Submitted Proposal to NASA's Astrophysics Strategic Mission

Large-Aperture Segmented Mirror Telescope Design Concept

(Image courtesy of John Frassanito & Associates, Inc. and the Future In-Space Operations Working Group)

Advanced Technology Large-Aperture Space (ATLAS) Telescope: A Technology Roadmap for the Next Decade

Principal Investigator: Dr. Marc Postman, STScI

Submitted in Response to NASA ROSES “Astrophysics Strategic Mission Concept Studies” (NNH07ZDA001N-ASMCS)

November 20, 2007

Table of Contents 1 The Imperative for a Large-Aperture UV/Optical Space Telescope .......................................2 2 Science Objectives and Expected Impact of an ATLAS Telescope ........................................3

2.1 Science Requirements Flowdown ...................................................................................6 3 Telescope Architecture..........................................................................................................7

3.1 Optical Telescope Assembly (OTA) Design ...................................................................7 3.2 Wavefront Sensing and Control (WFS&C).....................................................................8 3.3 Mirror Fabrication and Technologies..............................................................................9 3.4 Telescope Baffles .........................................................................................................10 3.5 Spacecraft Design.........................................................................................................11 3.6 Starlight Suppression....................................................................................................11 3.7 Science Instruments......................................................................................................11 3.8 Servicing Options and Mission Lifetime.......................................................................12

4 Cost Estimation Methodology .............................................................................................13 5 Technology Roadmap .........................................................................................................14 6 Concept Study Execution ....................................................................................................15

6.1 Statement of Work and Deliverables.............................................................................15 6.2 Management of Study...................................................................................................16 6.3 Schedule.......................................................................................................................16 6.4 Budget..........................................................................................................................16

7 References ..........................................................................................................................17 Biographical Sketches…………………………………………………………………………...18 Current & Pending Support……………………………………………………………………...57 Statements of Commitment……………………………………………………………………...67 Letters of Support………………………………………………………………………………104 Budget Justification: Narrative & Details……………………………………………………....109

Advanced Technology Large-Aperture Space Telescope: A Technology Roadmap for the Next Decade

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Use of information contained on this page is subject to the restriction on the title page of this proposal.

1 The Imperative for a Large-Aperture UV/Optical Space Telescope The greatest leaps in our understanding of the universe typically follow the introduction of radically new observational capabilities that bring previously unobserved phenomena into view. Some, such as the unambiguous detection of life on an Earth-like planet orbiting another star, will be profound yet conceivable. Others are entirely beyond our imagination. All forever change our view of our place in the universe. But even the great advances of knowledge that would follow finding faint traces of life on extra-solar planets will depend upon discoveries that can only be made by space-based telescopes with apertures of 8 meters or more. Large breakthroughs in astronomy often require large increases in telescope diameter relative to existing facilities. The information content gleaned from higher angular resolution, astrometric precision, and overall sensitivity, increases directly with telescope collecting area. A 16-m optical space telescope, for example, would revolutionize the study of galaxy evolution, enabling, for the first time, measurements of the kinematics of both the gaseous and stellar components of the smallest dwarf galaxies. It would yield such precise constraints on hierarchical structure formation models that a new era of “precision galaxy evolution” would ensue. Indeed, for many fields of astrophysics, the advantages of large apertures are so overwhelming that it is imperative to develop the technology permitting at least an order of magnitude increase in the collecting area of optical space telescopes. We propose here to create a roadmap of the key technology developments required to enable an Advanced Technology Large-Aperture Space Telescope (the ATLAS Telescope). To do so, we will focus our study on two telescope architectures that straddle the range in viable technologies: an 8-meter monolithic-aperture telescope and a 16-meter segmented-aperture telescope. The 8-m monolith represents an example of how NASA’s proposed Ares V Cargo Launch Vehicle enables a design that is technologically mature and ideally suited to high-contrast coronagraphy, albeit not scalable to larger apertures. The 16-m segmented telescope will provide a revolutionary new observational capability and generate a pathway to achieving very large apertures, potentially enabling the construction of telescopes that are more than 30 meters in diameter. The rapid progress on lightweight mirror technology that enabled the James Webb Space Telescope (JWST) is already being extended using new materials and processes, such as silicon carbide, corrugated and/or nanolaminate mirrors. Combined with advances in closed-loop wavefront control of active optics, ATLAS Telescope concepts are affordable for the 2020 era if the technological development continues appropriately. The primary deliverable of this study will be a technology development plan that will enable NASA, by the end of the coming decade (ca. 2019 – 2020), to enter phase A for a large UV/optical (0.11 – 2 µm) space telescope with an angular resolution 5 – 10 times better than JWST, but with a full life cycle cost that is no greater. The main focus of this study will be to greatly reduce the present cost of constructing large space-based mirrors, and we will identify the designs and technologies that enable the most cost-effective approaches. Our technology development plan is based on a comprehensive comparison of the 8-m and 16-m ATLAS Telescope mission options for mirror fabrication, wavefront sensing and control, optical design, thermal analysis, pointing control, spacecraft bus configuration, and science instrument capabilities and a roadmap to bring these to high technological readiness levels for flight.


