Advanced Technology Solar Telescope

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advanced technology solar telescope Inside front cover left blank Advanced Technology Solar Telescope

The Need: We are positioned for a new era of discovery about the Sun and the subtle ways it affects life on Earth. To 1 embark on this journey we must observe the Sun and its magnetic activities at higher spectral, spatial, and temporal resolutions that will be made possible with the 4-meter Advanced Technology Solar Telescope (ATST).

Getting the fi ne structures to see the big picture: Where modern telescopes can resolve features the size of the 3 state of Texas, ATST — using adaptive optics — will be able to resolve features as small as counties, 30 to 70 km (18-42 mi) across, and thus reveal the fi ne aspects of activities that control an entire star.

The Science: We now see the Sun as a complex of electrifi ed gases whose shape and fl ow are controlled by 4 magnetic fi elds. ATST will study the outer solar atmosphere where magnetic activities manifest themselves as sunspots and other changing phenomena that can affect life on Earth.

Sun and Climate: Discoveries that the solar constant varies — indeed, some aspects vary daily — sheds new light 5 on our understanding of how the Sun warms our home planet, and drives the need to understand those periods when solar variations have dramatically altered terrestrial climate.

ATST and the many colors of the Sun: ATST will take advantage of the full spectrum of solar radiation that is 8 passed by Earth’s atmosphere, from near ultraviolet through deep infrared, to provide powerful diagnostics of solar activities.

Looking beyond the Sun: Mysteries of the Sun are also illuminated by studying other stars, both sunlike and 10 beyond, just as astrophysics is enriched by using our Sun as a model for phenomena that cannot be resolved with current instruments.

11 The Telescope: ATST will be a world-class, 4-meter (13.1 ft) optical telescope that will serve the solar physics community to the middle of the 21st century and beyond. It will host an array of instruments — many using technologies still in the laboratory— and incorporate a world-class solar physics institute.

Smoothing the wrinkles out of our view of the Sun: Large solar telescopes have not been practical because of 13 limits imposed by atmospheric turbulence. Adaptive optics technologies can compensate for much of atmospheric blurring and produce sharp images and spectra at faster rates.

The ATST Team: ATST is being developed by a consortium led by the NSO and comprising the University of Chi- 17 cago, New Jersey Institute of Technology, University of Hawaii, and the High Altitude Observatory, plus elements of NASA, the U.S. Air Force, and other private and public institutions.

The National Solar Observatory: As the nation’s premier solar physics institution, the NSO provides international 18 1/2 relative size leadership in observing the Sun and in developing advanced new instruments, and offers research opportunities Advanced Technology Solar Telescope Collaborating Institutions National Solar Observatory from high school through post-graduate studies. High Altitude Observatory New Jersey Institute of Technology University of Hawaii University of Chicago University of Rochester University of Colorado University of California Los Angeles University of California San Diego California Institute of Technology National Solar Observatory Princeton University Stanford University Montana State University P.O. Box 62 950 N. Cherry Avenue Michigan State University California State University Northridge Harvard-Smithsonian Center for Astrophysics Colorado Research Associates Sunspot, NM 88349-0062 Tucson, AZ 85719-4933 Southwest Research Institution Air Force Research Laboratory Lockheed Martin 505-434-7000 520-318-8000 NASA/Goddard Space Flight Center AURANASA/Marshall Space Flight Center http://atst.nso.edu or http://www.nso.edu The National Solar Observatory operates under a cooperative agreement between the Association of Universities for Research in Astronomy, Inc. (AURA) and the National Science Foundation. © 2003 National Solar Observatory

Cover photo: The Sun in H as seen by the U.S. Air Force Improved Solar Optical Observing Network (ISOON) facility located at NSO’s Sun- spot, NM, site. Chapters in this book show the progression of the active region at center on April 30, May 1 (on the cover), and May 2, 2003. Anatomy of the Sun Cross-section At 1.4 million km, the Sun is 109 times wider of outer solar atmosphere Coronal Corona mass than Earth, yet much of what we know about its ejection interior comes from what we can see through Transition region, its outer atmosphere, as depicted here. Fusion Relative size of Earth { Chromosphere, occurs only in the core; heat and energy move Photosphere 12,723 km (7,908 mi) diameter through the radiative zone, and then drive the convection zone. These are concealed by view. We can only observe the photosphere, chromo- Corona Flare sphere, and transition region — with a combined Millions of kilometers into space depth less than 1/250th the radius of the Sun > 1 million K (1.8 million ˚F) (thinner even than the diameter of Earth). We can Transition region also observe the corona and activities like fl ares ~50-100 km (32-62 mi) deep, Core and coronal mass ejections which cross several rapid heating layers at once.

Seen only during solar eclipses or with special Radiative zone telescopes, the faint corona is the Sun’s tenuous outer atmosphere and extends deep into space. One of the mysteries of modern solar physics Convective zone is why the solar atmosphere suddenly soars in temperature in a thin transition region between the lower corona and the chromosphere. Coro- nal events are closely linked to magnetic activities in the lower atmosphere.

NASA/Marshall Space Flight Center NSO/AURA/NSF Extended magnetic fi eld lines arcing The chromosphere emits and absorbs a wide through the chromosphere and into the 1 range of colors as temperatures and pressure corona can reconnect, causing an ex- change. Yet through its depth, we can see how plosive fl are and/or releasing a coronal magnetic fi elds link features from the corona mass ejection (CME) into deep space. to the visible surface even as they twist and 2 Before onset Eruption onset change, as outlined by the inset image at far left. At immediate left, an abbreviated representation of layers of the atmosphere depicts 1) extreme 3 ultraviolet from iron (17.1 nm) at the bottom of the corona, 2) H (hydrogen-alpha; 656.3 nm) tracing out solar active regions, 3) calcium II K 4 (393 nm) showing magnetic fi elds, and 4) G- Confined eruption, ending Ejective eruption, middle band (430 nm) revealing features in the “visible surface.” In general, each spectral line relates to a specifi c temperature and thus the altitude of

NASA/Transition Region and Coronal Explorer (1), the source. Bart De Pontieu, Swedish Vacuum Telescope (3-4)

Gas inside the Sun is so dense that it reabsorbs light as quickly as it is emitted. The density decreases with distance from the surface until Chromosphere light at last can travel freely and thus gives the ~2,000-3,000 km (~1,243-1,864 mi) deep illusion of a “visible surface.” Escaping light also ~6,000-10,000 K (~10,340-17,540 ˚F) cools the gas, setting up granular circulation cells like an immense boiling pot (enlarged inset Photosphere (“visible surface”) of sunspot). Magnetic lines also emerge from the ~400 km (~240 mi) deep interior, suppressing convection (lower image) ~5,780 K (~9,944 ˚F) and forming sunspots of cool gas that often are NSO/AURA/NSF larger than Earth (size indicated by black circle Solar interior below inset). ~696,000 km (~432,567 mi) to center

~15.7 million K (~28.3 million ˚F) Heights and temperatures are approximate and variable; colors, shapes and positions are illustrative. NASA/ESA SOHO Consortium 1 The Need

ince George Ellery Hale’s 1908 discovery that sunspots Further, we must observe not just in familiar near-ul- coincide with strong magnetic fi elds, we have become traviolet and visible light, but exploit the relatively unex- increasinglyS aware of the Sun’s magnetic fi elds as an com- plored infrared solar spectrum. We must see the faint solar plex and subtle system. The familiar 11-year sunspot cycle corona in infrared, measure the polarization of sunlight is just the most obvious of its many manifestations. Recent with greater precision, and cover a large fi eld of view so advances in ground-based instrumentation have shown extended active areas can be studied. These capabilities will that sunspots and other large-scale phenomena that affect reveal hidden aspects of magnetic activities and help us life on Earth are intricately related to small-scale magnetic bridge the gap from what we know now to what we must processes whose inner workings are beyond what current learn. But doing so requires a large telescope to overcome telescopes can observe. limitations imposed by current instruments.

