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Page 1: Advanced Technology Solar Telescope...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

a d v a n c e d t e c h n o l o g y s o l a r t e l e s c o p ea d v a n c e d t e c h n o l o g y s o l a r t e l e s c o p e

Advanced

Technology

Solar

Telescope

Page 2: Advanced Technology Solar Telescope...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

Inside front coverleft blank

Page 3: Advanced Technology Solar Telescope...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

National Solar Observatory P.O. Box 62 950 N. Cherry Avenue Sunspot, NM 88349-0062 Tucson, AZ 85719-4933 505-434-7000 520-318-8000

http://atst.nso.edu or http://www.nso.eduThe 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

Advanced Technology Solar Telescope

AURA

The Need: We are positioned for a new era of discovery about the Sun and the subtle ways it affects life on Earth. To 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 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 magnetic fi elds. ATST will study the outer solar atmosphere where magnetic activities manifest themselves as sun-spots 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 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 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 beyond, just as astrophysics is enriched by using our Sun as a model for phenomena that cannot be resolved with current instruments.

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 tech-nologies 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 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-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 leadership in observing the Sun and in developing advanced new instruments, and offers research opportunities from high school through post-graduate studies.

1/2 relative size

Advanced Technology Solar TelescopeCollaborating Institutions

National Solar ObservatoryHigh Altitude ObservatoryNew Jersey Institute of TechnologyUniversity of HawaiiUniversity of ChicagoUniversity of RochesterUniversity of ColoradoUniversity of California Los AngelesUniversity of California San DiegoCalifornia Institute of TechnologyPrinceton UniversityStanford UniversityMontana State UniversityMichigan State UniversityCalifornia State University NorthridgeHarvard-Smithsonian Center for AstrophysicsColorado Research AssociatesSouthwest Research InstitutionAir Force Research LaboratoryLockheed MartinNASA/Goddard Space Flight CenterNASA/Marshall Space Flight Center

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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.

Page 4: Advanced Technology Solar Telescope...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

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Photosphere (“visible surface”)~400 km (~240 mi) deep~5,780 K (~9,944 ˚F)

Solar interior~696,000 km (~432,567 mi) to center~15.7 million K (~28.3 million ˚F)

Relative size of Earth12,723 km (7,908 mi) diameter

Chromosphere~2,000-3,000 km (~1,243-1,864 mi) deep~6,000-10,000 K (~10,340-17,540 ˚F)

Transition region~50-100 km (32-62 mi) deep, rapid heating

CoronaMillions of kilometers into space> 1 million K (1.8 million ˚F)

At 1.4 million km, the Sun is 109 times wider than Earth, yet much of what we know about its interior comes from what we can see through its outer atmosphere, as depicted here. Fusion occurs only in the core; heat and energy move through the radiative zone, and then drive the convection zone. These are concealed by view. We can only observe the photosphere, chromo-sphere, and transition region — with a combined depth less than 1/250th the radius of the Sun (thinner even than the diameter of Earth). We can also observe the corona and activities like fl ares and coronal mass ejections which cross several layers at once.

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 light at last can travel freely and thus gives the illusion of a “visible surface.” Escaping light also cools the gas, setting up granular circulation cells like an immense boiling pot (enlarged inset of sunspot). Magnetic lines also emerge from the interior, suppressing convection (lower image) and forming sunspots of cool gas that often are larger than Earth (size indicated by black circle below inset).

The chromosphere emits and absorbs a wide range of colors as temperatures and pressure change. Yet through its depth, we can see how magnetic fi elds link features from the corona to the visible surface even as they twist and change, as outlined by the inset image at far left. At immediate left, an abbreviated representation of layers of the atmosphere depicts 1) extreme 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 (393 nm) showing magnetic fi elds, and 4) G-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 the source.

Seen only during solar eclipses or with special telescopes, the faint corona is the Sun’s tenuous outer atmosphere and extends deep into space. One of the mysteries of modern solar physics 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 activi-ties in the lower atmosphere.

NSO/AURA/NSF

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

NSO/AURA/NSF

NASA/ESA SOHO Consortium

Before onset Eruption onset

Confined eruption, ending Ejective eruption, middle

NASA/Marshall Space Flight Center

Extended magnetic fi eld lines arcing through the chromosphere and into the corona can reconnect, causing an ex-plosive fl are and/or releasing a coronal mass ejection (CME) into deep space.

Anatomy of the SunCross-section

of outer solar atmosphere

Core

Radiative zone

Convective zone

Corona

Flare

Coronalmass

ejection Transition region, Chromosphere, Photosphere{

Heights and temperatures are approximate and variable; colors, shapes and positions are illustrative.

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Page 5: Advanced Technology Solar Telescope...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

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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.

