud cover (weather), and, potentially, seasonal variations in surface vegetation [6]. Constraints on variability require multiple visits to each target. The 8-m ATLAST (with internal coronagraph) will be able to observe ~100 different star systems 3 times each in a 5-year interval and not exceed 20% of the total observing time available to the community. The 16-m version (with internal coronagraph) could visit up to ~250 different stars 3 times each in a 5-year period. The 8-m or 16-m ATLAST (with a single external occulter) can observe ~85 stars 3 times each in a 5-year period, limited by the transit times of the occulter.

Employing multiple occulters would remove this limitation.



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