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2 Science Objectives and Expected Impact of an ATLAS Telescope Detecting signatures of life beyond the Solar System, dissecting the structure and formation history of galaxies from isolated dwarfs to rich clusters, and taking high resolution pictures of the outer reaches of the Solar System are just a few of the areas that will be revolutionized by the ATLAS Telescope, directly addressing 3 of the key objectives in NASA’s 2007 Astrophysics Science Plan: (1) understand stellar and galaxy evolution, (2) understand how individual stars form and affect planet formation; and (3) measure the properties of extrasolar planets. The ATLAS Telescope will be a critical facility for making breakthroughs in both the “Cosmic Origins” and “Exoplanet Exploration” science themes. The discovery potential brought about by two orders of magnitude increase in observing capability – sensitivity and information capacity through improved resolution – will ensure that an 8-m or 16-m ATLAS Telescope will qualitatively change and profoundly advance astronomy. Some examples follow. Life-bearing Extrasolar Planetary Systems With more than 250 known extrasolar planets and planetary systems, there is a general consensus that many terrestrial-mass planets orbit nearby stars, and some should have detectable signatures of life. If life alters the atmospheres of other planets as it does on Earth through the production of oxygen and carbon dioxide, for example, we could see these chemical signatures in spectra of the planets. Because the Earth’s atmosphere absorbs all the interesting spectral lines and these occur at wavelengths longer than 0.7 µm where the Earth’s airglow is bright, only a space telescope will be able to discern these signatures in the faint spectra of the planet. This technically challenging problem is nearly impossible to attack with telescopes a few meters in size, but the observational difficulties decline dramatically with increasing telescope diameter. A large space telescope has hundreds (8-m) to thousands (16-m) of candidate stars to search for life’s signatures among those with Earth-like planets in the habitable zones, orders of magnitude more than a 4-m telescope for any observational technique, as demonstrated in Table 1 (Beckwith 2007) for both the transit spectroscopy and coronagraphic imaging spectra.

At a minimum, the ATLAS Telescope will take spectra of terrestrial exoplanet atmospheres by observing transits (Table 1; Fig. 1; Valenti 2007). If additional wavefront corrections can be used to diminish the light from the star through a coronagraph or use of an external occulter, direct images will then bring into view hundreds to thousands of additional candidates star/planet systems as illustrated in Table 1. By

looking for atmospheric signatures in transits at the outset, the ATLAS Telescope will not rely on perfection of difficult starlight suppression techniques, but it will be an excellent test bed paving the way for the coronagraphic techniques that allow direct spectro-photometric imaging of our nearest neighbors.

Table 1: Exoplanet Host Star Sample Size vs. Telescope Diameter

#Coronagraphic Dtel

(meters) Expected#

Transits All Solar 2 1 3 0 4 11 27 10 8 85 216 78

16 682 1726 1092

Fig. 1: ATLAS-T enables the detection of signs of habitability in tens of Earth-like planets transiting their stars. Blue: model exoplanet spectrum; Black: simulated spectra of planet with a 16-m ATLAS-T over a 5-year mission; this star is the 51st brightest in the sample.

O2 H2O H2O H2O


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Fig. 2: The circles show where the Sun could be resolved in galaxies by 2.4, 8, and 16-m space telescopes. Many new spiral, elliptical, and dwarf galaxies come into view.

Stellar Populations and the Evolution of Galaxies The ATLAS Telescope will bring about a major revolution in the study of stars, allowing us to observe solar-luminosity stars in galaxies outside the local group. Previous revolutions were brought about by Edwin Hubble in the 1920’s, who first measured distances to stars beyond the Milky Way, and more recently by the Hubble Space Telescope (HST) that observed the very brightest stars in Virgo-cluster galaxies to measure the Hubble Constant to 10% accuracy. The Advanced Camera for Surveys on HST also made it possible to study entire populations of stars below 1 solar mass by resolving individual stars in M31, revealing separate populations from different accretion events – galaxy collisions – in the history of the galaxy (Brown et al. 2006). It is essential to discern solar-luminosity stars to measure the main sequence turnoffs of separate populations created over the

lifetime of the galaxy. The leap to galaxies beyond the local group will open up the entire Hubble sequence of elliptical and spiral galaxies to study and allow observers to reconstruct the history of these galaxies from accurate ages of the individual stars. In the era of JWST, where integrated populations of stars will be observed at very high redshift, we will need to understand the evolutionary history of these nearby galaxies in detail to make sense of the more distant samples. To obtain accurate stellar population ages, one needs stable, high-precision photometry for tens of thousands of stars spanning ~4 orders of magnitude of dynamic range in luminosity and in crowded fields extending over several arcminutes. While future (30-m) ground-based telescopes will provide high-resolution imaging, they will not do so with the contrast and stability over the wide fields that are required for this work. Figure 2 shows the reach of ATLAS-T compared to all previous telescopes for the study of stars in other galaxies. Enabling the Era of Precision Galaxy Evolution By its 250-fold improvement over the sensitivity of today's most powerful facilities and its ten-fold increase over the resolution of HST, the 16-m ATLAS-T will be the first observatory able to measure the dynamics of dark and baryonic matter separately in galaxies and to follow its evolution over cosmic time, determining how these structures grow. As the first observatory with the power to test rigorously the impact of dark matter on the formation of structure, the central assumption of Cold Dark Matter (CDM) theories, ATLAS-T will qualitatively change the study of galaxy evolution. The qualitative change brought about by ATLAS-T is its ability to use powerful new observational methods to test CDM predictions with the accuracy required for precision cosmology, an impossible task today and not even within the reach of JWST, which targets instead less precise measurements of more distant galaxies. It will be able to carry out absorption line studies using ordinary galaxies for background sources in the same manner that quasars are used today for UV/visible spectra. The vastly more abundant galaxies permit the study of intergalactic structure on dramatically finer scales than possible using quasars alone: there will be ~90 galaxies at redshifts around 3 that can serve as background light sources for ATLAS-T


