C PROJECT DESCRIPTION C.1 Overview Understanding the Sun has always been a vital quest of science because the Sun is the most significant astronomical object for humankind. Exciting observations of solar magnetic activity from recent space missions, such as Yohkoh, TRACE and SoHO, have stimulated both scientific and public interest in the Sun. They have highlighted the pervasive role magnetic fields play in determining the nature of the important physical processes driving solar activity and variability. Most importantly, these new observations have put new emphasis on the intrinsic relationship between small-scale physical processes and large-scale phenomena (e.g., coronal mass ejections). Meanwhile, because of improvements in computational capabilities, physical theory and numerical modeling are now addressing the fundamental scales and processes in the highly magnetized and turbulent plasma of the solar atmosphere. Our knowledge has now reached a point where a new solar telescope is needed to make important progress. Now is the time because of a fortunate confluence of new space observations and advanced numerical modeling. Together, these make it necessary to obtain observations of small-scale magnetism in order to understand the basic forces of solar activity. Recent breakthroughs in adaptive optics have eliminated the major technical impediment to making such observations. Even so, the current generation of solar telescopes (dating back to the 1960s) are too small and can neither resolve these small-scale physical processes nor accurately measure the magnetic field. Thus there is no unequivocal observational verification of current models or guidance for future modeling improvements. Above: Complex loop structure seen with TRACE in a flaring active region (A. Title). Below: Close up of solar flux tubes penetrating the solar surface seen with the Dunn Solar Telescope and adaptive optics (T. Rimmele). This impasse is recognized by the astronomy community, which has advanced strong scientific arguments for a large-aperture solar telescope. The most recent arguments are presented in the latest NSF/NASA Astronomy & Astrophysics Survey Committee (AASC) Decadal Survey (2000) and the NAS/NRC report on Ground-Based Solar Research: An Assessment and Strategy for the Future (1999). These reports make a strong and persuasive case for high-resolution studies of the solar atmosphere and the Sun s magnetic field. The generation of magnetic fields through dynamo processes, the amplification of fields through the interaction with plasma flows, and the dissipation of fields are still poorly understood. There is incomplete insight as to what physical mechanisms are responsible for heating the corona, what causes variations in the radiative output of the Sun, and what mechanisms trigger the flares and coronal mass ejections that affect the Earth, its climate, and its near space environment. Progress in answering these critical questions requires study of the interaction of the magnetic field and convection with a resolution sufficient to observe scales fundamental to these processes i.e., the pressure scale height, the photon mean-free path length, and the fundamental magnetic structure size. This proposal presents scientific rationale for a national investment in a new, ground-based, large-aperture solar telescope the Advanced Technology Solar Telescope (ATST). Such a telescope is required to provide high angular resolution and high sensitivity measurements that cannot be achieved any other way. The broad wavelength coverage (from the visible into the thermal infrared) provided by the proposed ATST also provides a unique capability and will allow observations spanning from the photosphere into the corona. Development of a 4-m solar telescope presents several challenges not faced by large nighttime telescopes. The enormous flux of energy from the Sun makes thermal control a paramount consideration, both to remove the heat without degrading telescope performance and to control mirror seeing. To achieve diffraction-limited performance, a powerful adaptive optics system is required that operates from the visible to infrared wavelength using solar structure as the wavefront sensing target. Low scattered light is essential for observing the corona but also to accurately measure the physical properties of small structures in, for example, sunspots. Highly efficient contamination control of the primary and secondary mirrors must therefore be addressed. The following major recent achievements in technology and instrumentation now make it possible to realize the ATST. A solar adaptive optics system in the visible and infrared is now in operation at NSO s Dunn Solar Telescope (DST). The Dutch Open Telescope (DOT) has an open-air design that provides diffraction-limited - 1 -
images. The development of high-precision vector polarimeters for the visible (NSO/HAO Advanced Stokes Polarimeter, the Swiss ETH ZIMPOL I and II polarimeters) and the infrared (e.g., NSO Near Infrared Magnetograph), and finally, the availability of large-format, high-speed detectors for the visible and infrared make it possible to do high-resolution imaging and precision spectroscopy and polarimetry over substantial fields of view. The ATST is a community wide program that will occur in two major phases. This proposal is for the design and development (D&D) phase. The early portion of the design phase will consist of concept development, refinement of science objectives, feasibility and engineering studies that address key technologies, specific conceptual designs for major subsystems, and critical design trade-offs and their effect on science drivers and costs. A concept design review (CoDR) will be held before detailed design work begins. The latter portion of the design phase consists of developing the detailed design and estimating definitive costs. A site survey will be carried out during the D&D phase. A site for the ATST will be selected based on the results of this survey. Dividing the project into a separate design and development phase is a model based on the ALMA (Atacama Large Millimeter Array) project. It permits better cost-control and will provide a more accurate estimate of the construction costs. The D&D phase will cost approximately $12.9M and includes technical trade studies, site selection, both telescope and instrument