The unknown

• Origin of interstellar matter

• Stellar flares

• Irradiance variations of sun-like stars

esatmospher• Accretion disk coronae

p-mode oscillations • Best solar optical telescope in the world

High spatial resolution

High photon flux

Thermal infrared

Act

ivit

yC

apab

ility

Imp

act

Transient eruptions:Coronal mass ejectionsand flares

Origin of solar variability Heating of corona and chromosphere;Origin of solar wind

Surface and atmospheric structure

The Need

Since George Ellery Hale’s 1908 discovery that sunspots coincide with strong magnetic fi elds, we have become

increasingly aware of the Sun’s magnetic fi elds as an com-plex and subtle system. The familiar 11-year sunspot cycle is just the most obvious of its many manifestations. Recent advances in ground-based instrumentation have shown that sunspots and other large-scale phenomena that affect life on Earth are intricately related to small-scale magnetic processes whose inner workings are beyond what current telescopes can observe.

At the same time, using advances in computer sci-ence and technology, scientists have developed intrigu-ing new theories about those small-scale processes, but without real-world observational data to verify their models. We are positioned for a new era of discovery about the Sun and how it affects life on Earth, how dis-tant stars work, and how we might control plasmas in laboratories.

To embark on that journey, we must observe the Sun and its magnetic activities at higher resolutions on three fronts:• Spatial — resolve fundamental scales of structures on

the solar surface and in its atmosphere,• Spectral — narrow slices of the solar spectrum for bet-

ter measurements of magnetic fi elds and thermody-namics, and

• Temporal — high cadence (frequent) images and spec-tra of rapidly developing events.

Further, we must observe not just in familiar near-ul-traviolet and visible light, but exploit the relatively unex-plored infrared solar spectrum. We must see the faint solar corona in infrared, measure the polarization of sunlight with greater precision, and cover a large fi eld of view so extended active areas can be studied. These capabilities will reveal hidden aspects of magnetic activities and help us bridge the gap from what we know now to what we must learn. But doing so requires a large telescope to overcome limitations imposed by current instruments.

NSO’s long range plan recognizes that progress in un-derstanding the Sun requires that it be treated as a global system, in which critical processes occur on all scales, from the very small (<100 km) to scales that encompass the whole Sun. This was recognized by the National Research Council in its recent decadal report, “Astronomy and As-trophysics in the New Millennium (2001)”:

The fi rst scientifi c goal for advancing the current under-standing of solar magnetism is to measure the structure and dynamics of the magnetic fi eld at the solar surface down to its fundamental length scale.

Despite the brightness of the Sun, solar physicists share a problem with their nighttime colleagues: “photon starva-tion.” While bright images of the solar disk, the corona, sunspots, and fl ares are the most familiar of solar observa-tions, defi nitive work is done at high spectral and spatial resolution while observing a small section of the Sun in a

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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).

Aperture 4 m (13.1 ft)Configuration Gregorian, off-axisMount Altitude-Azimuth (Alt-Az)Enclosure Ventilated corotating hybrid domeField of view Goal: 5 arc-min (300 arc-sec) Minimum: 3 arc-min (180 arc-sec)Image quality Seeing limited (no AO): under excellent seeing conditions,

<0.15 arc-sec @ = 1,600 nm Diffraction limited (with AO): under median seeing

conditions, Strehl ratio > 0.3 @ = 500 nmSpectral range 300-28,000 nm (0.3-28 µm)Polarization Accuracy: >10-4 of continuum intensity Sensitivity: 10-5 continuum intensity (limited by photon

statistics)Coronagraphic Near infrared through infraredScattered light <2.5 x 10-6, 1/10th solar radii from limb @ = 1,000 nmLifetime 30-40 years

ATST specifications

spectrally narrow subset of the available light. This is like looking through a microscope and switching to higher powers and inserting a color filter: as you get closer to the answer, you also reduce the available light, eventually close to blackout.

Also like their nighttime counterparts, solar physicists cope with atmospheric “seeing.” Looking through Earth’s atmosphere is like looking from the bottom of a swimming pool. Without corrective measures, current ground-based solar telescopes can reveal structures no smaller than a few hundred kilometers across on the surface of the Sun.

A larger telescope can solve photon starvation, but atmospheric seeing would limit it to the same spatial

resolution of smaller telescopes. Adaptive optics (AO), an emerging technology that corrects most of the atmospheric distortion, can enhance existing solar telescopes, but just to the diffraction limit set by their small apertures. Orbiting telescopes have a perfect seeing environment, but are ex-pensive and have limited lives.

A large ground-based telescope — such as the 4-meter aperture Advanced Technology Solar Telescope (ATST) with an integrated AO system — can simultaneously ad-vance solar resolution and spectral coverage and serve the science community for several decades. ATST will be a unique scientific tool providing an unprecedented combi-nation of spatial, temporal, and spectral resolutions across visible and infrared wavelengths. Just as early solar phys-ics research helped solve fundamental problems in atomic, nuclear, and gravitational physics, ATST is expected to have a broad impact on astrophysics, plasma physics, and solar-terrestrial relations.