At the same time, using advances in computer sci- NSO’s long range plan recognizes that progress in un- ence and technology, scientists have developed intrigu- derstanding the Sun requires that it be treated as a global ing new theories about those small-scale processes, but system, in which critical processes occur on all scales, from without real-world observational data to verify their the very small (<100 km) to scales that encompass the models. We are positioned for a new era of discovery whole Sun. This was recognized by the National Research about the Sun and how it affects life on Earth, how dis- Council in its recent decadal report, “Astronomy and As- tant stars work, and how we might control plasmas in trophysics in the New Millennium (2001)”: laboratories. The fi rst scientifi c goal for advancing the current under- To embark on that journey, we must observe the Sun standing of solar magnetism is to measure the structure and and its magnetic activities at higher resolutions on three dynamics of the magnetic fi eld at the solar surface down to fronts: its fundamental length scale. • Spatial — resolve fundamental scales of structures on the solar surface and in its atmosphere, Despite the brightness of the Sun, solar physicists share • Spectral — narrow slices of the solar spectrum for bet- a problem with their nighttime colleagues: “photon starva- ter measurements of magnetic fi elds and thermody- tion.” While bright images of the solar disk, the corona, namics, and sunspots, and fl ares are the most familiar of solar observa- • Temporal — high cadence (frequent) images and spec- tions, defi nitive work is done at high spectral and spatial tra of rapidly developing events. resolution while observing a small section of the Sun in a

Transient eruptions: Origin of solar variability Heating of corona and Surface and The unknown Coronal mass ejections chromosphere; atmospheric structure and flares Origin of solar wind Activity

High spatial resolution

High photon flux

Capability Thermal infrared

• Origin of interstellar • Irradiance variations of atmospheres p-mode oscillations • Best solar optical

Impact matter sun-like stars • Accretion disk coronae telescope in the world • Stellar flares

NSO/AURA/NSF ATST’s capabilities will allow us to probe a broad range of solar activities that have signifi cant social and scientifi c impacts.

1 NSO/AURA/NSF Adaptive optics (AO) is the leading technical advance that makes large-aperture solar telescopes practical. At the heart of such systems are thin, deformable mirrors (like the one at right of center) teamed with advanced software to iron out the wrinkles in our view of the Sun. While AO made large astronomical telescopes possible starting in the 1980s, solving the problem for the Sun has been an engineering challenge that we have met only in the last few years (page 13). spectrally narrow subset of the available light. This is like resolution of smaller telescopes. Adaptive optics (AO), an looking through a microscope and switching to higher emerging technology that corrects most of the atmospheric powers and inserting a color filter: as you get closer to the distortion, can enhance existing solar telescopes, but just to answer, you also reduce the available light, eventually close the diffraction limit set by their small apertures. Orbiting to blackout. telescopes have a perfect seeing environment, but are ex- pensive and have limited lives. Also like their nighttime counterparts, solar physicists cope with atmospheric “seeing.” Looking through Earth’s A large ground-based telescope — such as the 4-meter atmosphere is like looking from the bottom of a swimming aperture Advanced Technology Solar Telescope (ATST) pool. Without corrective measures, current ground-based with an integrated AO system — can simultaneously ad- solar telescopes can reveal structures no smaller than a few vance solar resolution and spectral coverage and serve the hundred kilometers across on the surface of the Sun. science community for several decades. ATST will be a unique scientific tool providing an unprecedented combi- A larger telescope can solve photon starvation, but nation of spatial, temporal, and spectral resolutions across atmospheric seeing would limit it to the same spatial visible and infrared wavelengths. Just as early solar physics research helped solve fundamental problems in atomic, ATST specifications nuclear, and gravitational physics, ATST is expected to Aperture 4 m (13.1 ft) Configuration Gregorian, off-axis have a broad impact on astrophysics, plasma physics, and Mount Altitude-Azimuth (Alt-Az) solar-terrestrial relations. Enclosure Ventilated corotating hybrid dome Field of view Goal: 5 arc-min (300 arc-sec) Minimum: 3 arc-min (180 arc-sec) Just a century after Hale’s breakthrough discovery, Image quality Seeing limited (no AO): under excellent seeing conditions, ATST will take us deeper into the heart of sunspots, flares, <0.15 arc-sec @ = 1,600 nm Diffraction limited (with AO): under median seeing and other key solar activities. ATST observations of the conditions, Strehl ratio > 0.3 @ = 500 nm small-scale processes at the solar surface and through the Spectral range 300-28,000 nm (0.3-28 µm) overlying atmosphere will help us understand the life cycle Polarization Accuracy: >10-4 of continuum intensity Sensitivity: 10-5 continuum intensity (limited by photon of magnetic fields. ATST will be a powerful, flexible sys- statistics) tem that will serve the U.S. and international solar physics Coronagraphic Near infrared through infrared Scattered light <2.5 x 10-6, 1/10th solar radii from limb @ = 1,000 nm communities as their main ground-based facility into the Lifetime 30-40 years middle of the 21st century and beyond. 

2 3 Simulation

0.”03 resolution (@550 nm, w/AO) 0.1

1” resolution (@550 nm, uncorrected) ATST 1 (2010) ) s d n o c e s - ) c

r 10 4 ) a 0 ( 4 0 0 e 2 l 0 ( a 2 ( c S I S O L FASR: Frequency-Agile Solar FASR E O Radiotelescope (2010) R S

E SDO: Solar Dynamics Observatory

100 T

S SOLIS: Synoptic Optical Long-term SDO Investigations of the Sun 300” fi eld of view (2005) STEREO: Solar-Terrestrial Relations Observatory

1000

1,920” disk 1920 EUV UV VIS NIR FIR Thermal Radio 10 eV 1 THz 1 GHz nm �m mm Getting the fi ne structures to see the big picture While many solar telescopes observe the whole solar disk, most namic (MHD) waves. Resolving these scales is of utmost impor- studies, including those planned for ATST, glean new knowledge tance to testing various physical models and thus understanding from small areas of the sun. ATST will have a large fi eld of view how the physics of the small scales ties into the larger problems. — up to 300 arc-seconds (5 arc-minutes) out of an average solar diameter of 1,920 arc-seconds —and will resolve down to 0.03 arc- An example is the question of what causes slight variations of the seconds in visible light (550 nm). This will reveal features as small as solar radiative output — the solar constant — that impacts the ter- 20 to 70 km wide at the solar surface. Atmospheric turbulence limits restrial climate. Solar radiation received at Earth increases with solar ground-based telescopes — without adaptive optics (see page 13) activity (see page 5). Since the smallest magnetic elements contrib- — to a resolution of about 1 arc-seconds, regardless of the size of ute most to this fl ux excess, it is of particular importance to study the telescope, and occasionally permits 0.5 arc-seconds for short and understand the physical properties of these dynamic structures. periods. By comparison, the human eye has a fi eld of view about Unfortunately, the 0.76 to 1.5-meter apertures of present solar tele- 263 times wider than the Sun (~140 degrees) and a resolution of scopes cannot provide measurements at such scales because of 60 arc-seconds. Getting to such a level of detail is but a means to their limited apertures. an end: Resolving fundamental astrophysical processes in the solar atmosphere. Further, because of the need to provide an observatory that can operate for decades and incorporate new technologies as they Specifi cally, scientists want to see down to the intrinsic scale, the become available, ATST needs to be ground-based. While space distance atoms and molecules move as these processes take place (with no atmosphere to absorb or blur light) offers the best possible at high temperatures. Two key measures of the intrinsic scale are seeing conditions for observing the Sun, planets, and the rest of the distance light travels before being reabsorbed by another atom the universe, size and technical evolution argue against building an (photon mean-free path) and the height across which atmospheric orbital ATST. Many of ATST’s new instruments will be require fi ne- pressure changes by a certain constant (pressure scale height). To tuning or frequent updates on site, while others will be added years resolve these fundamental length scales in the deepest accessible later as new detector technologies emerge. On-orbit servicing and layers of the solar atmosphere requires seeing structures as small as replacement are not affordable for a program like ATST. 70 km (42 mi) at the solar surface (an angular resolution of 0.1 arc- second or better). Finally, ATST will not operate alone. Just as a symphony cannot be appreciated by listening to one instrument, the Sun cannot However, modern numerical simulations with a resolution of be understood by observing just in visible light. ATST will work in 10 km plus inference from the best available images suggest concert with satellites and other ground-based observatories, such even smaller scales for crucial physical processes such as vor- as SDO, STEREO, SOLIS (which will be co-located with ATST), the tex flows, dissipation of magnetic fields, and magnetohydrody- proposed FASR observatory, and others.