detailed designs, and a well-costed construction plan. Rigorous project management practices will be applied throughout the project. A critical design review will occur in the fourth year and will be followed by a construction phase proposal. During the second phase, we will construct, integrate, and commission the telescope. The cost of the project, including the design phase, is estimated at $70M. During the D&D phase, we intend to develop national and international partnerships to provide part of the construction cost (see support letters in Section I). The ATST D&D project is summarized in this project description, which also includes a discussion of the extensive educational outreach opportunities presented by the ATST project. Because of its potential for revolutionizing solar physics, its key role in the suite of solar instruments that will investigate the Sun over the next few decades, and the technical challenges posed by its development, three appendices are included to fully describe the project. Appendix I provides a detailed description of the science objectives and their implication for telescope design. Appendix II is a discussion of the technical effort needed to develop an ATST design, including design trade-off studies and instrument design. Appendix III describes the management plan, organizational structure, and work breakdown. To enhance the flow of the project description, the bibliography in Section D is referenced only in the detailed science write-up in Appendix I as well as the technical description in Appendix II. C.2 Science Drivers C.2.1 Why Does Solar Physics Need a Large-Aperture Telescope? The solar atmosphere provides an ideal laboratory to study the dynamic interaction of magnetic fields and plasma. Magnetoconvection is a fundamental process that is at the heart of many key problems of solar astronomy and astrophysics in general. For example, understanding the evolution of magnetic flux in the lower atmosphere is essential in addressing the most pressing problems in solar physics, such as the origin of magnetic fields, the irradiance variability, and heating of the corona. Magnetic fields provide channels for energy and momentum transport, thereby closely coupling the dynamics of the upper atmosphere to the convectively driven dynamic behavior of the magnetic field near the surface of the Sun. The photosphere represents a crucial interaction region where energy is easily transformed from one form to another. For example, kinetic energy from convective motion can be easily transformed into magnetic energy. The energy stored in the magnetic field is eventually dissipated at higher layers of the solar atmosphere, sometimes in the form of violent flares and coronal mass ejections (CMEs) that ultimately affect the Earth and drive space weather. The different layers of the solar atmosphere, namely the photosphere, the chromosphere and the corona are connected through the magnetic field and therefore have to be treated as one system, rather than individual Simulation of interaction between convection and flux tubes (O. Steiner). Phase-diversity reconstruction of solar photosphere showing bright structures associated with magnetic fields (Paxman, Seldin, Keller) - 2 -
layers. The ATST is a crucial tool needed for trying to understand this complex physical system. A main driver for a large-aperture solar telescope is the need to spatially resolve the fundamental astrophysical processes at their intrinsic scales in the solar atmosphere. It has long been argued that the fundamental spatial scales are the photon mean-free path and the pressure scale height. To resolve both fundamental length scales in the deepest accessible layers of the solar atmosphere, a resolution of 70 km or 0.1 arcsec is required. However, modern numerical simulations with a resolution of 10 km as well as inference from the best available images have suggested that crucial physical processes (e.g., vortex flows, dissipation of magnetic fields, magnetohydrodynamic (MHD) waves) occur on even smaller scales. Resolving these scales is of utmost importance to be able to test various physical models and thus understand how the physics of the small scales ties into the larger problems. An example is the question of what causes the variations of the solar radiative output, which impacts the terrestrial climate. The Sun s luminosity increases with solar activity. Since the smallest magnetic elements contribute most to this flux excess, it is of particular importance to study and understand the physical properties of these dynamic structures. Unfortunately, current solar telescopes cannot Figure 1 When observing the magnetic network and internetwork with high polarimetric sensitivity and high spatial resolution, about 10% of the Stokes-V line profiles (circular polarization) show an unusual shape. A "normal" V-profile, probably produced by a single magnetic element, is shown on the top. The other profiles show examples of unusual profiles. These unusual profiles are most likely caused by unresolved magnetic structures with opposite polarity within the resolution element and/or unresolved dynamic events. The ATST will, for the first time, clearly resolve these structures and dynamic events and allow us to compare the observations with numerical simulation. (M. Sigwarth.) even resolve such scales at visible wavelength because of their limited aperture. To illustrate the current state-of-the-art, Figure 1 shows precision polarimetric observations at the highest, currently achievable resolution using the HAO/NSO Advanced Stokes Polarimeter (ASP) and adaptive optics. Even after deploying the most modern technologies such as adaptive optics, much of the detailed physics remains hidden within the resolution element. The ATST is needed to be able to study these physical processes at the scales at which they occur. Figure 2 illustrates how, in the past, a substantial increase in resolution has helped to improve our physical understanding of solar phenomena. The ATST constitutes a quantum leap not only in terms of spatial resolution, but also in terms of wavelength coverage, of the diagnostic tools available and the capability to perform observations of very high sensitivity. - 3 -