Just a century after Hale’s breakthrough discovery, ATST will take us deeper into the heart of sunspots, flares, and other key solar activities. ATST observations of the small-scale processes at the solar surface and through the overlying atmosphere will help us understand the life cycle of magnetic fields. ATST will be a powerful, flexible sys-tem that will serve the U.S. and international solar physics communities as their main ground-based facility into the middle of the 21st century and beyond.

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1000

1920

100

10

1

0.1

Scale(arc-secon

ds)

EUV UV VIS NIR FIR Thermal Radio

ATST

SDO

�m mmnm

FASR

1 GHz1 THz10 eV

FASR: Frequency-Agile SolarRadiotelescope

SDO: Solar Dynamics ObservatorySOLIS: Synoptic Optical Long-term

Investigations of the SunSTEREO: Solar-Terrestrial Relations

Observatory

(2010)

(2010)

(2005)

STEREO

(200

4)SOLIS(200

4)

While many solar telescopes observe the whole solar disk, most studies, including those planned for ATST, glean new knowledge from small areas of the sun. ATST will have a large fi eld of view — 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-seconds in visible light (550 nm). This will reveal features as small as 20 to 70 km wide at the solar surface. Atmospheric turbulence limits ground-based telescopes — without adaptive optics (see page 13) — to a resolution of about 1 arc-seconds, regardless of the size of the telescope, and occasionally permits 0.5 arc-seconds for short periods. By comparison, the human eye has a fi eld of view about 263 times wider than the Sun (~140 degrees) and a resolution of 60 arc-seconds. Getting to such a level of detail is but a means to an end: Resolving fundamental astrophysical processes in the solar atmosphere.

Specifi cally, scientists want to see down to the intrinsic scale, the distance atoms and molecules move as these processes take place at high temperatures. Two key measures of the intrinsic scale are the distance light travels before being reabsorbed by another atom (photon mean-free path) and the height across which atmospheric pressure changes by a certain constant (pressure scale height). To resolve these fundamental length scales in the deepest accessible layers of the solar atmosphere requires seeing structures as small as 70 km (42 mi) at the solar surface (an angular resolution of 0.1 arc-second or better).

However, modern numerical simulations with a resolution of 10 km plus inference from the best available images suggest even smaller scales for crucial physical processes such as vor-tex flows, dissipation of magnetic fields, and magnetohydrody-

namic (MHD) waves. Resolving these scales is of utmost impor-tance to testing various physical models and thus understanding how the physics of the small scales ties into the larger problems.

An example is the question of what causes slight variations of the solar radiative output — the solar constant — that impacts the ter-restrial climate. Solar radiation received at Earth increases with solar activity (see page 5). Since the smallest magnetic elements contrib-ute most to this fl ux excess, it is of particular importance to study and understand the physical properties of these dynamic structures. Unfortunately, the 0.76 to 1.5-meter apertures of present solar tele-scopes cannot provide measurements at such scales because of their limited apertures.

Further, because of the need to provide an observatory that can operate for decades and incorporate new technologies as they become available, ATST needs to be ground-based. While space (with no atmosphere to absorb or blur light) offers the best possible seeing conditions for observing the Sun, planets, and the rest of the universe, size and technical evolution argue against building an orbital ATST. Many of ATST’s new instruments will be require fi ne-tuning or frequent updates on site, while others will be added years later as new detector technologies emerge. On-orbit servicing and replacement are not affordable for a program like ATST.

Finally, ATST will not operate alone. Just as a symphony can-not be appreciated by listening to one instrument, the Sun cannot be understood by observing just in visible light. ATST will work in concert with satellites and other ground-based observatories, such as SDO, STEREO, SOLIS (which will be co-located with ATST), the proposed FASR observatory, and others.

Getting the fi ne structures to see the big picture

0.”03 resolution(@550 nm, w/AO)

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

300” fi eld of view

1” resolution(@550 nm,uncorrected)

1,920” disk

Simulation

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Key ATST requirementsHigh 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-cal parameters, such as magnetic fi eld strength and direction, tempera-ture and velocity, on the short time scales involved.

Precise polarimetric measurements of solar magnetic fi elds.Access to a broad set of diagnostics, from visible to thermal infrared

wavelengths.Low scattered light observations in the infrared to allow measurements of

coronal magnetic fi elds.

The Science

Hale’s discovery in 1908 that magnetic fi elds perme-ate sunspots started a revolution that turned solar

science into a fi eld encompassing (and often advancing) many branches of physics. In particular, much of our solar research now involves magnetohydrodynamics (MHD), the study of plasmas (electrically conducting gases) whose shapes and fl ows are infl uenced by magnetic fi elds.