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spectra of the intervening galaxy halos within 100 kpc from the center of a disk galaxy at a redshift of 1 (Kacprzak et al. 2007). Spectra of the intergalactic medium (IGM) with such impact parameters will reveal the kinematics of the galactic halos through the hot gas in which MgII absorption systems originate, ubiquitous around bright galaxies. This method will allow mapping with great accuracy and high spatial resolution sampling the dynamics of the stellar disk and halo separately, breaking the disk+halo dynamical degeneracy and testing one of the most important predictions of the theory. Access to the dominant Lyman limit systems, as well as CIV and MgII absorbers, over the evolutionary redshift range 0.5 to 3 requires the combination of UV sensitivity, photometric stability, and diffraction-limited resolution available only to space telescopes; even 30-m class ground-based telescopes with adaptive optics systems cannot provide this combination of capabilities sufficiently well to make these observations. The ATLAS Telescope will be the first telescope to characterize the sub-structure in the halo of galaxies as a function of redshift, allowing a comparison between observation and theory at a high level of significance. It will allow us to measure dynamical masses up to redshifts of ~4 and distinguish mergers from distorted morphologies, producing images of gravitational lensing with unsurpassed accuracy. It will allow us to measure the strengths of galactic winds up to redshifts near 4, and the chemical enrichment of stars and gas in galaxies up to redshifts of ~6. Chemical Evolution and Feedback By manufacturing 90% of the heavy elements in the universe, stars play a direct role in its chemistry. However, stars are just one part of a more complex feedback cycle, where radiation, hot gas, and dynamic outflows act on surrounding galaxies and gas to create the luminous universe we observe today. Understanding this fundamental feedback cycle and its role in shaping galaxies is one of the most outstanding challenges in characterizing the evolution of the Universe. ATLAS-T’s high resolution (angular and spectral, both in the UV and optical) will enable precision measurements of the production sites of the heavy elements (hot stars, supernovae, emission nebulae) and provide unparalleled insights into the mechanisms that transport these chemical elements into the surrounding interstellar medium and the processes that expel metals from galaxies out into the Intergalactic Medium (IGM). ATLAS-T will probe the distribution of dark baryonic matter and heavy elements in IGM filaments and voids, tracing the cosmic web in UV absorption lines toward distant QSOs with unparalleled accuracy and sensitivity. The radial distribution of these heavy elements will reveal the nucleosynthetic sources produced by early star formation and reveal the interplay between the formation of structure and the processes of radiative, mechanical, and chemical feedback. Impact of an Observatory There is an enormous range of other science enabled by a large UV/optical telescope in space: high-resolution imaging of distant outer Solar System objects; resolving the broad line regions around active galactic nuclei, including the study of regions near black hole event

Fig. 3: (left) Image of the asteroid, Vesta, taken with HST; (right) A computer simulation of Vesta at a resolution similar to ATLAS-T.


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Fig. 4: Limiting S/N=10 point source sensitivities for ATLAS-T 8-m and 16-m in 1 hour. Sensitivities in the NIR for JWST and a 30m ground-based telescope are also shown, derived from expected MCAO performance. Ground-based sky absorption shown (bottom).

horizons through reverberation mapping; and resolving the disks around young and evolved stars alike through scattered star light, to name a few areas. Like HST, the ATLAS Telescope will have an impact on almost every subfield of astronomy. Moreover, the importance of having a large UV/optical telescope in space will grow with the expansion of capabilities in other fields. The ATLAS Telescope will be an essential tool working with ALMA, Con-X, and future large ground-based telescopes, providing an unsurpassed combination of sensitivity, resolution, and field of view for the most important problems in astronomy. Unanticipated discoveries with the ATLAS Telescope will almost certainly account for many of its most interesting scientific results, as all major advances in telescope technology have shown (e.g. Harwit’s Cosmic Discovery). As such, it will usher in a new era of discovery in astrophysics comparable to that created by the HST, an era that will be essential, as well, for maintaining strong support for NASA’s space science program far into the future.

2.1 Science Requirements Flowdown

The requirements on and goals for the 8-m and 16-m ATLAS-T performance characteristics are summarized in Table 2 below and are designed to enable a broad range of

scientific investigations,

including those highlighted

above. Neither the 8-m nor the 16-m aperture sizes are set solely by

scientific considerations.

An 8-m mirror is the largest circular monolithic mirror that will fit horizontally, with its support structure, in the

baseline fairing of the planned Ares V launch vehicle. The technical motivations for this choice are discussed in section 3.1. The 16-m aperture size is chosen to provide a ten-fold improvement over JWST’s resolution at its nominal 2 µm diffraction limit and, coupled with the six-fold improvement in collecting area, will detect a 0.3 nJy (32.7 AB mag) point source at 0.75 micron with a S/N ratio of 10 in 1 hour (Fig. 4). Additional performance requirements, including the temporal and spatial stabilities of the point spread function, photometry, and pointing, will be rigorously assessed as part of our proposed study. We will pay particular attention to the impact of performance on cost and on trades between different approaches to achieving the desired performance level (e.g., the placement of wavefront sensors and degree of correction applied). Details of the study trades are highlighted in the next sections.

Table 2: Top Level Science Requirement Summary

Parameter 8-m requirement 16-m

requirement 8-m goal 16-m goal

Angular Resolution

Diff. limited at 0.5µm (15.7 mas)