Figure 2 Illustration of how a significant gain in resolution impacts the observational picture and consequently our physical interpretation of solar phenomena. Above is a continuum image (tick marks in arcseconds) of the primary sunspot in AR7548, 1993.07.23, as obtained with the previous generation of polarimeters, at approximately 6" spatial resolution. The corresponding Stokes polarization spectra (I, Q, and V only) from the indicated pixel (x) are shown. Below, today's spectropolarimeters (the HAO/NSO ASP) produce images and Stokes spectra with 0.5"-1" spatial resolution. A sample of the stokes spectra (Q, and V only) are shown from sub-elements within the box in the same region. The variation in Stokes V, and hence the magnetic structure, over the "pixel" is dramatic; equally dramatic is the gain in physical understanding that results from these observations of higher resolution. Also dramatic will be the additional information gained when, with the ATST, the spatial resolution increases by an additional factor of 5-10, finally providing observations that clearly resolve the solar fine structure and help us understand the important physics behind them. (K. D. Leka.) The planned ATST design also permits exploitation of the infrared. The near-infrared spectrum around 1.5 µm has many advantages, particularly for precise measurements of the recently discovered weak, small-scale magnetic fields that cover the entire solar surface and could be the signature of local dynamo action. An aperture of 4 m is needed to clearly resolve these features at 0.1 arcsec in the near infrared. Furthermore, the infrared beyond 1.5 µm provides particularly powerful diagnostics of magnetic field, temperature, and velocity at the upper layers of the solar atmosphere. For example, observations using the CO lines at 4.7 µm have - 4 -
already changed significantly our picture of stellar chromospheres and have resulted in a better physical understanding of this important layer in the solar atmosphere. Space missions like Yohkoh, SoHO, and TRACE have advanced our knowledge of the Sun's corona enormously and have renewed interest in diverse coronal plasma problems ranging from how coronal mass ejections are formed and accelerated, to how photospheric magnetic fields drive the inverted coronal temperature structure. These successful space missions have further demonstrated the need for accurate measurements of the coronal magnetic field. The magnetic sensitivity of the IR lines and the dark sky conditions in the IR are important motivation to utilize the ATST for exploring the IR coronal spectrum. Exploiting the unique diagnostic tools that are now becoming accessible in the IR solar spectrum with newly developed IR detector technology requires the largest possible telescope aperture. The ATST will provide improvement by a factor of nearly three in linear spatial resolution over the largest currently available solar IR facility. This translates into a factor of nearly ten in the ability to discriminate small 2-D cool or hot spots on a diffuse background. This is a level at which new science becomes possible. The requirement for a large photon collecting area is an equally strong driver toward large aperture as is angular resolution. Observations of the faint corona are inherently photon starved. The fact that in many cases observations of structures and phenomena on the solar disk are also suffering from a lack of photons may be less obvious. The reason is that the solar atmosphere is highly dynamic. Small structures evolve quickly, limiting the time during which the large number of photons required to achieve measurements of high sensitivity can be collected to just a few seconds. The following sections summarize some of the most pressing science questions that will be addressed with the ATST. A detailed description of the ATST s science goals is given in Appendix I. C.2.2 Flux Tubes, the Building Blocks of Stellar Magnetic Fields Observations have established that the photospheric magnetic field is organized in small fibrils or flux tubes. These structures are mostly unresolved by current telescopes. Flux tubes are the most likely channels for transporting energy into the upper atmosphere, which is the source of UV and X-ray radiation from the Sun, which in turn affects the Earth s atmosphere. Detailed observations of these fundamental building blocks of stellar magnetic fields are crucial for our understanding not only of the activity and heating of the outer atmospheres of late-type stars, but also of other astrophysical situations such as the accretion disks of compact objects, or proto-planetary environments. Current solar telescopes cannot provide the required spectroscopy and polarimetry at an angular resolution to explore the enigmatic flux tube structures. Sunspot at very high spatial resolution obtained with NSO Dunn Solar Telescope (T. Rimmele). C.2.3 Magnetic Field Generation and Local Dynamos To understand solar activity and solar variability, we need to understand how magnetic fields are generated and how they are destroyed. The 11-year sunspot cycle and the corresponding 22-year magnetic cycle are still shrouded in mystery. Global dynamo models that attempt to explain large-scale solar magnetic fields are based on mean field theories. Dynamo action of a more turbulent nature in the convection zone may be an essential ingredient to a complete solar dynamo model. Local dynamos may produce the small-scale magnetic flux tubes recently observed to cover the entire Sun. This magnetic carpet continually renews itself on a time scale of a few days at most and its flux may be comparable to that in active regions. The ATST will make it possible to directly observe such local dynamo action at the surface of the Sun. The ATST will measure the turbulent vorticity and the diffusion of small-scale magnetic fields and determine how they evolve with the solar cycle. The ATST will address the following fundamental questions: How do strong fields and weak fields interact? How are both generated? How do they disappear? Does the weak-field component have global importance and what is its significance for the solar cycle? The ATST will address these fundamental questions by resolving individual magnetic flux tubes and observing their emergence and dynamics. It will measure distribution functions of field strength, field direction and flux - 5 -