U.S. Air Force ISOON (Sun), Applied Physics Laboratory/Johns Hopkins University (STEREO), New Jersey Institute of Technology (FASR), NSO/AURA/NSF (all others)

2 3 The Science

ale’s discovery in 1908 that magnetic fi elds perme- cool. When the gas density drops so light can travel freely, ate sunspots started a revolution that turned solar it forms what we see as the visible “surface” of the Sun, scienceH into a fi eld encompassing (and often advancing) the photosphere. Here twisted magnetic fi elds loop out many branches of physics. In particular, much of our solar of the convective zone, into space and back to form an ar- research now involves magnetohydrodynamics (MHD), ray of features, including sunspots, plages, fi laments, and the study of plasmas (electrically conducting gases) whose prominences. Magnetic fi elds reach through the overlying shapes and fl ows are infl uenced by magnetic fi elds. chromosphere and into the corona where they can become unstable and trigger coronal mass ejections, or simply open Sunspots, it turns out, are the best-known manifesta- into interplanetary space. When massive fi elds pierce the tions of large magnetic system found in the outer third or visible surface they often form darkened areas — sunspots so of the Sun. Hydrogen fusion, the source of the Sun’s — where the magnetic fi eld keeps hot gas from rising from energy, occurs only in the core. The remainder of the Sun the interior. is a massive blanket serving two roles. First, it compresses the core to keep fusion going, and moderates the fl ow of All these affect life on Earth. The 11-year sunspot energy from the core into space. The outer region of the cycle (actually a 22-year magnetic cycle) is one of the bet- blanket is the convective zone where giant gas cells circu- ter-known phenomena. But the various forces that drive late like water in a boiling pot, bringing heat to the surface. the cycle, and determine its intensity and its relationship At the same time, solar rotation moves the cells around the with conditions around and on Earth, remain poorly un- Sun, somewhat like massive weather systems. Because the derstood. Historical evidence indicates that changes in the gas is electrically conducting, this motion produces a series of massive dynamos generating magnetic fi elds that stretch and shear, disconnect and reconnect.

(Earth’s own magnetic fi eld has a strength of about 0.5 gauss. A simple bar magnet has a fi eld of about 3,000 gauss, but it drops sharply with distance, and is almost undetect- able a few feet away. The magnetic fi elds inside sunspots range upwards to 4,000 gauss and span a volume several times larger than Earth. This means sunspots are produced by immensely powerful dynamos.)

Magnetic activities below the photosphere are hidden from view because the gas is optically dense: atomic particles are so tightly packed that photons — from gamma rays down to radio waves — are absorbed almost as soon as they are emitted. If not for this, the Sun would rapidly Key ATST requirements High angular resolution of 0.03 arc-seconds at 550 nm (mid-visible) and 0.08 arc-seconds at 1,600 nm (near-infrared) to resolve and study the fundamental astrophysical processes at the spatial scales needed to verify model predictions. High photon fl ux to provide accurate and precise measurements of physi- Robert F. Stein, Michigan State University cal parameters, such as magnetic fi eld strength and direction, tempera- A series of images from a computational fl uid dynamics model de- ture and velocity, on the short time scales involved. picts cool gas descending from lanes between granules within the Precise polarimetric measurements of solar magnetic fi elds. Sun’s visible surface. The parcel in this model is 1,200 km square Access to a broad set of diagnostics, from visible to thermal infrared wavelengths. and 2,500 km deep. High-resolution images of activities in the solar Low scattered light observations in the infrared to allow measurements of atmosphere are needed to validate computer predictions and place coronal magnetic fi elds. them on a fi rm footing.

4 5 Sun and climate

We think of the Sun as constant, but physical and historical re- areas. On balance, plages and network lines outweigh sunspots, cords show variations, the most famous being the sunspot number, and the Sun thus appears brighter at sunspot maximum. One aspect a count of actual sunspots and sunspot groups (above). A 22-year of increased solar activity is that solar ultraviolet emissions have a Hale cycle — two 11-year cycles with solar magnetic poles revers- greater brightness range than the Sun as a whole. As a result, ozone, ing every 11 years — appears to be the norm for our Sun, based an important greenhouse gas in the stratosphere, varies in direct on just four centuries of data with some hiccups. For example, the response to daily changes in the solar ultraviolet irradiance that forms Maunder Minimum, a 70-year period (1645-1715, spanning about three complete Hale cycles) without spots, coincided with severe winters and shortened summers as Europe experienced a period known as the Little Ice Age (about 1500-1850). The period was brutal enough that Henry VIII and his court were able to ride horses across the frozen Thames River in 1536, and London had Frost Fairs on the Thames as late as 1814. Pieter Bruegel’s “The Hunters in the Snow,” painted in 1565, is widely believed to depict severe conditions during this era.The Maunder Minimum is more than a co- incidence. Although consistently accurate sunspot counts only started a few decades after the discovery of sunspots in 1612, scientific data (such as tree ring analyses and carbon-14 in organic remains) and historical records of harvests and first frosts point to sunspot lows coinciding with other cold spells, such as the Spörer Minimum (1415-1510) and Dalton Minimum (1795-1820), and sunspot highs

Pieter Bruegel the Elder, Kunsthistorisches Museum Wien, Vienna 200 Little Ice Age (~1500-1850) Dalton 100 Maunder Minimum Minumum 1645-1715 1795-1820 0

Sunspot Number 1600 1650 1700 1750 1800 1850 1900 1950 2000 Dave Hathaway, NASA/Marshall Space Flight Center

with warm spells such as the Medieval Maximum (900-1300) when 1369 Greenland was green and colonized by the Vikings, as compared to

today’s glacial island. )

2 1368 Total solar irradiance Annual sunspot number 150 (Climate is a general description of long-term environmental con- 1367 ditions across regions or the entire planet. Weather refers to local 100 short-term variations in conditions. However, climate sets the stage 1366 for types of weather.) 50 1365 We are still uncovering the links between solar activity and terres- Solar Irradiance (W/m 0 Sunspot (Wolf) number trial climate because neither is fully understood. Earth’s climate sys- 1364 Cycle 21 Cycle 22 Cycle 23 tem is highly complex, comprising large heat sinks like the oceans, 1363 that form a buffer and delay (sometimes by years or decades) a 78 80 85 90 95 00 03 response to shifts in solar irradiance. NASA/Goddard Space Flight Center & Claus Frolich, World Radiation Laboratory

On the solar side of the question, we learned in the last few by splitting oxygen molecules (O2) into free atoms that recombine decades how variable the “constant” can be, and thus refer to total to form ozone (O3). The implication is that periods like the Maunder solar irradiance (TSI). Irradiance values averaged from satellite data Minimum could allow natural depletion of ozone and lead to global (different colors in the graph indicate different satellites) show daily cooling. variations as solar active regions arise and disappear and move across the solar disk. Annual sunspot numbers for cycles 21-23 are Further, only recently have we established that the Sun’s total superimposed on the graph at right to show how the two correspond. energy output varies slightly with the “normal” sunspot cycle, as we understand it. Such data can only be collected by satellites The paradox in sunspot variations is that the Sun is brighter when above Earth’s atmosphere (right) and thus exposed to the full it has more spots. What was revealed by modern instruments (and range of solar energy. The data clearly show a variation with sun- not readily apparent to observers relying on just their eyes) is that spot numbers. We also know that geomagnetic storms, driven by sunspots are accompanied by brightened areas, plages (French for solar activities, provide an additional, variable energy input through beach) and by bright network lines connecting magnetically active the polar caps.

4 5 tions and waves and determine how energy is transferred from turbulent gas motions to the magnetic field.