Sunspots, it turns out, are the best-known manifesta-tions of large magnetic system found in the outer third or so of the Sun. Hydrogen fusion, the source of the Sun’s energy, occurs only in the core. The remainder of the Sun is a massive blanket serving two roles. First, it compresses the core to keep fusion going, and moderates the fl ow of energy from the core into space. The outer region of the blanket is the convective zone where giant gas cells circu-late like water in a boiling pot, bringing heat to the surface. At the same time, solar rotation moves the cells around the Sun, somewhat like massive weather systems. Because 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 par-ticles 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

cool. When the gas density drops so light can travel freely, it forms what we see as the visible “surface” of the Sun, the photosphere. Here twisted magnetic fi elds loop out of the convective zone, into space and back to form an ar-ray of features, including sunspots, plages, fi laments, and prominences. Magnetic fi elds reach through the overlying chromosphere and into the corona where they can become unstable and trigger coronal mass ejections, or simply open into interplanetary space. When massive fi elds pierce the visible surface they often form darkened areas — sunspots — where the magnetic fi eld keeps hot gas from rising from the interior.

All these affect life on Earth. The 11-year sunspot cycle (actually a 22-year magnetic cycle) is one of the bet-ter-known phenomena. But the various forces that drive the cycle, and determine its intensity and its relationship with conditions around and on Earth, remain poorly un-derstood. Historical evidence indicates that changes in the

Robert F. Stein, Michigan State University

A series of images from a computational fl uid dynamics model de-picts cool gas descending from lanes between granules within the Sun’s visible surface. The parcel in this model is 1,200 km square and 2,500 km deep. High-resolution images of activities in the solar atmosphere are needed to validate computer predictions and place them on a fi rm footing.

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We think of the Sun as constant, but physical and historical re-cords show variations, the most famous being the sunspot number, a count of actual sunspots and sunspot groups (above). A 22-year Hale cycle — two 11-year cycles with solar magnetic poles revers-ing every 11 years — appears to be the norm for our Sun, based on just four centuries of data with some hiccups. For example, the 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 start-ed 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

with warm spells such as the Medieval Maximum (900-1300) when Greenland was green and colonized by the Vikings, as compared to today’s glacial island.

(Climate is a general description of long-term environmental con-ditions across regions or the entire planet. Weather refers to local short-term variations in conditions. However, climate sets the stage for types of weather.)

We are still uncovering the links between solar activity and terres-trial climate because neither is fully understood. Earth’s climate sys-tem is highly complex, comprising large heat sinks like the oceans, that form a buffer and delay (sometimes by years or decades) a response to shifts in solar irradiance.

On the solar side of the question, we learned in the last few decades how variable the “constant” can be, and thus refer to total solar irradiance (TSI). Irradiance values averaged from satellite data (different colors in the graph indicate different satellites) show daily variations as solar active regions arise and disappear and move across the solar disk. Annual sunspot numbers for cycles 21-23 are superimposed on the graph at right to show how the two correspond.

The paradox in sunspot variations is that the Sun is brighter when it has more spots. What was revealed by modern instruments (and not readily apparent to observers relying on just their eyes) is that sunspots are accompanied by brightened areas, plages (French for beach) and by bright network lines connecting magnetically active

Sun and climate

areas. On balance, plages and network lines outweigh sunspots, and the Sun thus appears brighter at sunspot maximum. One aspect of increased solar activity is that solar ultraviolet emissions have a greater brightness range than the Sun as a whole. As a result, ozone, an important greenhouse gas in the stratosphere, varies in direct response to daily changes in the solar ultraviolet irradiance that forms

by splitting oxygen molecules (O2) into free atoms that recombine to form ozone (O3). The implication is that periods like the Maunder Minimum could allow natural depletion of ozone and lead to global cooling.

Further, only recently have we established that the Sun’s total energy output varies slightly with the “normal” sunspot cycle, as we understand it. Such data can only be collected by satellites above Earth’s atmosphere (right) and thus exposed to the full range of solar energy. The data clearly show a variation with sun-spot numbers. We also know that geomagnetic storms, driven by solar activities, provide an additional, variable energy input through the polar caps.

78 031363

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Sol

ar Ir

radi

ance

(W/m

2 )

Sun

spot

(Wol

f) nu

mbe

r80 85 90 95 00

1363

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Total solar irradiance

Cycle 21 Cycle 22 Cycle 23

Annual sunspot number

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Maunder Minimum1645-1715

Little Ice Age (~1500-1850)

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spot

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ber

DaltonMinumum1795-1820

NASA/Goddard Space Flight Center & Claus Frolich, World Radiation Laboratory

Dave Hathaway, NASA/Marshall Space Flight Center

Pieter Bruegel the Elder, Kunsthistorisches Museum Wien, Vienna

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sunspot cycle impact Earth’s climate, although modulated by terrestrial events such as volcanoes (see page 5). While a number of instruments monitor the Sun’s total output, ad-vanced instruments like ATST are needed so scientists may unravel fundamental drivers that determine that output.