tube sizes and compare these with theoretical models. The ATST will observe plasma motions and relate them to the flux tube dynamics. C.2.4 Interaction of Magnetic Fields and Mass Flows In sunspots, the total magnetic field is large enough to completely dominate the hydrodynamic behavior of the local gas, a regime very different from that of the rest of the solar photosphere. Numerical simulations and theoretical models predict dynamical phenomena, such as oscillatory convection in the strong-field regions of sunspot umbrae, flows at the speed of sound along penumbral filaments and oscillations and wave phenomena. To verify the predictions of numerical simulations of sunspots and ultimately answer such fundamental questions as Why do sunspots exist? require an extremely capable instrument. High-resolution (<0.1 arcsec) vector polarimetry combined with high sensitivity (requires high photon flux) and low-scattering optics are required. Understanding the interaction of magnetic flux and mass flows is crucial for our understanding of the behavior of magnetic fields from the scales of planetary magnetospheres, to star-forming regions, to supernova remnants, to clusters of galaxies. Sunspots allow us to test those theories in a regime where magnetic fields dominate mass flows. C.2.5 Inhomogeneous Stellar Upper Atmospheres Measurements of CO absorption spectra near 4.7 µm show surprisingly cool clouds that appear to occupy much of the low chromosphere. Only a small fraction of the volume apparently is filled with hot gas, contrary to classical static models that exhibit a sharp temperature rise in those layers. The observed spectra can be explained by a new class of dynamic models of the solar atmosphere. However, the numerical simulations indicate that the temperature structures occur on spatial scales that cannot be resolved with current solar infrared telescopes. A test of the recent models requires a large-aperture solar telescope that provides access to the thermal infrared. Such observations would further explore the dynamical basis of the thermal bifurcation process, a fundamental source of atmospheric Coronal loop as seen with TRACE (A.Title). inhomogeneities in late-type stars. Spicules, the forest of hot jets that penetrate from the photosphere into the chromosphere, are clearly a MHD phenomenon that is not understood nor adequately modeled. Their role in the mass balance of the atmosphere is uncertain. Combined with UV observations (like those of TRACE), the ATST will allow us to resolve their nature. C.2.6 Magnetic Fields and Stellar Coronae The origin and heating of the solar corona, and the coronae of late-type stars, are still mysteries. Most of the proposed scenarios are based on dynamic magnetic fields rooted at the 0.1-arcsec scale in the photosphere. However, none of the processes has been clearly identified by observations or theory. EUV and X-ray observations have gained in importance, but ground-based observations are still critical, not only to determine the forcing of the coronal fields by photospheric motions, but also for the measurement of the coronal magnetic field strength itself. This is important for developing and testing models of flares and coronal mass ejections, which propel magnetic field and plasma into inter-planetary space and induce geomagnetic disturbances. In particular, precise measurements of the coronal magnetic field strength and topology are needed in order to distinguish between different theoretical models. The ATST, with its large aperture, low scattered light characteristics, and the capability to exploit the solar infrared spectrum will provide these critical measurements. C.2.7 Cross-Disciplinary Impacts The processes discussed in the previous sections involve fundamental physics, fluid dynamics, plasma physics, and magnetohydrodynamics (MHD), which are processes that occur throughout the Universe. The Sun provides a unique laboratory for studying these key astrophysical processes in detail under astrophysical conditions. For example, such important processes as MHD waves, MHD turbulence, magnetoconvection, magnetic reconnection, and magnetic buoyancy occur in other stars, nebulae, galactic centers, and intergalactic interactions, but can be observed in detail only on the Sun. Solar flares serve as a prototype for many high- - 6 -
energy phenomena and particle acceleration mechanisms in the Universe. The cause of the Sun's million-degree corona must be understood before we can confidently interpret the EUV and X-ray emission from distant astrophysical objects. Almost all astronomical objects (stellar and galactic) are active, producing intense suprathermal emissions, both quasi-steady and transient. Detailed observations of small-scale solar magnetic structures will help us understand other astronomical phenomena such as accretion disks of compact objects and proto-planetary environments. The study of solar plasma-magnetic field interaction (magnetoconvection) has the potential of revolutionizing our understanding of solar and stellar structure, stellar activity and mass loss, stellar atmospheres, and how the Sun generates the variability that affects the Earth. Several specific examples are given in Appendix I. In most cases, our scientific understanding of these solar phenomena is limited by our inability to resolve them at spatial scales less than a few tenths of an arcsecond. Presently no models of solar active regions exist that can produce accurate quantitative predictions of when and where activity will occur on the Sun, or predict what the magnitude of the emissions resulting from that activity will be. The accuracy of solar activity predictions is limited by incomplete understanding of the underlying physical processes in the evolving solar atmosphere. The ATST will provide data needed to develop models for the magnetic evolution of active regions, the triggering of magnetic instabilities, and the origins of atmospheric heating events. These models will provide a much better capability for understanding and predicting when and where activity will occur on the Sun and for predicting the level of enhanced emissions expected from these events. These in turn will permit solarterrestrial models to be refined to include critical solar data instead of the proxies that are used now. Outfitted with modern technology, the ATST will provide the necessary sensitivity and spatial and temporal resolution for the next epoch in solar research the detailed