By exploiting the broad spectral coverage planned for the ATST, we can observe how these processes can vary with height and measure their role in determining the structure, dynamics, and heating of the chromosphere and the corona. The near-IR spectrum around 1,500 nm has many advantages particularly for precise measurements of the recently discovered weak, small-scale magnetic fields that cover the entire solar surface and which could be the signature of local dynamo action. A 4-meter aperture is needed to clearly resolve these features at 0.1 arc-seconds in

Lockheed Martin the near-infrared. Furthermore, the infrared beyond 1,500 The 11-year sunspot cycle is depicted in a series of magnetograms nm provides particularly powerful diagnostics of magnetic that reveal solar activity. Bright areas indicate north polarity; dark field, temperature, and velocity at the upper layers of the areas, south polarity. Constant magnetic activity is a feature of the solar atmosphere. ATST’s infrared capabilities will be used Sun, but concentrations of activity — which also produce sunspots, to measure the cool chromospheric component and provide flares, and other phenomena — rise and fall about every 11 years. critically needed measurements of coronal magnetic fields. sunspot cycle impact Earth’s climate, although modulated The dynamics and heating of these outer layers of the solar by terrestrial events such as volcanoes (see page 5). While a atmosphere in turn result in the violent flux expulsions number of instruments monitor the Sun’s total output, ad- we see as flares and mass ejections. All of these processes vanced instruments like ATST are needed so scientists may are tied together by the behavior of magnetic flux in the unravel fundamental drivers that determine that output. dynamic plasma at the solar surface. ATST also will impact other areas of astrophysics, space science, and plasma ATST will primarily study only the outer layers of the physics and help refine space weather models. Some of the Sun’s atmosphere, the photosphere, chromosphere, and major scientific questions driving ATST follow. corona (the inner workings are inferred from oscillations in the photosphere). Nevertheless, all the matter and energy Flux tubes that reach Earth from the Sun have to travel through these Observations have established that the photospheric regions which display a dazzling and intriguing array of magnetic field is organized into countless small fibrils or scientific puzzles that allow us to infer what is happen- flux tubes. The majority of these structures are too small ing inside. ATST’s enhanced resolution in space, time, and for current telescopes to resolve. Flux tubes are the most wavelength will let scientists see what lies beyond the likely channels for transporting energy from the solar in- reach of current telescopes. Further, ATST will extend our terior into the upper atmosphere, which is the source of understanding into the thermal infrared spectrum, thus ultraviolet and X-ray radiation from the chromosphere, providing deeper insight into the solar atmosphere.

The solar atmosphere provides an ideal laboratory to study the dynamic interaction of magnetic fields and plasma. Beginning with the generation of the magnetic fields themselves and their cyclic behavior, ATST will ob- Bellan Plasma Group, California Institute of Technology serve the small-scale processes at the solar surface that play From telescope to laboratory a critical role in understanding the overall sunspot cycle. Combining results from ATST and laboratory experiments will play ATST will contribute to understanding magnetic flux emer- an important role in furthering our understanding of turbulent flow gence through the turbulent boundary between the solar in plasmas. Generating plasma-filled twisted magnetic flux tubes in a laboratory (above) serve as scaled experiments of the dynam- interior and atmosphere. The ATST will reveal the nature ic magnetic phenomena observed on the Sun. Such tests offer of solar flux tubes, which are generally believed to be the insights into magnetic helicity and topology, triggering of instabili- fundamental building blocks of magnetic structure in the ties, magnetic reconnection physics, conversion of magnetic en- atmosphere and the progenitors of solar activity. ATST will ergy into particle energy, importance of magnetic boundary condi- observe the interaction of flux tubes with convective motions, and interactions between adjacent magnetic structures.

6 7 Oscar Steiner, Kiepenheuer-Institut für Sonnenphysik Computer model of a magnetic flux tube rising from the convective zone into the photosphere. These are believed to be an important conduit for energy flowing from the solar interior to the hot outer SOHO Consortium, ESA, NASA atmosphere. Color contours indicate convective regions, and black Unlike Earth with its simple pair of north and south magnetic poles, lines depict field strength. Flux tubes are below the limit of resolution the Sun is carpeted with countless small north and south magnetic in current telescopes. joined by field lines that loop through the solar atmosphere and transition region, and solar corona, which in turn affects the corona and back to the surface. They are born, fragment, drift, and Earth’s outer atmosphere. Fine-scale observations of these disappear in a day or two, arising from mechanisms that are poorly understood at present. fundamental building blocks of stellar magnetic fields are crucial for our understanding not only of the activity and strong fields and weak fields interact? How are both gener- heating of the outer atmospheres of stars, but also of other ated? How do they disappear? Does the weak-field compo- astrophysical situations such as the accretion disks around nent have global importance and what is its significance for compact objects like pulsars, or environments where dust the solar cycle? ATST will measure distribution functions accretes into planetary systems. Current solar telescopes of field strength, field direction and flux tube sizes for com- cannot provide the required combination of spectroscopy, parison with theoretical models, and will observe plasma polarimetry, and angular resolutions to explore the enig- motions and relate them to the flux tube dynamics. matic flux tube structures. Magnetic fields and mass flows Magnetic field generation and local dynamos The total magnetic field of sunspots is large enough To understand solar activity and solar variability, we to dominate the hydrodynamic behavior of the local gas, 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 theories involving large averages (mean field). Dynamo action of a more turbulent nature in the convection zone may be an essential ingredient in a complete solar dynamo model. Surface dynamos may produce the small-scale magnetic flux tubes composing a “magnetic carpet” that continually renews itself every few days at most and having magnetic flux levels 10 to 100 times more than that in active regions.

ATST will make it possible to observe such local dynamo action at the surface of the Sun. ATST will measure the turbulent vorticity and the diffusion of small-scale Thomas Rimmele, NSO/AURA/NSF High-resolution image of a sunspot reveals tubelike structures that magnetic fields and determine how they evolve with the make the penumbra and extend from the granulated solar surface 11-year solar cycle. By resolving individual magnetic flux into the cooler umbra at center. Sunspots are formed by magnetic tubes and observing their emergence and dynamics ATST field lines emerging through the surface and blocking downward will address fundamental questions including: How do convection of the darker, cool gas.

6 7 ATST and the many colors of the Sun "Black" light Max. eye Red laser Incandescent Radiant heat ~400-345 sensitivity pointer lamp peak ~550 ~650 ~999

300 400 500 600 700 800 900 (nm) ~380 ~770 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000 10,000 20,000 28,000

UV Visible light Near IR Thermal IR

ATST’s spectral range — 300 to 28,000 nm — will span virtually the emissions by iron at 630.25 nm. Vibrations and rotations of hot atoms entire optical spectrum passed by Earth’s atmosphere. Within this are and molecules produce a complex series of lines in the infrared that tens of thousands of emission and absorption lines, each telling a dif- ATST will explore. Although hydrogen and helium compose almost ferent part of the solar activity story. The wavelength and appearance 99 percent of the Sun’s mass, we learn the most from spectral lines of each line is determined by the state of the elements composing the generated by wisps of carbon, oxygen, silicon, iron, and other atoms solar atmosphere. An atom or molecule can emit many different lines, that the Sun captured as it formed 4.5 billion years ago. Like colored often in a series, and lines can be shifted by motion to or from the dyes dropped into a river, these additional chemicals let us trace what observer, or by intense magnetic fi elds. For example, the spectrum is happening on the Sun. (The vertical line in each reference image below was taken across a sunspot. The broadening inside the yel- shows the position of the slit; the horizontal lines are for reference; low ellipse is Zeeman splitting where an intense magnetic fi eld affects thus, the spectrum is offset from the reference images.)