ATST will primarily study only the outer layers of the Sun’s atmosphere, the photosphere, chromosphere, and corona (the inner workings are inferred from oscillations in the photosphere). Nevertheless, all the matter and energy that reach Earth from the Sun have to travel through these regions which display a dazzling and intriguing array of scientific puzzles that allow us to infer what is happen-ing inside. ATST’s enhanced resolution in space, time, and wavelength will let scientists see what lies beyond the reach of current telescopes. Further, ATST will extend our understanding into the thermal infrared spectrum, thus 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-serve the small-scale processes at the solar surface that play a critical role in understanding the overall sunspot cycle. ATST will contribute to understanding magnetic flux emer-gence through the turbulent boundary between the solar interior and atmosphere. The ATST will reveal the nature of solar flux tubes, which are generally believed to be the fundamental building blocks of magnetic structure in the atmosphere and the progenitors of solar activity. ATST will observe the interaction of flux tubes with convective mo-

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 the near-infrared. Furthermore, the infrared beyond 1,500 nm provides particularly powerful diagnostics of magnetic field, temperature, and velocity at the upper layers of the solar atmosphere. ATST’s infrared capabilities will be used to measure the cool chromospheric component and provide critically needed measurements of coronal magnetic fields. The dynamics and heating of these outer layers of the solar atmosphere in turn result in the violent flux expulsions we see as flares and mass ejections. All of these processes are tied together by the behavior of magnetic flux in the dynamic plasma at the solar surface. ATST also will im-pact other areas of astrophysics, space science, and plasma physics and help refine space weather models. Some of the major scientific questions driving ATST follow.

Flux tubesObservations have established that the photospheric

magnetic field is organized into countless small fibrils or flux tubes. The majority of these structures are too small for current telescopes to resolve. Flux tubes are the most likely channels for transporting energy from the solar in-terior into the upper atmosphere, which is the source of ultraviolet and X-ray radiation from the chromosphere,

Bellan Plasma Group, California Institute of Technology

From telescope to laboratoryCombining results from ATST and laboratory experiments will play an important role in furthering our understanding of turbulent flow in plasmas. Generating plasma-filled twisted magnetic flux tubes in a laboratory (above) serve as scaled experiments of the dynam-ic magnetic phenomena observed on the Sun. Such tests offer insights into magnetic helicity and topology, triggering of instabili-ties, magnetic reconnection physics, conversion of magnetic en-ergy into particle energy, importance of magnetic boundary condi-tions, and interactions between adjacent magnetic structures.

Lockheed Martin

The 11-year sunspot cycle is depicted in a series of magnetograms that reveal solar activity. Bright areas indicate north polarity; dark areas, south polarity. Constant magnetic activity is a feature of the Sun, but concentrations of activity — which also produce sunspots, flares, and other phenomena — rise and fall about every 11 years.

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transition region, and solar corona, which in turn affects the Earth’s outer atmosphere. Fine-scale 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 stars, but also of other astrophysical situations such as the accretion disks around compact objects like pulsars, or environments where dust accretes into planetary systems. Current solar telescopes cannot provide the required combination of spectroscopy, polarimetry, and angular resolutions to explore the enig-matic flux tube structures.

Magnetic field generation and local dynamosTo 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 theories in-volving 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 dy-namo action at the surface of the Sun. ATST will measure the turbulent vorticity and the diffusion of small-scale magnetic fields and determine how they evolve with the 11-year solar cycle. By resolving individual magnetic flux tubes and observing their emergence and dynamics ATST will address fundamental questions including: How do

strong fields and weak fields interact? How are both gener-ated? How do they disappear? Does the weak-field compo-nent have global importance and what is its significance for the solar cycle? ATST will measure distribution functions of field strength, field direction and flux tube sizes for com-parison with theoretical models, and will observe plasma motions and relate them to the flux tube dynamics.

Magnetic fields and mass flowsThe total magnetic field of sunspots is large enough

to dominate the hydrodynamic behavior of the local gas,

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 atmosphere. Color contours indicate convective regions, and black lines depict field strength. Flux tubes are below the limit of resolution in current telescopes.

Thomas Rimmele, NSO/AURA/NSF

High-resolution image of a sunspot reveals tubelike structures that make the penumbra and extend from the granulated solar surface into the cooler umbra at center. Sunspots are formed by magnetic field lines emerging through the surface and blocking downward convection of the darker, cool gas.

SOHO Consortium, ESA, NASA

Unlike Earth with its simple pair of north and south magnetic poles, the Sun is carpeted with countless small north and south magnetic joined by field lines that loop through the solar atmosphere and corona and back to the surface. They are born, fragment, drift, and disappear in a day or two, arising from mechanisms that are poorly understood at present.

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393.3682 & 396.8492, Calcium II K & H: Deep violet (right) emitted and absorbed by calcium ions in the chromosphere. Useful for monitor-ing higher altitudes of the chromosphere.

431-430, G-band: A rich complex of absorption lines dominated by vibrational and rotational modes of carbon-hydrogen (CH) molecules (right). G-band images show bright points indicating concentrations of magnetic fl ux in the photosphere. The bandpass also is rich in lines of iron, titanium, and other elements.

589.59 & 589.99, Sodium D1, D2: Orange-yellow double line emit-ted and absorbed by sodium. These lines are used to measure motions in the solar atmosphere.

630.25, Iron (Fe I): Deep red color emitted (or absorbed) by hot, neutral iron in the photosphere (see top). Excellent for tracking magnetic activity on the Sun.