elucidation of the physics of solar and stellar activity and variability. Gamma-ray Magnetic Energy Domain X-ray EUV UV Visible Near-IR Thermal IR Radio 0.02 0.2 S p a c e Convective Energy Domain Solar-B SDO 2 20 HESSI Solar-B ATST 200 Rotational Energy Domain SOHO SOLIS FASR 2000 Wavelength Figure 3 ATST compared to other solar assets. C.3 The ATST in Context While many areas of solar physics such as helioseismology, measurements of coronal structure, and studies of small-scale surface dynamics and fields have grown independently, we now understand that the Sun is a highly coupled system. Physical processes that occur on large and small scales, in the interior and at the surface, interact to produce its complex behavior. It is not possible for a single telescope alone to address the broad range of question that must be answered to understand the Sun. Only by combining the data from many instruments, many time scales, and many spatial dimensions, will true progress be made. - 7 -
The Decadal Survey panel report on solar astronomy emphasized the importance of this interplay and the important role the ATST would play. When coupled with high-energy and other observations from space, e.g., SOLAR-B, Solar Dynamics Observatory (SDO), High Energy Solar Spectroscopic Imager (HESSI)), with ground-based radio observations (Very Large Array (VLA), Very Large Baseline Array (VLBA), Frequency- Agile Solar Radio (FASR) Telescope), and with long-term synoptic observations (SOLIS, ISOON, GONG, BBSO, Mees, Marshall), which place the current observations in the context of the solar cycle, the ATST optical/infrared observations will enable a complete picture of the Sun to be developed from its interior to interplanetary space. Figure 3 shows the complementary nature of the ATST and many of the assets that are available now and that will be available in the future. C.4 Telescope and Instrument Requirements The final design of the ATST and its first light instruments will be the product of the proposed design and development (D&D) phase, during which a detailed flow down of the design requirements for the telescope and instruments from the science requirements will be performed. The straw man of the basic telescope requirements summarized below and the resulting technical challenges have been determined during several workshops involving large parts of the community. Appendix II provides a more detailed description of the proposed scope and technical approach to the D&D phase. The science requirements also determine the site requirements, which are also discussed in more detail in Appendix II of this proposal. C.4.1 Resolution The 4-m aperture combined with adaptive optics (AO) will provide diffraction-limited spatial resolution (0.03 arcsec at 500 nm, 0.08 arcsec at 1.6 µm) within the isoplanatic patch, which is of order 10 arcsec in the visible and significantly larger in the infrared. Recent experience with solar adaptive optics systems at Sacramento Peak and at the Canary Islands demonstrates that AO substantially improves the image quality over a much larger field of view (arcminutes) by correcting the near-ground seeing that often dominates the day-time seeing. Sub-arcsecond resolution can be achieved over these much larger fields of view. These data can be further processed using post-facto image processing techniques such as phase diversity to arrive at diffraction-limited observations. In the future Multi-Conjugate Adaptive Optics (MCAO) is likely to directly achieve diffraction-limited resolution over field of views of several arcminutes. Primary Mirror 4 meters Wavelength Range 0.3 to 35 µm Field of View 5 arc minutes Diffraction Limited Resolution 0.03 arc sec @ 0.5µ 0.08 arc sec @1.6µ Adaptive Optics Visible and infrared Low Scattered Light <10ppm @ λ>1µm, r=1.1rsun Low and stable instrumental Polarization Spatial resolution is only one aspect. The science goals require an instrument that enables high-resolution observations in the most general sense. High resolution includes high-spatial, high-temporal, and high-spectral resolution. All three of these are important to our understanding of small-scale phenomena on the Sun. C.4.2 Photon Flux and Sensitivity Why solar observations are often photon starved: Achieving high spatial, temporal and spectral resolution simultaneously requires a high throughput of photons, which in turn requires a large telescope aperture. The requirement for high throughput is an equally strong driver toward large aperture as is angular resolution. The number of photons per angstrom per second per diffraction-limited angular resolution element is independent of aperture size. Photospheric structures can move or evolve with speeds close to the sound speed of about 6 km/s. So for features 0.1 arcsec or smaller, one must collect photons within a few seconds to avoid spatial smearing. Thus, while a 1-m telescope could, in principle, achieve 0.1-arcsec diffraction-limited resolution in the optical, in practice, a 4-m aperture is necessary to accumulate sufficient photons to allow an accurate measurement of, for example, the vector magnetic field over a time scale in which the dynamic scene does not evolve significantly. Another strong driver for the large telescope aperture are observations of the faint corona. - 8 -
C.4.3 Polarization Accuracy Accurate measurements of field strength and direction require a telescope with low instrumental polarization. In order to achieve an accuracy of 10-4 in the polarization measurements, instrumental polarization can only be of order 1%. Even at that level any instrumental polarization must be accurately calibrated as a function of telescope pointing. C.4.4 Low Scattered Light A low scattered light facility is a requirement for the envisioned coronal capabilities of the ATST but also for many other observations. For example, sunspot observations, and in particular observations of the umbra, and umbral and penumbral fine structure, also require instruments with low-scattering optics. Large sunspots with field strength in excess of 3 kg often have residual intensities of less than 10%. In order to accurately measure physical parameters in the umbra, the umbral signal must be at least an order of magnitude above the noise introduced by scattered light from the surrounding photosphere. This requires scattered light from the instrumentation to