TBD

Wavelength Range

0.115 – 2 µm 0.115 – 2 µm 0.115 – 3 µm 0.115 – 3 µm

Field of view 8 x 8 arcmin 5 x 5 arcmin 10 x 20 arcmin 8x16 arcmin Non-sidereal

Tracking Track KBOs Track KBOs TBD TBD

Field of Regard

Entire sky over course of 1 year

Entire sky over course of 1 year Same as baseline

Same as baseline


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3 Telescope Architecture We will now describe the key trade studies and design considerations we will undertake as part of our ATLAS Telescope concept study. We highlight the most important items in bold face. Constraints imposed on the telescope designs by available or planned launch vehicle fairing diameters and mass-to-orbit capabilities must be taken into account. For instance, a monolithic circular 8-m aperture telescope will not fit into existing EELVs with their 5-m fairing diameters (or proposed EELV modifications up to 7-m fairings1), and would, thus, require a segmented design. However, most of the technologies required for an 8-m segmented telescope will be well in hand from JWST heritage. Consequently, we do not propose to study an 8-m segmented primary design but rather an 8-m monolithic design. The principal reasons for studying a monolithic 8-m aperture primary are (1) such mirrors currently exist with the required optical quality, (2) monolithic mirrors are favored for high contrast imaging, and (3) monolithic apertures of this size will fit within the 10-m baseline fairing (8.4-m inner diameter) of the planned Ares V heavy lift vehicle. Furthermore, the huge mass-to-orbit capability of the Ares V (60 metric tons to L2) permits us to use existing, rather heavy optics, so that a costly mirror development program can be avoided. We will develop a 16-m segmented telescope architecture and technology plan. A 16-m aperture segmented telescope could conceivably be launched on either a large EELV or on the Ares V. For the EELV option, mass is a crucial constraint, which drives us to consider lightweight optics, for which a technology development plan is needed (see section 3.3). During the study, we will consider the trades for packaging the 8-m ATLAS-T into an Ares V fairing and the 16-m ATLAS-T into Ares V and EELV fairings. The 8-m telescope described in this proposal can be launched with the secondary mirror and light baffle fully deployed in the Ares V fairing. The 16-m segmented telescope, however, will require some deployment of either the primary and/or secondary mirror and the light shield/baffle in orbit. We will consider the trade-offs between astronaut-assisted assembly and fully automated deployment for the 16-m telescope. Infrastructure for astronaut-assisted assembly is envisioned as part of NASA’s Vision for Space Exploration, although probably not until well into the 2020 decade.

3.1 Optical Telescope Assembly (OTA) Design

For the 8-m monolithic aperture telescope, we conducted a preliminary investigation (using MSFC internal funds) over the past year of a mission concept intended for launch on an Ares V. We baselined the use of an existing ground-based 8-m mirror because it has a number of specific advantages: (1) it has been demonstrated that one can polish such a mirror to a surface figure of better than 10 nm rms with straight forward active control and (2) the blank cost is relatively inexpensive if you do not need any or much light weighting. During this initial 8-m aperture study, we considered two optical design architectures: a Ritchey-Chrétien Cassegrain (RC) and a Three-Mirror Anastigmat (TMA) with a fine steering mirror. A RC design was examined for its excellent on- and off-axis image quality, compact size and versatility. The RC optical design has a 1 arc minute field of view. However, to achieve the desired field of view (Table 2), a refractive corrector is required in the scientific instrument suite - although this then limits the spectral range. To achieve the desired spectral coverage over a several arc minute field of view, a TMA design had to be investigated. We found a TMA configuration that was diffraction limited over a

1 United Launch Alliance, August 2007 presentation.


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field of view of 8.4 by 12 arc minutes. During the study, we would optimize this design for the 8-m ATLAS Telescope and investigate the folding of the optics and packaging to be consistent with our instrument module. For the 16-m segmented aperture telescope, we evaluated the performance of three telescope designs: a Cassegrain, and two TMAs. The rms wavefront error over the field of view was used as a merit function. All three designs were constrained to have an entrance pupil diameter of 16-m, a focal ratio of 12 (yielding an effective focal length of 192 m), and a reference wavelength of 633 nm. The rms wavefront error was calculated over the full 5 x 5 arc minute FOV. We discovered that the two-mirror Cassegrain could not maintain performance over the desired FOV. The TMA design form has enough degrees of freedom to produce images with near-zero spherical aberration, coma, and astigmatism and a flat focal plane. However, higher order aberrations still contribute to the wavefront error. General aspheric surfaces are required to control the high order aberrations over the full field. Another feature of the TMA designs is the option of locating a fold mirror at the exit pupil where the mirror could function as a fast steering mirror (FSM) for pointing correction or a deformable mirror (DM) for wavefront correction. In our initial 16-m ATLAS-T designs, we found a TMA design with 40 nm rms wavefront error (i.e., diffraction limited at 0.5 µm). However, this design is not optimized and there are still many important trades to investigate. For example, depending on the requirements of the associated instruments, one might be able to relax the wavefront requirement on the telescope and do wavefront correction at the instrument level. Also, the various deployment schemes might drive the telescope design. These issues, along with thermal considerations, PSF stability, and pointing stability, will be part of the optical design work during the study phase. For both the 8-m and 16-m telescopes, we will consider on-axis and off-axis designs during the study phase. Exoplanet science objectives suggest that control of diffracted starlight and maximization of entrance pupil area are important considerations; these requirements alone would tend to favor an unobscured (i.e., off-axis) design. The off-axis design may also have advantages for more compact payload storage in smaller fairings. A detailed comparison between on-axis and off-axis designs will be performed, as well as studying the method of deployment or emplacement.

3.2 Wavefront Sensing and Control (WFS&C)

Actuator control of the primary will be required for both the 8-m monolith and the 16-m segmented telescopes. Actuator density for the monolith is less than that for the segmented architecture because the mirror surface is constrained to be continuous. Members of our team studied the optimal mounting and actuation locations for the strawman TPF-C design (a 3.5 x 8-m elliptical monolithic mirror). They estimated that an actuator density of ~1/m2 was sufficient to give acceptable wavefront correction of gravity distortion to permit ground testing. The 8-m ground-based mirror is larger and stiffer than the TPF-C mirror, so we may need less than one actuator per m2. Team members who will contribute to this study have been at the forefront of the TRL6 wavefront phasing demonstration at the JWST testbed. We will examine the potential limits on wavefront correction in this architecture. The requirement of diffraction limited performance at 0.5 µm wavelength may require further correction in the instruments and/or higher authority control on the primary mirror (PM). During the study, we will consider the following trades for the 8-m monolithic mirror: (1) actuator density, location, stroke and forces, (2)


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Fig. 5a: The 0.75-m actuated hybrid mirror.