656.28, Hydrogen-alpha (H ): Deep red color (right) emitted or absorbed by hot, neutral hydrogen in the chromosphere. Excellent for tracking active regions on the Sun.

854.2, Calcium II: Absorbed by the same atom as the 393 and 396 lines (bottom); highly sensitive to magnetic fi elds and thus an ideal wavelength for magnetic fi eld measurements in the chromosphere.

Sample spectral lines of interest

300

400

~38

0

500

600

700

~77

080

0

900

1,00

0

2,00

0

3,00

0

10,0

00

28,0

00

20,0

00

4,00

0

5,00

0

6,00

0

7,00

0

8,00

0

9,00

0

Visible lightUV Near IR Thermal IR

(nm)

"Black" light~400-345

Red laserpointer~650

Radiant heatIncandescentlamp peak

~999

Max. eyesensitivity

~550

ATST and the many colors of the Sun

A yardstick for lightFor consistency, the unit of measure used for light throughout this book is the nanometer, a billionth of a meter (25.4 million nm/inch). This is the preferred 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 convenience, scientists working in infrared often use microns (µm), a millionth of a meter (1,000 nm).

Solar thumbnails: NSO, USAF/ISOON (H-alpha),

K.S. Balasubramaniam, NSO/AURA/NSF

Continuum H core H wing 629.4 629.6 629.8 630.0nm

630.2 630.4 630.6

1074.6 & 1079.8, Iron (Fe XIII): Yields highly sensitive measure-ments of plasma velocities in coronal loops.

1083.00, Helium: Helium atoms (right) in the chromosphere about 2,000 km above the photosphere indicate coronal intensities be-cause this infrared spectral line is infl uenced by X-rays.

4700, Carbon monoxide (CO): A current mystery of the Sun is why portions of its atmosphere cool enough to allow some atoms — such as carbon and oxygen — to combine and form molecules (simulated at right). Ob-servations already have signifi cantly changed our understanding of the chromospheres of stars and led to a better understanding of the solar atmosphere. CO also emits at other wavelengths.

12320, Magnesium I: These are the most magnetically sensitive lines in the solar atmosphere. By observing Zeeman splitting, we can measure magnetic fi elds down to 100 Gauss. Combined with other data, the Mg I observations will help us reconstruct the three-dimensional structure of magnetic fi elds in the photosphere and lower chromosphere.

ATST’s spectral range — 300 to 28,000 nm — will span virtually the entire optical spectrum passed by Earth’s atmosphere. Within this are tens of thousands of emission and absorption lines, each telling a dif-ferent part of the solar activity story. The wavelength and appearance of each line is determined by the state of the elements composing the solar atmosphere. An atom or molecule can emit many different lines, often in a series, and lines can be shifted by motion to or from the observer, or by intense magnetic fi elds. For example, the spectrum below was taken across a sunspot. The broadening inside the yel-low ellipse is Zeeman splitting where an intense magnetic fi eld affects

emissions by iron at 630.25 nm. Vibrations and rotations of hot atoms and molecules produce a complex series of lines in the infrared that ATST will explore. Although hydrogen and helium compose almost 99 percent of the Sun’s mass, we learn the most from spectral lines generated by wisps of carbon, oxygen, silicon, iron, and other atoms that the Sun captured as it formed 4.5 billion years ago. Like colored dyes dropped into a river, these additional chemicals let us trace what is happening on the Sun. (The vertical line in each reference image shows the position of the slit; the horizontal lines are for reference; thus, the spectrum is offset from the reference images.)

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a regime very different from that of the rest of the solar photosphere. Earth’s magnetic fi eld is about 0.5 gauss at the surface. Fields within sunspots can range from 1,500 to 3,000 gauss. This is about the strength of a bar magnet, but across a volume several times larger than Earth, thus hinting at the immense power at work to form the fi eld. Numerical simulations and theoretical models predict dy-namic phenomena, such as oscillatory convection in the strong-fi eld regions of sunspot umbrae, fl ows that move at the speed of sound along penumbral fi laments, and oscilla-tions and wave phenomena.

To verify these predictions and ultimately to answer such fundamental questions as “Why do sunspots exist?” will require an extremely capable instrument. High-resolu-tion (better than 0.1 arc-seconds) vector polarimetry com-bined with high sensitivity and low-scattering optics are required. Understanding the interaction of magnetic fl ux and mass fl ows is crucial for our understanding of the be-havior of magnetic fi elds on scales ranging from planetary magnetospheres to clusters of galaxies. Sunspots allow us to test those theories in a regime where magnetic fi elds dominate mass fl ows.

Stellar upper atmospheresA 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

spectra near 4,700 nm show surprisingly cool clouds that appear to occupy much of the lower chromosphere. Ap-parently only a small fraction of the volume is fi lled with hot gas, contrary to classical static models that exhibit a sharp temperature rise in those layers. The observed spec-tra 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-ap-erture solar telescope that provides access to the thermal infrared.