be of order 1% or less. Coronal observations require scattered light to be limited to less than 10-5 of solar disk intensity at 1.1 solar radii and for infrared wavelength. C.4.5 Wavelength Coverage In order to address the scientific problems stated above, a wide range of diagnostic tools has to be applied. From the beginning, the ATST will be a well-instrumented telescope that allows the combination of different instruments covering a large wavelength range from the UV to the thermal infrared (0.35 35 µm). A first complement of ATST instruments will include: Visible and IR imaging cameras; Medium- and high-dispersion spectrographs for visible and near-ir; Thermal-IR spectrograph; Visible and IR polarimeters; Narrow-band filter systems. Instrumentation will be designed using a modular approach, which allows use of the same modules to assemble different instruments. For example, the same polarimetry package can be used in combination with a spectrograph or a narrow-band imaging filter system. C.4.6 Field of View In order to allow studies of the evolution of entire active regions, the ATST will provide a field of view (FOV) of 5 arcmin. The quality of the telescope optics will be <0. 1 within this FOV. C.4.7 Location The best affordable site in terms of seeing, sky clarity and sunshine hours will be chosen in order to maximize the telescope performance and minimize the cost of adaptive optics. C.5 Summary: ATST Parameters and Example Science Drivers Table 1 presents a flow down from ATST science goals, to observables, to the requirements these place on the telescope design. A more detailed discussion of the science goals that are summarized in Table 1 can be found in Appendix I. One of the early objectives of the D&D phase is to finalize this table. The table also presents telescope and site parameters and the instrumentation needed to achieve the desired capabilities. C.6 Straw Man Telescope Design Solar physicists have developed many unusual telescopes and instruments specifically for solar observations. To focus the ATST D&D effort, we will begin with a straw man optical layout that draws on previous experience with solar telescopes and recent design studies as mentioned in Section 1 of Appendix II. Table 2 summarizes the basic telescope and performance parameters. Many existing solar telescopes are off-axis designs (e.g., the Dunn Solar Telescope (DST)). - 9 -
Table 1. Summary Science Objectives and Flow Down to Instrument Requirements Science Goals Observational Requirements Telescope Requirements Telescope and Site Parameters Instruments Gain Physical Understanding of: Magnetoconvection Dynamo processes: Small-scale dynamo Origin of weak field and importance for solar cycle Flux emergence and dissipation Formation, destruction, internal structure of flux tubes/sheets Generation of acoustic oscillations Chromospheric and coronal heating Observe fundamental astrophysical processes at the scales at which they occur Measure B and plasma parameters with sufficient spatial and temporal resolution and sufficient accuracy to test theoretical models, e.g., flux tubes Observe: visible and near-infrared spectrum, rich diagnostics Spatial resolution: 0.05 at 500 nm 0.1 at 1.6 µm Large photon flux Accurate polarimetry (>10-4) Low scattered light Aperture: 4 m Seeing control: Adaptive optics Thermal control Low and stable instrumental polarization < 1% Good seeing site VIS/NIR spectrographs and polarimeters VIS/NIR narrow-band filters VIS/NIR Detectors MHD waves Triggering of activity Structure of sunspots Gain Physical Understanding of: Structure and dynamics of upper atmosphere: Shock wave heating COmosphere MHD-wave and topological heating Prominence formation and eruption 3D-structure of magnetic field: Chromospheric and coronal magnetic fields Measure B and plasma parameters in upper atmospheric layers Observe: CO lines at 4.8 µm (transition zone from β>1 to β<1, T diagnostics) MgI at 12 µm (B, upper photosphere) HeI 1.0830 µm (B, chromosphere) Coronal lines at e.g., 1.0747 and 3.9µm, (B, corona) High resolution IR access (>12microns) Large photon flux Accurate polarimetry Low scattered light < 10e-5 at 1.1 solar radii Coronagraphic capabilities in IR Aperture: 4 m or larger Open-air design Dust Control Adaptive Optics Unobscurred light path Low sky brightness NIR/thermal IR detectors NIR/Thermal IR spectrograph and polarimeter Active Region Evolution Understand process of activity build-up and triggering Dynamo processes Measure B and plasma parameters at different layers in atmosphere and over entire active region Long time series Large FOV (>5 arcmin) High resolution over large FOV AO & site with large isoplanatic patch or MCAO All of the above Explore the unknown New discoveries E.g., Explore IR spectrum >12 µm Flare spectra in IR Flexibility, adaptability Multi-observing stations Ability to implement new ideas All of the above Users furnished Solar/stellar connection Solar system objects Astroseismology Extra-solar planets Stellar spectroscopy Dedicated instrument, long time series of high-resolution stellar spectroscopy Coronagraphic observations Large dynamic range observations High spatial and spectral resolution Large dynamic range Good instrumentation Low scattered light Coronagraph Largest possible aperture Nighttime operations High-resolution spectrograph Nighttime AO VIS/IR detectors - 10 -
The f-ratio of these telescopes, however, is rather long (e.g., DST, f/72) and therefore issues such as heat-stop design become relatively simple. Such long f-ratios, however, are impractical for a 4-m aperture telescope. The concept of a 4-m, fast primary off-axis telescope is an exciting new concept for solar physics. It offers the opportunity to develop an extremely capable telescope that can address all of the scientific problems discussed in the Science section (Appendix I), and promises to revolutionize our understanding of the Sun and stars. Table 2: Straw Man Telescope Aperture: 4m Optical configuration: Gregorian, off-axis FOV: 5 arcmin Optical Quality: <0. 