WFS&C authority on the PM versus distributed with deformable mirrors, and (3) dedicated WFS&C versus dual-use of science cameras for WFS. A 16-m telescope will require WFS&C under two different circumstances: (1) initial alignment, and (2) periodic alignment correction. The goal of initialization is to establish the required wavefront performance following launch and deployment. Periodic alignment will be required to maintain this performance in the presence of thermal drifts, mechanical disturbances, changes in the telescope structure geometry, etc. Although extensive work on WFS&C issues has been performed to date by various groups, the development of a WFS&C scheme optimized for a 16-m segmented telescope will be investigated thoroughly during the study phase, as it is a potential cost driver. We will leverage the JWST experience as we conduct this aspect of the study. The initialization and maintenance processes make use of proven dispersed fringe sensing techniques, including dispersed Hartman sensing for coarse segment phasing and either phase retrieval algorithms or a Shack-Hartmann camera for wavefront improvement. Long term maintenance of the telescope alignment and segment phasing could be accomplished through periodic WFS&C or through the use of a laser metrology subsystem, which forms an optical truss that continuously monitors changes in the telescope configuration. Much of the required technology for this subsystem has been demonstrated by SIM, JWST, and the Palomar testbed to a degree probably beyond what is needed. The metrology subsystem has the nanometer accuracy and continuous operation capabilities needed to assure a very stable PSF. This performance can be further enhanced using a small, full-aperture deformable mirror within an instrument. Clearly, there is a broad spectrum of sensors, actuators, and algorithms for implementing precise WFS&C. Selecting the most effective and cost-efficient suite of WFS&C techniques will be a major topic of research in our study.

3.3 Mirror Fabrication and Technologies

Building a 16-m segmented aperture space telescope requires a new paradigm for the fabrication of its mirrors. Two promising developments will be highlighted here. During our study we will develop a roadmap to bring a large-scale segmented aperture using these advanced mirror technologies to TRL-6. The development of meter-class actuated hybrid mirrors (AHM), shown in Fig. 5a, specifically addresses the problem of providing tens of square

meters of optical quality, lightweight, space-qualifiable mirrors at reduced cost and fabrication schedule compared to traditional glass mirrors (e.g., Hickey et al. 2002). These mirrors have areal densities of 12-15 kg/m2 (with control elements) and fabrication schedules of months. These advantageous attributes result from a combination of three distinct technologies: (1) a metallic nanolaminate facesheet that provides a high optical-quality reflective surface, (2) a silicon carbide (SiC) facesheet that provides structural support and houses actuators to provide an adaptive surface figure, and (3) wavefront sensing that provides active figure control. JPL, LLNL and NGST/Xinetics have been developing

AHMs for several years. Nanolaminate materials are multi-layer metallic foils grown by sputter deposition, with atomic-scale control, made off of a master convex mandrel. The SiC mirror substrates provide a high stiffness-to-weight structure with excellent thermal stability. Near net


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Fig. 6: Sunshade Design Options: Flat (left), Cone (center), "Sugar Scoop" (right)

Fig. 5b: A 0.5-m corrugated mirror under development at ITT

shape forming by re-usable, replication molds enables the SiC substrates to be fabricated in a few weeks to finished dimensions. Actuators are embedded in the substrate ribs in a manner that eliminates external forces and moments, thus obviating the need for a separate reaction structure. Two 75-cm AHMs have been fabricated and optically tested both in air and vacuum. Based on these tests, a surface figure of 100 nm is already achievable for meter class optics but further wavefront error reduction will be needed to achieve the required ATLAS-T performance. This optic has also been shown to survive a 15 g rms random vibration test with no degradation in optical performance. An alternative segmented, replicated mirror architecture that also warrants investigation is the ITT corrugated mirror approach (Strafford et al. 2006; Fig. 5b). This glass approach is built up using thermal forming of 5 thin glass sheets into a plano assembly that is then thermally slumped over a convex mandrel. Two scales of corrugated cores prevent print through of the structural load into the optical figure. The entire sequence of forming operations can be done rapidly so that mirror blank fabrication times would be a few weeks in production. Each segment is formed to an accuracy within the capture range of an interferometer, so that a few weeks or less would be needed to finish the mirror. Each mirror would need low authority actuation for phasing, of order 7-14 actuators per segment for the accuracies needed for the 16-m ATLAS-T. Areal densities including actuators would be in the range of 15 kg/m2. The rapid fabrication and high re-use of materials of this approach would work to bring the cost of a large space primary mirror below a cost density of $1M/m2 as required to make a 16-meter approach affordable. A technology demonstrator has passed acoustic testing at GSFC and is at TRL4, with a 0.6-m mirror assembly in preparation for vibration testing in January 2008. Near-term extension of the technology will include completion and cryogenic testing at NASA/MSFC of a 0.6-m spherical mirror, and extension of the process from borosilicate glass to ULE fused silica glass for optimal thermal performance at room temperature.

3.4 Telescope Baffles

Several design options have been generated for protecting large aperture telescopes in L2 orbits from thermal heating by solar radiation. This protection is particularly important for telescopes that need the high thermal stability required for

stable point spread functions. Figure 6 shows some of the options we plan to evaluate during our study. The flat sunshield on the left is the JWST design. The conical sunshade design shown in the center was developed for the TPF-C mission, as was the “sugar scoop” configuration shown on the right. All three designs utilize lightweight membranes with high reflectivity coatings and a 2 to 3 degree dihedral between layers to allow radiant energy to experience multiple reflections


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and escape through the openings. For all three options it will likely also be necessary to have a cylindrical light baffle around our UV/optical telescope to keep the light from off-axis stars and scattered starlight from reaching the focal plane of our instruments. During the study, we will consider the trade-offs between these 3 sunshade designs including cost, complexity of manufacture, and deployment procedures once in orbit.