Continuum H core H wing 629.4 629.6 629.8 630.0 630.2 630.4 630.6 nm K.S. Balasubramaniam, NSO/AURA/NSF Sample spectral lines of interest

393.3682 & 396.8492, Calcium II K & H: Deep 1074.6 & 1079.8, Iron (Fe XIII): Yields highly sensitive measure- violet (right) emitted and absorbed by calcium ments of plasma velocities in coronal loops. ions in the chromosphere. Useful for monitor- ing higher altitudes of the chromosphere. 1083.00, Helium: Helium atoms (right) in the chromosphere about 2,000 km above the 431-430, G-band: A rich complex of absorption photosphere indicate coronal intensities be- lines dominated by vibrational and rotational cause this infrared spectral line is infl uenced modes of carbon-hydrogen (CH) molecules by X-rays. (right). G-band images show bright points indicating concentrations of magnetic fl ux in 4700, Carbon monoxide (CO): A current mystery of the Sun is the photosphere. The bandpass also is rich in why portions of its atmosphere cool enough to allow some atoms lines of iron, titanium, and other elements. — such as carbon and oxygen — to combine and form molecules (simulated at right). Ob- 589.59 & 589.99, Sodium D1, D2: Orange-yellow double line emit- servations already have signifi cantly changed ted and absorbed by sodium. These lines are used to measure our understanding of the chromospheres of motions in the solar atmosphere. stars and led to a better understanding of the solar atmosphere. CO also emits at other 630.25, Iron (Fe I): Deep red color emitted (or absorbed) by hot, wavelengths. neutral iron in the photosphere (see top). Excellent for tracking magnetic activity on the Sun. 12320, Magnesium I: These are the most magnetically sensitive lines in the solar atmosphere. By observing Zeeman splitting, we 656.28, Hydrogen-alpha (H ): Deep red color can measure magnetic fi elds down to 100 Gauss. Combined (right) emitted or absorbed by hot, neutral with other data, the Mg I observations will help us reconstruct the hydrogen in the chromosphere. Excellent for three-dimensional structure of magnetic fi elds in the photosphere tracking active regions on the Sun. and lower chromosphere. Solar thumbnails: NSO, USAF/ISOON (H-alpha), A yardstick for light 854.2, Calcium II: Absorbed by the same atom For consistency, the unit of measure used for light throughout this book is the as the 393 and 396 lines (bottom); highly nanometer, a billionth of a meter (25.4 million nm/inch). This is the preferred sensitive to magnetic fi elds and thus an ideal unit from the International System of Units (“metric system”). The earlier prefer- ence for wavelengths of visible light was the Angstrom (Å, 0.1 nanometer). For wavelength for magnetic fi eld measurements convenience, scientists working in infrared often use microns (µm), a millionth in the chromosphere. of a meter (1,000 nm).

8 9 a regime very different from that of the rest of the solar spectra near 4,700 nm show surprisingly cool clouds that photosphere. Earth’s magnetic fi eld is about 0.5 gauss at appear to occupy much of the lower chromosphere. Ap- the surface. Fields within sunspots can range from 1,500 parently only a small fraction of the volume is fi lled with to 3,000 gauss. This is about the strength of a bar magnet, hot gas, contrary to classical static models that exhibit a but across a volume several times larger than Earth, thus sharp temperature rise in those layers. The observed spec- hinting at the immense power at work to form the fi eld. tra can be explained by a new class of dynamic models of Numerical simulations and theoretical models predict dy- the solar atmosphere. However, the numerical simulations namic phenomena, such as oscillatory convection in the indicate that the temperature structures occur on spatial strong-fi eld regions of sunspot umbrae, fl ows that move at scales that cannot be resolved with current solar infrared the speed of sound along penumbral fi laments, and oscilla- telescopes. A test of the recent models requires a large-ap- tions and wave phenomena. erture solar telescope that provides access to the thermal infrared. To verify these predictions and ultimately to answer such fundamental questions as “Why do sunspots exist?” Another mystery comes from spicules, forests of hot will require an extremely capable instrument. High-resolu- jets that penetrate from the photosphere through the chro- tion (better than 0.1 arc-seconds) vector polarimetry com- mosphere and then fade in a few minutes. These are be- bined with high sensitivity and low-scattering optics are lieved to be an MHD phenomenon that is not understood required. Understanding the interaction of magnetic fl ux nor adequately modeled. Combined with extreme ultravio- and mass fl ows is crucial for our understanding of the be- let observations from space, ATST will allow us to resolve havior of magnetic fi elds on scales ranging from planetary their structure and dynamics and understand their nature magnetospheres to clusters of galaxies. Sunspots allow us and roles. to test those theories in a regime where magnetic fi elds dominate mass fl ows.

Stellar upper atmospheres A bizarre feature of the Sun is the presence of carbon monoxide molecules. Normally their components exist as free atoms. But some regions of the chromosphere are cool enough to allow molecules to form for brief periods. Indeed, measurements of carbon monoxide absorption

Allen Gary, NASA/Marshall Space Flight Center (left) and Alan Title, Lockheed Martin (right) Coronal loops — great arcs of hot gas carried into the upper solar atmosphere along magnetic fi eld lines — are an important source of ultraviolet and X-ray emissions from the Sun. Understanding their origins and fi ne-scale structure at lower altitudes in the photosphere will require observations in the infrared part of the spectrum. Magnetic fi elds and the solar corona The origin and heating of the solar corona, and the coronae of other stars, are still mysteries. Most of the proposed scenarios are based on dynamic magnetic fi elds rooted at the 0.1-arc-second scale in the photosphere. How- ever, neither observation nor theory has clearly identifi ed the appropriate processes. Extreme ultraviolet and X-ray observations have gained in importance, but ground-based observations are still critical, not only to determine the forc-

Bruce Lites, High Altitude Observatory, and the Swedish Vacuum Telescope ing of the coronal fi elds by photospheric motions, but also Mesas with bright cliff sides are a recent discovery in solar physics. to measure the coronal magnetic fi eld strength itself. This is The structures seen here are granules, columns of hot gas rising to important for developing and testing models of fl ares and the visible surface, then cooling and descending back into the interi- coronal mass ejections, which propel magnetic fi eld and or. High-resolution images near the solar limb revealed that the sides plasma into interplanetary space and induce geomagnetic of granules are hotter and brighter where magnetic fi elds penetrate the solar atmosphere. disturbances. In particular, precise measurements of the

8 9 coronal magnetic field and its topology are needed in order to distinguish between different theoretical models. ATST — with its large aperture, low scattered light characteris- tics, and the capability to exploit the solar infrared spectrum — will provide these critical measurements.

Space weather ATST can play an important role in understanding and modeling the origins of solar activities that drive space weather events (also called solar-terrestrial physics). Large flares can be accompanied by highly energetic particles that K.Strassmeier, Vienna, NOAO/AURA/NSF endanger astronauts and satellites. Coronal mass ejections Looking beyond the Sun (CMEs) produce interplanetary blast waves that also accel- Solar physics is central to astrophysics, with the Sun providing a erate energetic particles and carry mass and magnetic fields unique “laboratory” for studying key astrophysical processes that that impact Earth. The response of Earth’s magnetic field occur elsewhere in the Universe but can’t be observed directly. can disrupt electrical power grids on Earth, damage satel- Magnetic fields are ubiquitous and important throughout the Uni- lites, endanger the health of air crews crossing the polar verse. Detailed observations of flux tubes are crucial for our un- cap and astronauts in orbit, and have other impacts on hu- derstanding not only of the activity and heating of the outer atmospheres of our Sun and other stars, but also the accretion disks of man society. It appears that the microstructure of magnetic compact objects, or environments where planets are about to form. fields on the Sun plays an essential role in the physics of these large-scale phenomena. Studies of the Sun also play a fundamental role in basic physics and in the physics of stars. The Sun provides our best opportunity ATST will provide precision data on magnetic fields and for observing MHD and plasma phenomena under astrophysical atmospheric parameters needed to develop models for the conditions. MHD waves, MHD turbulence, magnetoconvection, magnetic evolution of active regions, the triggering of mag- magnetic reconnection, and magnetic buoyancy occur in many astrophysical systems but can be observed only on the Sun. netic instabilities, and the origins of atmospheric heating events. These models will provide a much better capability Solar flares serve as a prototype for many high-energy phe- for understanding and predicting when and where activity nomena and particle acceleration mechanisms in the Universe. will occur on the Sun and for predicting the level of en- The cause of the Sun’s million-degree corona must be understood hanced emissions expected from these events. These in turn before we can confidently interpret EUV and X-ray emissions from will let us refine solar-terrestrial models to include critical distant stars. Our scientific understanding of these solar phenom- solar data instead of the proxies that are used now.  ena is limited by our inability to resolve them at spatial scales less than a few tenths of an arc-second. The study of solar plasma- magnetic field interaction has the potential to revolutionize our understanding of solar and stellar atmospheres, stellar activity and mass loss, and how solar variability affects Earth.