1 over FOV Adaptive Optics: Strehl >0.5 within isoplanatic patch Wavelength Coverage: 350 nm - 35 microns Polarization Accuracy: Better than 10E-4 of Intensity Coronagraphic: in the IR Scattered Light: <10E-5 at r/rsun = 1.1 and >1µm During the conceptual design phase of the ATST D&D project, we will identify the specific optical requirements imposed by each scientific goal, prioritize these requirements, determine how well the straw man configuration meets these requirements and develop trade-offs and alternatives as necessary. Some compromises and trade-offs may be necessary. A weighted decision matrix based on the science requirements will be used to select the final optical configuration best suited for ATST. C.6.1 Design Challenges There are several critical technology areas where a moderate up-front investment will largely reduce the risk involved in developing and using these technologies, aid in selecting the components that will ensure scientific requirements can be met, and/or allow accurate cost estimates. Crucial technologies required for the ATST have recently been developed and successfully demonstrated at existing solar telescopes of smaller aperture (AO, IR, open-air design). Previous initial designs studies (e.g., CLEAR) did not identify any technical show stoppers and with the selected straw man concept, additional work is needed in the following critical areas. C.6.1.1 Mirror Seeing Most existing solar telescopes avoid image degradation caused by heating of the optical surfaces by placing the optics in an evacuated tube. However, the science drivers for the ATST require an open-air design. Therefore a major design consideration is the thermal control of the primary and secondary optics to ambient temperature in order to avoid mirror seeing. Maintaining the surface of the primary and secondary mirrors at ambient - 11 -
temperature, despite the roughly 100 W/m 2 of energy that they each absorb, is particularly critical for the ATST in avoiding the generation of seeing, or image degradation within the telescope s optical path (Appendix II, Section 3.4). C.6.1.2 Energy Removal Approximately 12 kw of power must be effectively removed from the input beam at a with AO without AO telescope focus without degrading the telescope s performance. A heat stop must be designed for the prime focus to remove the tremendous solar heat load. The heat stop is a critical item for a large-aperture, fast solar telescope. A prototype should be developed and tested under realistic conditions (Appendix II, Section 3.7). C.6.1.3 Adaptive Optics The design of the ATST must include an adaptive optics system that operates from the visible to the thermal infrared wavelengths using solar structure as wavefront sensing target. AO will enable solar astronomers to perform diffraction-limited imaging, and, more importantly, to resolve the fundamental scales in spectroscopic and polarimetric observations of solar fine structure. NSO has invested substantial resources in demonstrating the first solar AO system that works with solar granulation as the wavefront sensor target. This low-order (24 subaperture Shack Hartmann) AO system achieves diffraction-limited imaging in good seeing conditions. A 4-m class ATST will require a much larger AO system with several hundred degrees of freedom. To minimize the risk involved in the development of large solar AO systems, this development will be performed in steps. Comparison of the images of solar granulation that were taken at 500 nm with the Dunn Solar Telescope shows the improvement offered by AO. The image on the left has been corrected with a low-order AO system. The uncorrected image on the right was obtained simultaneously, but shows part of the same field of view at a slightly higher magnification (T. Rimmele) In collaboration with the New Jersey Institute of Technology/Big Bear Solar Observatory (BBSO), the Kiepenheuer Institute for Solar Physics (KIS), and the Air Force Research Laboratory (AFRL), an AO system with about 80 subapertures is being developed. A goal is to develop scalable technology that can be utilized for the much larger ATST AO system (Appendix II, Section 7). AO will be combined with post-facto image reconstruction techniques, such as phase diversity and speckle reconstruction. This combination has already produced stunning imagery of small-scale magnetic elements recorded at the DST. C.6.1.4 Contamination Control and Scattered Light Accurate, high spatial resolution polarimetry of features such as flux tubes, magnetic pores, and sunspots, as well as pointing off the solar disk to measure coronal magnetic fields, require a low-scattered-light telescope. To meet the relatively stringent scattered-light requirements of the ATST, highly efficient contamination control of the primary and secondary mirrors must be addressed (Appendix II, Section 3.8). C.6.1.5 Instrumentation Designing an initial set of focal plane instrumentation is an important part of the overall design and development effort. A first step will be to refine the straw man for the initial or first light suite of instruments (Appendix II, Section 9.2) in close collaboration with partners and the solar community. There is wide agreement on what some of these instruments will be. For example, visible and Velocities inside a sunspot measured with the AO system at the Dunn Solar Telescope (T. Rimmele) near-infrared spectropolarimeters, as well as visible tunable filters, are considered essential instruments. Other instruments, such as an IR fiber spectrograph have to be better defined. The university partners, HAO and NASA collaborators will do much of the instrumentation design work. The partner contributions are described in detail in Appendix III. C.6.2 Site Selection The choice of a site for the ATST is an important aspect in its design. Although there is no technical challenge involved, the selection of an excellent site for the ATST within a relatively short period of time may influence - 12 -