3.5 Spacecraft Design

The ATLAS Telescope spacecraft (SC) bus will need to be designed to accommodate a large astronomical payload in an L2 halo orbit. The JWST bus provides a good baseline model to use, with the exception that the isolation of the Command and Data Handling (C&DH) system from the focal plane, which was needed to maintain JWST’s cold operating temperature, will not be required for an optical/UV telescope operating at 250K or warmer. The electrical power subsystem’s solar arrays for JWST provide ~2 kW of power for spacecraft and payload electronics, which may not be adequate for ATLAS-T. We will quantify the SC bus mass and power requirements in this study. We will also require at least a factor of ~5 improvement in bandwidth over the current 28 Mb/sec achieved from JWST’s high gain Ka-band antenna and communications system in order to support the Gigapixel imagers envisioned for ATLAS-T. The propulsion capabilities of the SC bus will depend on telescope mass but also, in the case of the 16-m, on the specifics of the deployment procedure of the primary mirror. If it is astronaut-assembled or deployed in LEO, it will require a larger propulsion system to get it out to L2. The 8-m ATLAS-T should be able to do a direct insertion into L2 if an Ares V vehicle is used. We will quantify the SC propulsion requirements in this study. No major technology development is seen as required for the SC bus; however, the overall design of the SC bus will be unique.

3.6 Starlight Suppression

A key scientific goal for the ATLAS Telescope is the direct detection and characterization of terrestrial planets around other stars. Hence, we will not preclude the use of starlight suppression systems. In this study, we will assess current and future coronagraphic methods for 8-m monolithic and 16-m segmented telescope options, in both on-axis and off-axis telescope designs. On-axis segmented designs will be the most challenging due to diffraction effects. We will also consider possible approaches that include internal and externally occulted coronagraphs, and their performance limitations. We will develop requirements for the level of suppression needed to achieve the science goals that includes: (1) computing the suppression depth (contrast) as a function of wavelength and inner and outer working angles, (2) setting requirements on wavefront, amplitude and polarization sensing and control performance for both an internal coronagraph and an external occulter, (3) setting a lower bound on expected coronagraphic performance as a function of the number/size of segments, segment gap width, throughput, and the Lyot stop size/shape, and (4) deriving the distance, size and shape of an external occulter for a 16-m segmented telescope, and subsequently set the requirements on manufacturing, deployment, and alignment errors for such an external occulter.

3.7 Science Instruments

Given that most of the technology challenges preventing ATLAS-T from being built today reside in the areas of mirror fabrication and optical control of very large-apertures in space, our study rightly focuses more on the telescope system design options and costs than on the science


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instrumentation. Furthermore, we anticipate that many instrument technology developments will occur over the next decade, from work done both for space- and ground-based observatories. Thus, the instrument section of our study will use a plausible baseline suite of science instruments for the 8-m and 16-m ATLAS Telescope designs. We will use these baseline instruments to constrain the observatory science module’s mass and power requirements as well as the science operations requirements (e.g., data storage, communications bandwidth). We will not undertake, in this study, detailed designs for the science instruments. Table 3 summarizes our baseline instrument suite.

Table 3: Baseline Science Instrument Suite

Instrument Wavelength Coverage Angular and Spectral

Resolution Telescope Detectors/Modes

UV: 0.11 – 0.28µm R = 2 – 3, plus selected narrow band filters

~32 Megapixels

Visible: 0.3 – 1.1µm R = 5 – 100 Nyquist @

0.5µm ~6 Gigapixels Direct Imager

NIR: 1.0 – 3.0µm R = 5 – 100 Nyquist

@1.2µm

8-m and 16-m

~1 Gigapixel

Integral Field Spectrograph

0.3 – 1.7µm, split with dichroic

R = 20 – 200, 800 x 800 microlens array

8-m and 16-m

De-focused mode for Exoplanet

Transits

Coronagraph Same as IFS above R = 20 – 200,

100 x 100 microlens array 8-m and

16-m Exoplanet

Characterization

UV/Optical Spectrograph

Two channels: 0.11 – 0.3µm (UV), 0.3 – 1.0µm (Vis)

R = 2000, 5000, and hi-res modes: 30000 & 100,000

8-m and 16-m

Multi-object capability

NIR Spectrograph

1.0 – 3.0µm R = 500, 2000, 10000 16-m Multi-object

capability

Our team will conduct the basic systems engineering studies to quantify the macroscopic parameters of the instruments such as dimensions, shape, volume, mass, power, focal plane size, data volume, thermal requirements, mechanisms, calibration sources, etc. We will identify the nature of the detectors and focal planes in terms of physical dimensions, format, pixel size, power and thermal control. These characteristics will be needed for the cost-estimation exercises at the IDC, and for sizing accommodations in the spacecraft bus. We will identify where the needs of these instruments stretch existing technologies and will include these in our recommendations for technology development. Requirements for facilities needed to assemble, align, transport and test the instruments will also be developed.

3.8 Servicing Options and Mission Lifetime

ATLAS-T is envisioned to be a large observatory, representing a major investment of national resources, and providing versatile scientific capabilities for a generation of astrophysical research. It is incumbent on the designers of such a facility to make it as affordable as possible, to minimize implementation risks and difficulties, and to ensure the maximum scientific performance. While we won’t have resources in our study to do any serious exploration of servicing technologies, our study will follow a design approach that benefits the implementation phase, as well as the in-orbit lifetime and scientific productivity. That is, we will develop a design based on concepts of modularity, simple interfaces and accessibility. We see three broad benefits to this approach: (1) It should simplify the fabrication, assembly and testing of the


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observatory. (2) Partitioning enables opportunities for multiple partners to participate. (3) It enables, but does not require, in-orbit servicing. Our goal is to develop an observatory design that ensures a minimum five-year science mission following the completion of all commissioning activities. All consumables, reliability projections and limited lifetime items should be mindful of this requirement. No servicing or maintenance should be required or planned to accomplish the core science mission. However, as HST has shown, space observatories can be scientifically competitive for at least 20 years and with the impressive capabilities expected for ATLAS-T, we don’t want to preclude such longevity.