ATST, employing a stable, high-resolution spectrograph to look at other stars at night, could offer a unique platform for dedicated programs in the solar-stellar connection. This active sub-discipline of contemporary astrophysics lets us test ideas from narrow solar studies against stars of different mass, age, rotation, and evolution. Solar-stellar studies also offer unique insights into the range of possible behavior of a star like the Sun that solar observations alone are unable to achieve. Areas of interest include: Astroseismology, measure global oscillations in stars. Doppler imaging, resolve starspots (like HD 12545, above) and fine magnetic structures on stars. NASA/SOHO Stellar convection, the relationship between rotation and convection Coronal mass ejections, or CMEs, occur when magnetic field lines in other stars. open into space and spew large bubbles of hot solar gas across the Magnetic fields in late-type stars, magnetic field properties of many star solar system. These normally entrap magnetic field lines and can types. interact with Earth’s magnetosphere and disrupt communications Planets and small solar system bodies, extend polarization mea- and power systems as well as causing the best-known Sun-Earth surements to planetary astronomy. effect, the aurora borealis.

10 11 The Telescope

he ATST project is being developed by a collaboration with its unprecedented combination of high spectral, spa- of observatories, universities, and research laboratories tial, and temporal resolution. It also will serve as the focal ledT by the National Solar Observatory. Funding is provided point for systematic campaigns involving other ground- by the National Science Foundation. The Association of based and orbiting observatories. ATST will become a ma- Universities for Research in Astronomy (AURA) manages jor component of the solar facilities operated by the NSO the project through the NSO, based in Tucson, AZ, and and available to U.S. and international solar astronomers. Sunspot, NM. Principal partners are the New Jersey Insti- tute of Technology, the University of Hawaii, the University Telescope design of Chicago, and the High Altitude Observatory; another 17 The need to provide solar observers with greatly im- collaborators are involved (see page 17 for a complete list). proved images, spectra, and data in turn leads to demand- ing observing requirements that drive ATST’s design. These ATST recently completed a conceptual design review include: that defi ned major aspects of the telescope and its support facilities. Nevertheless, many aspects are presented here • A planned 300x300 arc-seconds fi eld of view (minimum are subject to change as the project evolves. of 180x180 arc-seconds) so the telescope can observe large active areas as they evolve. When completed, the ATST will be the world’s premier • Spatial resolutions of 0.03 arc-second at a wavelength ground-based facility for observing and studying the Sun of 500 nm (green) and 0.08 arc-seconds at 1,600 nm

Conceptual design for the ATST observatory depicts it inside a new hybrid enclosure designed to reduce costs while providing the best possible observing conditions. The “snorkel” atop the enclosure moves in step with the mount so the telescope always has a clear view of the Sun. The design is subject to change as it is refi ned. The structure under the telescope houses optical benches and other support systems for science instruments, and will rotate with the telescope to ensure a stable view. The building to the right will include offi ces and work spaces as well as mirror recoating facilities.

Mark Warner, NSO/AURA/NSF

10 11 Secondary mirror Secondary mirror assembly (70 cm [27.6 in.]))

Light from Sun Baffle Field stop

Heat stop assembly System Primary mirror interconnects assembly

Pier and Coude

Feed optics

Deformable mirror Azimuth cable wrap To Coude labs Primary mirror (instruments) (4 meters [13.1 ft]) Mark Warner, NSO/AURA/NSF

Primary optical system for the ATST includes coupling optics to focus solar images for instruments mounted in the two-story Coude lab.

(near-infrared) when using adaptive optics to compen- support structures that would scatter light and overwhelm sate for atmospheric blurring. the faint coronal light. The large unobstructed aperture and • Temporal resolution to reveal events spanning a few broad spectral coverage further require that the telescope seconds (e.g., fast movies rather than long time expo- be open and reflective (an evacuated telescope would sures). require impractically large windows that also would not • Spectral resolution to enable precision, sensitive mea- transmit infrared radiation). surements of magnetic activities in the solar atmosphere from near ultraviolet into the thermal infrared. This combination of requirements led the ATST team to • Sufficient light flux and control of scattered light to re- select an open-air, off-axis Gregorian telescope with a 4-me- veal faint structures and activities in the corona. ter (13.1-ft) primary mirror. This design differs from famil- • Sufficient light flux to allow high-resolution polarim- iar astronomical telescopes. In the Hubble Space Telescope, etry, studies using polarized light to determine the whose basic design dates back to the 1800s, a secondary strength and directions of magnetic fields within solar mirror is placed in front of the primary mirror’s focal point structures. to enlarge the image and redirect the light into science instruments. Most nighttime telescopes have similar arrange- Meeting these criteria involves several challenges. ments. However, the large heat flux and scattered light Spatial resolution improves as the diameter of the primary requirements push ATST to an off-axis design. mirror increases. High spectral and temporal resolutions also require a large flux of light, thus driving the design In a Gregorian design, often used in solar physics, the towards a large aperture. Observing the faint solar corona secondary mirror is placed beyond prime focus. A field means not only a large aperture, but an optical path clear of stop — actively cooled for ATST — is placed at prime focus

12 13 Basic concept for Shack-Hartmann Adaptive Optics System Uncorrected Deformable mirror light

Actuators

Correlation & Corrected Reconstruction light Digital Signal Processors Beam splitter Shack-Hartmann CCD camera lenslet array Adjust mirror so each subaperture matches one reference aperture

Thomas Rimmele & Kit Richards, NSO/AURA/NSF Uncorrected and corrected images show the striking improvement possible with the NSO’s adaptive optics system. Smoothing the wrinkles out of our view of the Sun

A key advance making the ATST possible is adaptive optics (AO) erture image is displaced due to the distortion induced by seeing. technology that allows the telescope to operate near its diffraction An image-processing computer selects one subaperture as the limit, the theoretical best that a telescope can deliver. Diffraction- reference image to determine how the other subapertures should limited resolution is rarely achieved by large telescopes on Earth be shifted to compensate for the atmosphere. This has been a because the turbulent atmosphere acts like a continually changing significant challenge because the low contrast between solar rubber lens that bends starlight out of shape before it reaches the features does not provide clear landmarks that could be used for ground. Astronomers and solar physicists alike refer to being “see- corrections. ing limited” because the telescope is kept from operating near its theoretical capability. Larger apertures alone can make the problem The control computer then commands actuators mounted on the worse because the telescope is looking through a larger column of back of a small deformable mirror in the main light path. In a sense, disturbed air. the actuators are like miniature loudspeakers. They reshape the mirror by as little as a few wavelengths of light — or even fractions of “Seeing” has evolved from a qualitative description into a series wavelengths — in response to varying electric signals. A tilt/tip mir- of mathematical techniques and formulas. The first critical measure ror in the light path recenters the image in case of major distortions. is the point spread function or PSF. Telescopes do not focus light The result is a striking stabilization of the image and removal of blur- into perfect points but into an Airy disk, a series of rings (each dim- ring due to Earth’s atmosphere. mer than the next) generated by the way light interferes with itself. A perfect telescope will put 84 percent of the light from a star into the The solar Shack-Hartmann approach (illustrated above) was first center or disk 0, 7 percent into disk 1, 3 percent in disk 2, and so employed on the 75 cm (29.5 inch) Dunn Solar Telescope at the on. Larger telescopes produce narrower Airy disks, as do shorter National Solar Observatory at Sacramento Peak at Sunspot, NM, wavelengths. Other measures include the Zernike modes, a mea- in 1998. The low-order AO24 system used 24 subapertures and 97 sure of how distorted the wavefront is, and the Fried parameter, a actuators, and corrected the mirror 1,200 times a second (1,200 measure of atmospheric distortion. Hertz, Hz). Soon after its first demonstration it was improved and placed into routine operation on the Dunn. In 2003, the same team Controlling and correcting seeing has been the goal of AO, a so- demonstrated the more advanced high-order AO76 version with 76 phisticated technology that has emerged over the last two decades subapertures and 97 actuators at 2,500 Hz. and has enhanced the work of nighttime astronomy. However, nighttime AO is not suited to solar observations because the Sun This demonstrated the feasibility of scaling the Shack-Hartmann is too bright and extended to provide pointlike sources like stars. concept upwards by several orders of magnitude to meet ATST’s ATST will employ a different approach, which has been in develop- requirements, an anticipated 1,000 subapertures. AO technology is ment since the 1990s. improving spatial resolution of existing solar telescopes, and allows them to operate close to their diffraction limits. Using AO, the ATST In the Shack-Hartmann AO design, small lenses divide the team anticipates providing resolutions down to 0.03 arc-second in incoming image into multiple images, each using a different sub- green light (550 nm). This is equivalent to 1/64,000th the apparent aperture of the image of a sunspot or other feature. Each subap- diameter of the Sun.