schedule, and to some extent cost, of the project. The dominant site requirements are: minimal cloud cover, many continuous hours of sunshine, excellent average seeing and many continuous hours of excellent seeing, good infrared transparency, and if a coronal option is chosen, frequent coronal skies. An ATST site survey working group (SSWG) has been formed to ensure quality site evaluation and selection. This group oversees the ATST siting activities and ultimately recommends a site for the ATST. Members of this committee include experts in the areas of atmospheric seeing, site testing, and infrared and coronal observations from both the US and international community. The committee s charge includes definition of relevant site test parameters, methods of measuring these parameters, and criteria to be used for site selection. The committee has already presented a straw man of the ATST site test plan (Appendix II, Section 10). This plan will be further refined during the first months of the D&D effort. Because of the time it requires to perform a thorough site test campaign, the instruments needed to carry out the site test measurements should be constructed and deployed as early as possible. NSO has started to conduct an exploratory site-testing program using scintillometers and is currently building a solar differential image motion monitor (S-DIMM), which will be used to determine the seeing parameter r 0 at the test sites. C.6.3 Trade and Design Studies Table 3 summarizes the major trade and design studies that will be performed during the D&D phase. The table captures the main issues that are unique to the ATST development. The items highlighted in bold text indicate the straw man design. A detailed discussion can be found in Appendix II, Section 3. Technical Issue Optical configuration Primary and secondary mirror temperature control Heat stop Contamination control Coatings Adaptive optics Primary and secondary mirror materials Guiding and tracking Mount configuration Enclosure Polarimetry Post-focus instruments Focal locations of instruments Control systems Site selection Table 3: Trade and Design Studies Major Alternatives On- or off-axis Apalanar or classical type optics Instrument locations F-ratio Air, liquid, or solid-state cooling Heating of front surface or passive Air-flow across mirror or not Absorbing vs reflective Air or liquid cooling medium Sheath flow over open mirror or overpressure in tube, electrostatic, frequent CO 2 cleaning Aluminum, protected silver, or multi-layer metallic layer Degrees of freedom Location in optical train ULE, Zerodur, or SiC Guiding telescope optically or mechanically coupled to main telescope Blind pointing Equatorial, Alt-az or alt-alt-az Conventional dome or LEST-type or rolling enclosure or exoskeleton Various polarization modulating schemes covering the full wavelength range Design first-light instrumentation Building blocks approach Gregorian, Coudé, other Monolithic or distributed Test mountain vs lake vs marine environment - 13 -
C.7 Implementation C.7.1 ATST Development and Management Plan The structure of the ATST project design and development followed by construction, and integration is a construct suggested by the NSF/AST, based on their recent experience with the ALMA project. This experience resulted in the conclusion that paper studies alone are not sufficient for a successful design effort and that trade studies and prototyping must accompany design. With trade studies and prototypes, the performance and cost of the facility to be constructed can be realistically assessed. During the fifth fiscal year of the project, the D&D phase will produce the final drawings, leading to a seamless transition to the construction phase of the project. Appendix III gives a detailed discussion of the ATST program and management plans as summarized below. C.7.2 Design and Development Phase The following program provides a staged development effort for the ATST. Validation of critical technologies, required for design development, to support fiscal planning and to ensure optimum scientific return from the telescope will occur in the first half of the design phase and will take advantage of previous efforts to define large-aperture solar telescopes. The technical issues to be explored are shown in Table 3. NSO has already started exploring a few of these issues and is committed to a substantial investment of its current resources to developing the ATST program. Formal concept definition will be developed, including definitive trade-off studies, engineering design development, critical risk assessment and mitigation planning, and the development of procurement and management plans. Major components of the D&D phase are the following: Formation of the core project team. A full-time team will be established during the first year with the use of contracted assistance for specific design tasks. NSO will house this team and coordinate the various design tasks, many of which will be conducted at the various ATST partner organizations. During the second year, we will ramp up to full staffing for D&D phase procurement and ongoing design and contract management efforts. Design development. Specific design studies will be carried out for major subsystems, including optics, structures, facility, adaptive optics, primary focal plane instrumentation and cameras, and controls. NSO will begin technical development of the more complex AO system needed for a 4-m aperture. Risk mitigation and cost control development. Development studies and/or experiments on industrial processes and equipment designs, which can result in meaningful mitigation of technical risk and program cost control. Conceptual design review. A CoDR will be held early in the second year of the design phase. The review will include results of initial studies, including concept development and analysis, and provide a decision point for design concept selection. It will allow the science advisory committee the opportunity to assess how well the concepts meet the scientific requirements and to redirect the design efforts if necessary. Preliminary design review. A PDR will be held during the third year of the design phase. The review will include results of all studies to date, including site testing and selection and design development and analysis. It will provide a decision point for the NSF regarding project status, and will allow the scientific advisory committee an opportunity to assess how well the developed design is meeting the scientific requirements. Critical design review. During the fourth year, the design phase will culminate in the presentation of the full ATST design, including instrumentation. A plan for constructing and commissioning the ATST will be presented, as well as an accurate assessment of costs to build and operate the telescope. The CDR will permit NSF and its partners to finalize their commitments for constructing the full-up telescope facility. A successful CDR will result in the submission of a construction phase proposal to the NSF/MRE (Major Research Equipment) program. - 14 -