4 Cost Estimation Methodology We will estimate the ATLAS-T mission costs during the study using a combination of parametric and analogous cost estimating methodologies. Since we are pre-Phase A and since much of the required technology is below TRL5, the estimates must also be viewed as immature and an uncertainty factor of at least 40% will be applied to our estimates. The cost estimates will be linked with the master schedule and all assumptions used in the costing exercise will be vetted with the study team. The parametric Price H cost model will be used to develop a cost based on hardware component design, part counts, and mass and design maturity. Price H works for any hardware system whether it is space hardware or ground based hardware. In cases where a hardware component has no suitable cost precedent, a strategy for costing for the unusual component will be developed by consulting our industry partners who have expertise in related technologies. The Price H parametric approach has a confidence level value associated with the output, allowing a corresponding uncertainty to be computed. A second approach to bolster confidence in our final mission cost estimate is the analogous hardware and systems. With this approach similar historical work, hardware or development programs are used to develop an estimate. This analogous cost could be for integration and test or for a purchased instrument or component. These costs may be scaled based on projections regarding the mission. The analogous costs are compared with the parametric modeling and programmatic rules of thumb to either give confidence in the cost or indicate areas that need more study. Reserve/Contingency will be at least 30% of the total cost less launch vehicle and EPO by year for phase B through D and 30% of the total cost less E/PO for Phase E. Launch vehicle costs will be taken from NASA HQ documentation. An uncertainly range of 40% of the subtotal before reserves will be applied to account for the pre-Phase A level of maturity of the concepts developed. The end result is a cost with enough credibility that it can be used for strategic mission planning and prioritization in a decadal review.

Table 4: Summary of Key ATLAS Telescope Trade Studies Trade Study Topic Leads Priority

Optical Design 8m: Content (GSFC), Stahl (MSFC); 16m: Korechoff (JPL) High Mirror Technology & Fabrication AHM: Hickey (JPL) & NGST; Corrugated Mirrors: Content (GSFC) High WFS&C 8m: Content (GSFC); 16m: Redding (JPL) High Sunshade Design NGST team (Polidan, Lillie) High Deployment & Assembly NGST and BATC (Kilston, Ebbets) teams High Spacecraft Bus Purves (GSFC) Medium Science Instruments BATC team, Woodgate (GSFC) Medium Integration & Testing 8m: Stahl & Hopkins (MSFC);16m: Oseas (JPL) Medium Starlight Suppression Lyon (GSFC), Krist (JPL), Cash (U. Colorado) Low/Medium Servicing BATC team with Grunsfeld (JSC) Low


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5 Technology Roadmap The key goal of this study is to define a development roadmap to bring the technologies required to make the ATLAS Telescope affordable (e.g., comparable to JWST cost in FY08 dollars) as part of a well-balanced NASA astrophysics program in the 2020 era. The most significant tall poles are affiliated with the manufacture, deployment, and control of the OTA and these are summarized in Table 5. These are focused on achieving the desired mirror areal density of 15 kg/m2 or less, large aperture fabrication, and wavefront sensing and control, principally for the 16-m segmented telescope design. Figure 7 illustrates a preliminary cut at a technology development roadmap for our most critical components. This roadmap will, naturally, be expanded and fleshed out in considerably more detail by the end of our study. A full aperture test facility may also be needed as part of the development for the 16-meter telescope, but it is

generally supported under agency institutional funds or infrastructure, and is thus not included herein. For the purpose of completeness, however, its design has been referenced here and we will devote some time in our study to assess the challenges of pre-flight testing of a 16-m space telescope although this topic is worthy of a full study in its own right. The technology roadmap shown in Figure 7 concentrates initially on accelerating investments in the area of mirror fabrication to achieve the desired areal density, and offers two possible approaches (corrugated versus nanolaminates as discussed in section 3.4). Wavefront sensing and control technology development will commence a few years later as the funding becomes available, starting with the coarse phasing and fine phasing trades for image based approaches. Finally, alignment maintenance using laser metrology will go through the cycle of architecture, design,

Table 5: Significant Technology Tall Poles

Telescope Requirement

Technology Category

Needed Technology

Relevant Telescope

Design

Needed Product to Achieve TRL6

Current TRL

Heritage

High Contrast Optics Surface Quality Roughness < 2

nm 16-m

Replicated Superpolished

Surface 4

Nanolaminate Mirrors

Segmented Large-

Aperture Optics/Fabrication 2.5-m Segments 16-m

Actuated Hybrid Mirror Scale-up; Nanolaminate or

Corrugated Mirrors

4 JWST

Architecture

Low Mass Optics/Material Areal Density <

15 kg/m2 16-m

Areal Density System

Demonstrator 4

Nanolaminate & Corrugated

Mirrors

Image Resolution

Optics Rms WFE < 35

nm, zero g performance

8-m and 16-m

Test Facility 4 Hubble

Low Cost Density

Optics/Fabrication Cost/Area <

$1M/m2 16-m Multi-segment

demo made to low fixed cost

4 Nanolaminate & Corrugated

Mirrors High Contrast

Imaging Wavefront

Sensing & Control WFS&C << 0.001 wave

8-m and 16-m

WFS capable in limited field

2 AO; JWST; JPL HCIT

Optical Alignment

Maintenance

Wavefront Sensing & Control

Laser Metrology; Distance Gauge (nm accuracy); Beam launcher

mass << 100 gm

16-m

Subscale Testbed of Laser Truss and Prototype

Gauge

4 SIM

Telescope Thermal

Protection Baffles Sunshade

8-m and 16-m

Lightweight Membrane with High Reflectivity

Coating

4 JWST


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and prototype fabrication. The technology development timeline and its attendant funding wedge in Fig. 7 are approximate at this time, pending further analysis during the proposed study phase. It is based largely on a JWST-like 7 year focused technology program, with companion system studies running in parallel to ensure that technologies and manufacturing processes are working to the appropriate mission requirements.