12 13 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 Scientific and Technical Advisory Groups Start Concept & design CoDR PDR CDR AO Adaptive Optics program CDR Critical Design Review Schedule extension if Site surveys Select finalists Final selection CoDR Concept Design Review mirror is procured at Technology development PDR Preliminary Design Review start of construction High-order AO development AO76 demonstration Mirror production Construction Note: Schedule subject to change Integration Integration Operations First light

NSO/AURA/NSF Preliminary development schedule for ATST anticipates “first light” and operations in 2010 assuming an authority to proceed in 2005. where it intercepts most of the light and allows only light designs. The slim contours of the dome were selected to from the area being studied to pass to the secondary mir- minimize dome heating by sunlight while allowing con- ror. To keep ATST’s optical path clear, the secondary mir- trolled flow of ambient air for cooling. It will also allow a ror is offset from the center of the primary. For example, full range of pointing angles for ATST operations. the points that stars have in many astronomical photos are caused by light diffracted by the secondary mirror sup- Science instruments ports. Such unwanted light would spoil ATST observations No telescope produces science without instruments. This of the faint corona as well as complicate the heat rejection is particularly true for solar telescopes where we to observe problem. Thus, the primary mirror actually is an off-axis the same object with a large variety of different instruments. segment of a virtual 12-meter mirror. The primary will be However, cost constraints may severely limit the number of a 100-mm (4 in) thin mirror mounted on active supports to instruments that will be available in the beginning of ATST’s maintain the appropriate parabolic curve as loads on the operations. ATST will use a new modular instrument ap- mirror shift during Sun tracking. The secondary mirror will proach, where a variety of instruments can be assembled focus light downward through a series of relay mirrors that from a limited number of common optical modules. ATST eventually conduct the light into the science gallery below also will be designed so the science community can develop the main structure. and install its own instruments to meet specific needs.

Building the telescope without a barrel or other shroud U V will allow cooling by ambient air and avoids heat traps. Even so, ensuring proper ventilation and cooling through the telescope and within the dome will be a significant design challenge. Like all large, modern night telescopes, ATST will use an altitude-azimuth (alt-az) mount.

The telescope will sit atop a two-story turntable. Relay I optics will transmit the image from the secondary mirror to Q science instruments on the two lower levels. Telescope and instruments will rotate together to track the Sun across the sky and thus provide a stable image of the Sun. This science gallery provides standardized instruments as well as stations where physicists from around the world can bring their own instruments to observe through ATST (and allow S. Kasiviwanathan, NSO/AURA/NSF Magnetic fields polarize light as it is emitted by a hot gas. From on-site fine-tuning, an important consideration). Alterna- measurements of plane (Q, U) and circular (V) polarizations, as tively, scientists can use complete instruments or instru- well as intensity (I) — the Stokes parameters — can let scientists ment modules that will be added to ATST over time as part calculate the direction and strength of the magnetic field. This im- of the observatory’s capabilities. age, covering an area about 1/50th the diameter of the Sun, was taken with the Diffraction-Limited Spectropolarimeter at the Dunn Current plans call for enclosing ATST with a new Solar Telescope at Sacramento Peak. More powerful versions of the DLSP will be employed by ATST. hybrid enclosure that draws features from other existing

14 15 teams. International collaborations are also being sought for both telescope and instrument design and construction. Priority instruments for ATST’s initial years include (lead institution in parentheses): Visible light polarimeter (High Altitude Observatory): Take advantage of the rich diagnostic potential of spectral lines in the visible part of the spectrum. It will measure all four Stokes polarization parameters and allow diffraction-limited vector magnetograms. Near-IR polarimeter (University of Hawaii). Take advantage of the greater Zeeman splitting of near-infrared (1,000 to 5,000 nm [1-5 µm]) lines to accurately measure lines from the photosphere, chromosphere, and corona. Tunable infrared filter (New Jersey Institute of Technology). Used as a part of a magnetograph for measurements of solar magnetic field at deeper layers in solar atmo- NSO/AURA/NSF sphere. The two-level structure beneath the ATST will provide eight locations — four on each level — for large, specialized science instruments Broadband filter system (Lockheed Martin). This first-light that will rotate with the telescope structure. In addition, a smaller instrument will be used to commission the telescope ninth position is available at the Gregorian focus. and provide background images supporting other in- ATST maximum allowable instrumentation sizes vestigations. Visible tunable filter/polarimeter (NSO and NASA/ Location L x W x H1 Mass cm (in) kg (lbs) Marshall Space Flight Center and the Kiepenheuer-In- stitut für Sonnenphysik ). Make two-dimensional vec- 500 x 240 x 240 3,000 Upper Coude2 (196 x 94 x 94) (6,600) tor magnetograms and spectral images. Thermal IR instrument (NASA/Goddard Space Flight Cen- 450 x 240 x 240 3,000 Lower Coude (177 x 94 x 94) (6,600) ter). The GSFC solar group is developing designs for cameras and spectrographs that will exploit ATST’s 250 x 210 x 40 1,000 Gregorian bench capabilities in the thermal infrared. (98 x 83 x 16) (2,200) 1 Equipment must pass through 2.5 m-square doors, so allowances must be made for handling gear. Also, equipment In addition, the ATST control system will incorporate legs can be separated by no more than the 1.6 m (63 in) width of the Virtual Instrument concept. A Virtual Instrument is a col- the reinforced flooring. lection of components operating concurrently. Typically the 2 The upper Coude platform is narrower than the lower platform, majority of a Virtual Instrument’s components will be on thus reducing instrument length by 50 cm. the same optical bench, although a Virtual Instrument may The ATST will support up to nine instruments in three be spread across multiple benches, share a bench with other areas, the upper and lower Coude platforms, and the Gre- instruments, or even include off-site components. This will gorian focus. The Coude platforms will form the lower lev- allow greater flexibility for adapting instruments to explore els of the large rotating structure supporting the telescope new results in a multi-bench environment. proper. Each will have four instrument ports. The upper Coude platform is slightly smaller than the lower platform Site selection and thus will accommodate a slightly shorter instrument. A telescope with as much promise and capability as The Gregorian instrument bench will be located on the op- the ATST must be placed at a location that provides the tic support structure directly below the elevation axis, just best possible conditions within Earth’s atmosphere. A above the Coude levels. In addition, instruments design- leading selection criterion is high altitude to place the ers will have to meet other constraints, such as a 1.6-meter telescope above much of the atmosphere (always the first width for floor supports, and handling allowances for step in reducing the blurring problem), hence the choice doors. of mountains in each case. Other criteria include minimal cloud cover, many continuous hours of sunshine, excel- Agreements for instrument design development have lent average seeing conditions (including many continu- been established with some of the co-principal investigator ous hours of excellent seeing), good infrared transparency,

14 15 and no dust or aerosols to scatter light and obscure the corona. The selected site will represent the best blend of these factors.

Six sites — four on continental North America and two on islands — selected from more than 50 proposed sites are being evaluated: • Big Bear Solar Observatory, California, • Mees Solar Observatory, Haleakala, Hawaii, • NSO/Sacramento Peak, New Mexico, • Observatorio Astronomico Nacional, San Pedro Martir, Baja California, Mexico, • Observatorio Roque de Los Muchachos, La Palma, Ca- nary Islands, Spain, and • Panguitch Lake, Utah.