Another mystery comes from spicules, forests of hot jets that penetrate from the photosphere through the chro-mosphere and then fade in a few minutes. These are be-lieved to be an MHD phenomenon that is not understood nor adequately modeled. Combined with extreme ultravio-let observations from space, ATST will allow us to resolve their structure and dynamics and understand their nature and roles.

Magnetic fi elds and the solar coronaThe origin and heating of the solar corona, and the

coronae of other stars, are still mysteries. Most of the pro-posed 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-ing of the coronal fi elds by photospheric motions, but also to measure the coronal magnetic fi eld strength itself. This is important for developing and testing models of fl ares and coronal mass ejections, which propel magnetic fi eld and plasma into interplanetary space and induce geomagnetic disturbances. In particular, precise measurements of the

Bruce Lites, High Altitude Observatory, and the Swedish Vacuum Telescope

Mesas with bright cliff sides are a recent discovery in solar physics. The structures seen here are granules, columns of hot gas rising to the visible surface, then cooling and descending back into the interi-or. High-resolution images near the solar limb revealed that the sides of granules are hotter and brighter where magnetic fi elds penetrate the solar atmosphere.

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.

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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 spec-trum — will provide these critical measurements.

Space weatherATST 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 endanger astronauts and satellites. Coronal mass ejections (CMEs) produce interplanetary blast waves that also accel-erate energetic particles and carry mass and magnetic fields that impact Earth. The response of Earth’s magnetic field can disrupt electrical power grids on Earth, damage satel-lites, endanger the health of air crews crossing the polar cap and astronauts in orbit, and have other impacts on hu-man society. It appears that the microstructure of magnetic fields on the Sun plays an essential role in the physics of these large-scale phenomena.

ATST will provide precision data on magnetic fields and atmospheric parameters needed to develop models for the magnetic evolution of active regions, the triggering of mag-netic 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 en-hanced emissions expected from these events. These in turn will let us refine solar-terrestrial models to include critical solar data instead of the proxies that are used now.

Looking beyond the SunSolar physics is central to astrophysics, with the Sun providing a

unique “laboratory” for studying key astrophysical processes that occur elsewhere in the Universe but can’t be observed directly. Magnetic fields are ubiquitous and important throughout the Uni-verse. Detailed observations of flux tubes are crucial for our un-derstanding not only of the activity and heating of the outer atmo-spheres of our Sun and other stars, but also the accretion disks of compact objects, or environments where planets are about to form.

Studies of the Sun also play a fundamental role in basic physics and in the physics of stars. The Sun provides our best opportunity for observing MHD and plasma phenomena under astrophysical conditions. MHD waves, MHD turbulence, magnetoconvection, magnetic reconnection, and magnetic buoyancy occur in many as-trophysical systems but can be observed only on the Sun.

Solar flares serve as a prototype for many high-energy phe-nomena and particle acceleration mechanisms in the Universe. The cause of the Sun’s million-degree corona must be understood before we can confidently interpret EUV and X-ray emissions from distant stars. Our scientific understanding of these solar phenom-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 evolu-tion. 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.Stellar convection, the relationship between rotation and convection

in other stars.Magnetic fields in late-type stars, magnetic field properties of many star

types. Planets and small solar system bodies, extend polarization mea-

surements to planetary astronomy.

NASA/SOHO

Coronal mass ejections, or CMEs, occur when magnetic field lines open into space and spew large bubbles of hot solar gas across the solar system. These normally entrap magnetic field lines and can interact with Earth’s magnetosphere and disrupt communications and power systems as well as causing the best-known Sun-Earth effect, the aurora borealis.

K.Strassmeier, Vienna, NOAO/AURA/NSF

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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

The Telescope

The ATST project is being developed by a collaboration of observatories, universities, and research laboratories

led by the National Solar Observatory. Funding is provided by the National Science Foundation. The Association of Universities for Research in Astronomy (AURA) manages the project through the NSO, based in Tucson, AZ, and Sunspot, NM. Principal partners are the New Jersey Insti-tute of Technology, the University of Hawaii, the University of Chicago, and the High Altitude Observatory; another 17 collaborators are involved (see page 17 for a complete list).

ATST recently completed a conceptual design review that defi ned major aspects of the telescope and its support facilities. Nevertheless, many aspects are presented here are subject to change as the project evolves.

When completed, the ATST will be the world’s premier ground-based facility for observing and studying the Sun

with its unprecedented combination of high spectral, spa-tial, and temporal resolution. It also will serve as the focal point for systematic campaigns involving other ground-based and orbiting observatories. ATST will become a ma-jor component of the solar facilities operated by the NSO and available to U.S. and international solar astronomers.

Telescope designThe need to provide solar observers with greatly im-

proved images, spectra, and data in turn leads to demand-ing observing requirements that drive ATST’s design. These include:

• A planned 300x300 arc-seconds fi eld of view (minimum of 180x180 arc-seconds) so the telescope can observe large active areas as they evolve.