Figure 7: Very Preliminary Technology Development Roadmap

6 Concept Study Execution The joint engineering, technology, and scientific expertise on this study team ensure a successful study leading to a viable roadmap for development and space qualification of all the key ATLAS-T technologies. Our team members, including 3 NASA centers and 2 industry partners (Northrop Grumman, Ball Aerospace), are leaders in inventing the technology used by, building the hardware for, operating, and performing the science with many of the world’s best ground and space-based UVOIR telescopes including Gemini, Spitzer, HST, Kepler, and JWST.

6.1 Statement of Work and Deliverables

We will perform the trade studies described in section 3 (highlighted in bold face) and summarized in Table 4. We will refine the science requirements for stability of the point spread function, wavefront error tolerances, science and pupil plane instrument performance, and


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pointing stability. We will use the outcome of these refined requirements and studies to estimate life cycle costs, using methodologies in section 4, for the 8-m and 16-m ATLAS-T mission configurations. For both the 8-m and 16-m concepts, we will identify the configurations that minimize cost without substantially sacrificing the key scientific objectives discussed in section 2. We will define a technology development roadmap that, if adopted, will bring all critical elements to TRL6 by the end of the coming decade (ca. 2020). Finally, we will prepare both mid-study and final reports for NASA in accordance with the NRA requirements.

6.2 Management of Study

P.I. M. Postman will be fully responsible for the management of the study and will coordinate all activities during the study. The P.I. will be assisted by a Study Manager (B. Milam) and a Missions Systems engineer (G. Mosier), both from GSFC. The Study Manager’s role is to ensure that all milestones are completed within the study cost and schedule. The Systems Engineer’s role is to make sure the technical requirements are reasonable, justified and the resulting concepts are optimized to meet the requirements. Leads for all trade study and technology development activities are already identified and will be in regular contact with the P.I. The Space Telescope Science Institute’s Office of Sponsored Programs will oversee the industry partner subcontracts and has had extensive experience supporting NASA mission concept studies comparable to this one in scope. The P.I. will synthesize the critical outcomes, coordinate the writing of the final study report, and present the results to NASA. Descriptions of the specific roles of all team members are given in the budget detail and narrative section.

6.3 Schedule

The key milestones for the study are shown below. The work falls into 4 phases: research and planning, Integrated Design Capability (IDC) preparation and execution, evaluation of costs and designs, and final study report preparation. The milestones are scheduled to support a timely IDC run in order to prepare for the end of year Decadal Presentation. The presentation will

contain enough detail to enable an informed decision regarding Decadal priorities and technology readiness.

6.4 Budget

Our study budget is split 60%/40% across FY2008 and FY2009 (assuming a 3/2008 start). Details are given in the budget narrative and justification section. Our cost breakdown as follows: JPL is 21%, IDC analysis (required by NRA) is 20%, NGST is 18%, GSFC is 14%, MSFC is 12%, BATC is 10%, and STScI is 5%. Almost the entire STScI budget is travel support for 3 team meetings in Baltimore plus attendance by the P.I. at the end-of-study workshop being run by NASA. A small amount of STScI funding is for labor associated with final production of the study report.

Early 3/08

Funding in place, Study Kick-off meeting at STScI

3/08 Define trade space, refine science requirements, develop mirror technology investigation

4/08 Refine telescope point designs as needed from above.

Late 5/08 Complete IDC (GSFC) pre-work; initial IDC run (1 week)

7/08 Team meeting at STScI; prepare interim Report

9/08 Final IDC run and preliminary mission life-cycle cost

10/08 Team meeting. Review initial draft of technology roadmap.

11/08 Begin final report preparation.

1/09 Revise IDC and Cost reports. Finalize technology roadmap.

3/1/09 Deliver final report to NASA


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7 References Beckwith, S. V. W., “Detecting Life-bearing Extra-solar Planets with Space Telescopes,” 2007, astro-ph arXiv:0710.1444, submitted to ApJ. Brown, T., et al., “The Detailed Star Formation History in the Spheroid, Outer Disk, and Tidal Stream of the Andromeda Galaxy,” 2006, ApJ, 652, 323. Harwitt, M., “Cosmic Discovery: The Search, Scope, and Heritage of Astronomy,” 1981, New York: Basic Books. Hickey, G. S., Lih, S.-S., Barbee, T. W., Jr., 2002, SPIE, 4849, 63. (“Development of Nanolaminate Thin Shell Mirrors”) Kacprzak, G. C., Churchill, C. W., Steidel, C.C., Murphy, M.T., “Halo Gas Cross Sections and Covering Fractions of MgII Absorption Selected Galaxies,” 2007, astro-ph: arXiv:0710.5765, AJ, in press. Strafford, D. N., DeSmitt, S. M., Kupinski, P. T., & Sebring, T. A., 2006, SPIE, 6273, 0R. (“Development of Lightweight, Stiff, Stable, Replicated Glass Mirrors for the Cornell Caltech Atacama Telescope (CCAT)”) Valenti, J., 2007, private communication. Based on earlier work done to explore potential of JWST/NIRSpec to characterize extrasolar planets around cool stars. See 2005 B.A.A.S., 37, 1350. This proposal builds upon prior studies of large optical/UV space telescope concepts, most notably: Shull, M. et al., 1999, “Space Ultraviolet-Visible Observatory (SUVO)”, The Emergence of the Modern Universe: Tracing the Cosmic Web (Boulder: UVOWG), http://origins.colorado.edu/uvconf/UVOWG.html Green, J. et al., 2005, “Modern Universe Space Telescope (MUST),” NASA Vision Mission Concept Study for a 10-m optical/UV space telescope.

Documents

Submitted Proposal to NASA's Astrophysics Strategic Mission