Sites are evaluated with identical sets of instruments, including: • Solar Dual Image Motion Monitor to measure differen- tial motion of the solar image (caused by atmospheric motion) and to derive the Fried parameter (a measure NSO/AURA/NSF Each candidate ATST site is equipped with a semi-automated ob- of seeing distortions), servatory, like this one at Big Bear Lake, CA, to record seeing condi- • A Shadow Band Array and Ranger to estimate turbu- tions over a period of at least a year. lence several hundred meters above the site, • A coronal photometer to determine sky brightness that manent staff and visiting scientists. A small visitor center would compete with the faint corona, and will be included and sized to match access and anticipated • Dust and water vapor monitors to measure dust ac- visitors. cumulation on optics and humidity that may impact infrared observations. When design and development of the ATST are well Economic and other factors also will be considered. Fi- under way, the National Solar Observatory will seek part- nal site selection is scheduled for mid- to late 2004. ners to develop and build a solar physics research institute to complement the telescope facility. Although plans are Facilities and science institute tentative, the NSO anticipates relocating its headquarters Adjacent to the ATST building will be facilities to op- at or near a university with a world-class solar physics or erate and maintain the ATST (including periodic mirror astrophysics program to nurture ATST’s growth in the com- recoating), plus offices, computers, and housing for per- ing decades. 

Candidate sites for ATST

Panguitch Lake, Utah

Big Bear Solar Observatory, NSO/Sacramento Peak, New Big Bear Lake, California Mexico Observatorio Astronomico Observatorio Roque de Los Nacional, San Pedro Martir, Baja Muchachos, La Palma, California, Mexico Mees Solar Observatory, Canary Islands, Spain Haleakala, Hawaii

NSO/AURA/NSF Six ATST candidate sites were being evaluated during 2002-03. Each was equipped with an observing station, similar to the one shown in the inset, designed to provide at least a year’s worth of data on observing conditions.

16 17 ATST Team

International partnerships under development

1/2 relative size

Management Stephen Keil National Solar Observatory, Sunspot; Principal investigator AdvancedThomas Technology Rimmele* National Solar Solar Te Observatory,lescope Sunspot; Project scientist CollaboratingJim Institutions Oschmann National Solar Observatory, Tucson; Project manager Co-principal investigators (lead) Tim Brown,National David Elmore, Solar PhilipObservatory Judge, Michael KnoelkerHigh, Bruce Altitude Lites, SteveObservatory Tomczyk High Altitude Observatory Carsten Denker,New Philip Jersey Goode Institute, Haimin Wangof Technology New Jersey Institute of Technology Jacques Beckers, FaustoUniversity Cattaneo, of HawaiiRobert Rosner University of Chicago Roy Coulter,University Jeffrey Kuhn of ,Chicago Haosheng Lin University of Hawaii University of Rochester NSO science team University of Colorado K.S. Balasubramaniam, Alexei Pevtsov, Han Uitenbroek National Solar Observatory, Sunspot Mark Giamapapa, Jack Harvey, Frank Hill, ChristophUniversity Keller of, CaliforniaMatthew Penn Los AngelesNational Solar Observatory, Tucson University of California San Diego California InstituteScientific of Technology collaborators NathanPrinceton Dalrymple, University Richard Radick Air Force Research Laboratory Stanford UniversityPaul Bellan California Institute of Technology Cristina Cadavid, GaryMontana Chapman, State Stephen University Walton California State University, Northridge Michigan StateKimberly University Leka Colorado Research Adriaan van Ballegooijen, Ruth Esser, Peter Nisenson, John Raymond Harvard-Smithsonian Center for Astrophysics CaliforniaHector State Socas-Navarro University NorthridgeHigh Altitude Observatory Harvard-SmithsonianManolo Collados CenterVera Institutofor Astrophysics de Astrofisica de Canarias Colorado ResearchMichael Sigwarth Associates Kiepenheuer-Institut für Sonnenphysik Thomas BergerSouthwest, Theodore ResearchTarbell, Alan Institution Title Lockheed Martin Solar and Astrophysics Laboratory Air Force ResearchRobert Laboratory Stein Michigan State University Lockheed MartinDana Longcope Montana State University NASA/GoddardDonald Space Jennings Flight NASA Center Goddard Space Flight Center John Davis, Allen Gary, Ron Moore NASA Marshall Space Flight Center NASA/Marshall Space Flight Center Chio Cheng Princeton University Craig Deforest, Don Hassler Southwest Research Institute Alexander Kosovichev Stanford University Jan Stenflo Swiss Federal Institute of Technology, Zurich Institute of Astronomy Roger Ulrich University of California, Los Angles Andrew Buffington, Bernard Jackson University of California, San Diego Thomas Ayres, Juri Toomre University of Colorado Luigi Smaldone Università di Napoli Federico II John Thomas The University of Rochester Blue indicates member of Science Working Group (*chair)

16 17 NSO/AURA/NSF The NSO’s premier principal sites are located at Sunspot, atop Sacramento Peak, NM (left) and Kitt Peak, southwest of Tucson, AZ (right). The 76 cm Dunn Solar Telescope (at rear, left), is the premier U.S. facility for high-resolution solar physics and has pioneered solar adaptive optics and high-resolution, ground-based solar physics as a necessary prelude to the ATST. The 1.5-meter McMath-Pierce Solar Telescope on Kitt Peak, AZ (foreground, right), is the largest unobstructed-aperture optical telescope in the world, and is the only solar telescope in the world that routinely investigates the relatively unexplored infrared domain beyond 2,500 nm (2.5 µm). National Solar Observatory

The mission of the National Solar Observatory is to advance Understanding the coupling between the interior and surface. our understanding of the Sun as an astronomical object and as the Coupling between surface and interior processes that lead to dominant external influence on Earth and by providing forefront ob- irradiance variations and the build-up of solar activity. servational opportunities to the research community. The NSO has Understanding the coupling of the surface and the envelope: two principal sites located at Sunspot, NM, and Kitt Peak, AZ. The transient events. Mechanisms of coronal heating, flares, and Sunspot and Kitt Peak sites represent separate origins through, re- coronal mass ejections which lead to effects on space weather spectively, the U.S. Air Force and the National Science Foundation. and the terrestrial atmosphere. These were combined under one organization in the 1970s. Today Exploring the unknown. Fundamental plasma and magnetic field NSO comprises world-class telescopes, the Dunn Solar Telescope processes on the Sun in both their astrophysical and laboratory at Sunspot and the McMath-Pierce Solar Telescope at Kitt Peak, context. and the Global Oscillation Network Group (GONG). The Advanced Technology Solar Telescope will incorporate the NSO’s mission includes the operation of cutting edge facili- current capabilities of the Dunn and the McMath-Pierce telescopes ties, the continued development of advanced instrumentation both while providing an unprecedented extension in spatial, temporal, in-house and through partnerships, conducting solar research, and and spectral resolution. As a necessary prelude to the ATST and as educational and public outreach. indispensable facilities for current research in solar physics, NSO will operate the McMath-Pierce and the Dunn Solar telescopes through NSO accomplishes this mission through: first light at the ATST. • Leadership for the development of new ground-based facilities that support the scientific objectives of the solar and solar-terrestrial physics community; • Advancing solar instrumentation in collaboration with university researchers, industry, and government laboratories; • Synoptic observations that permit solar investigations from the ground and space in the context of a variable Sun; • Providing research opportunities for undergraduate and graduate students, helping develop classroom activities, working with teachers, and mentoring high school students; and NSO/AURA/NSF • Innovative staff research. NSO also sponsors and manages the Global Oscillation Network Group (GONG) which studies the internal structure and dynamics of The broad research goals of NSO include: the Sun by helioseismology — the measurement of acoustic waves Understanding the mechanisms generating solar cycles. Mecha- that penetrate the solar interior — by using six stations around the world, such as the one atop Tiede, Canary Islands (left). In addition, nisms driving the surface and interior dynamo and the creation NSO is commissioning the Solar Optical Long-term Investigations and destruction of magnetic fields on both global and local of the Sun (SOLIS) facility (right) that will operate atop Kitt Peak and scales. eventually be co-located with ATST.

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18 19 National Solar Observatory Sunspot, NM 88349-0062 http://atst.nso.edu/

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