• Spatial resolutions of 0.03 arc-second at a wavelength of 500 nm (green) and 0.08 arc-seconds at 1,600 nm

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(near-infrared) when using adaptive optics to compen-sate for atmospheric blurring.

• Temporal resolution to reveal events spanning a few seconds (e.g., fast movies rather than long time expo-sures).

• Spectral resolution to enable precision, sensitive mea-surements of magnetic activities in the solar atmo-sphere from near ultraviolet into the thermal infrared.

• Sufficient light flux and control of scattered light to re-veal faint structures and activities in the corona.

• Sufficient light flux to allow high-resolution polarim-etry, studies using polarized light to determine the strength and directions of magnetic fields within solar structures.

Meeting these criteria involves several challenges. Spatial resolution improves as the diameter of the primary mirror increases. High spectral and temporal resolutions also require a large flux of light, thus driving the design towards a large aperture. Observing the faint solar corona means not only a large aperture, but an optical path clear of

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

Mark Warner, NSO/AURA/NSF

Primary mirror(4 meters [13.1 ft])

To Coude labs(instruments)

Secondary mirror(70 cm [27.6 in.]))

Fieldstop

Feed optics

Deformablemirror

Light from Sun

Feed optics

Azimuth cable wrap

Pier andCoude

Systeminterconnects

Heat stopassembly

Secondarymirrorassembly

Primary mirrorassembly

Baffle

support structures that would scatter light and overwhelm the faint coronal light. The large unobstructed aperture and broad spectral coverage further require that the telescope be open and reflective (an evacuated telescope would require impractically large windows that also would not transmit infrared radiation).

This combination of requirements led the ATST team to select an open-air, off-axis Gregorian telescope with a 4-me-ter (13.1-ft) primary mirror. This design differs from famil-iar astronomical telescopes. In the Hubble Space Telescope, whose basic design dates back to the 1800s, a secondary mirror is placed in front of the primary mirror’s focal point to enlarge the image and redirect the light into science in-struments. Most nighttime telescopes have similar arrange-ments. However, the large heat flux and scattered light requirements push ATST to an off-axis design.

In a Gregorian design, often used in solar physics, the secondary mirror is placed beyond prime focus. A field stop — actively cooled for ATST — is placed at prime focus

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Basic concept for Shack-HartmannAdaptive Optics System

Uncorrectedlight

Correctedlight

Beamsplitter

Shack-Hartmannlenslet arrayCCD camera

Deformable mirror

Actuators

Correlation &ReconstructionDigital SignalProcessors

Adjust mirror so each subaperture matches one reference aperture

A key advance making the ATST possible is adaptive optics (AO) technology that allows the telescope to operate near its diffraction limit, the theoretical best that a telescope can deliver. Diffraction-limited resolution is rarely achieved by large telescopes on Earth because the turbulent atmosphere acts like a continually changing rubber lens that bends starlight out of shape before it reaches the ground. Astronomers and solar physicists alike refer to being “see-ing limited” because the telescope is kept from operating near its theoretical capability. Larger apertures alone can make the problem worse because the telescope is looking through a larger column of disturbed air.

“Seeing” has evolved from a qualitative description into a series of mathematical techniques and formulas. The first critical measure is the point spread function or PSF. Telescopes do not focus light into perfect points but into an Airy disk, a series of rings (each dim-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 center or disk 0, 7 percent into disk 1, 3 percent in disk 2, and so on. Larger telescopes produce narrower Airy disks, as do shorter wavelengths. Other measures include the Zernike modes, a mea-sure of how distorted the wavefront is, and the Fried parameter, a measure of atmospheric distortion.

Controlling and correcting seeing has been the goal of AO, a so-phisticated technology that has emerged over the last two decades and has enhanced the work of nighttime astronomy. However, nighttime AO is not suited to solar observations because the Sun is too bright and extended to provide pointlike sources like stars. ATST will employ a different approach, which has been in develop-ment since the 1990s.

In the Shack-Hartmann AO design, small lenses divide the incoming image into multiple images, each using a different sub-aperture of the image of a sunspot or other feature. Each subap-

Smoothing the wrinkles out of our view of the Sun

erture image is displaced due to the distortion induced by seeing. An image-processing computer selects one subaperture as the reference image to determine how the other subapertures should be shifted to compensate for the atmosphere. This has been a significant challenge because the low contrast between solar features does not provide clear landmarks that could be used for corrections.

The control computer then commands actuators mounted on the back of a small deformable mirror in the main light path. In a sense, the actuators are like miniature loudspeakers. They reshape the mir-ror by as little as a few wavelengths of light — or even fractions of wavelengths — in response to varying electric signals. A tilt/tip mir-ror in the light path recenters the image in case of major distortions. The result is a striking stabilization of the image and removal of blur-ring due to Earth’s atmosphere.

The solar Shack-Hartmann approach (illustrated above) was first employed on the 75 cm (29.5 inch) Dunn Solar Telescope at the National Solar Observatory at Sacramento Peak at Sunspot, NM, in 1998. The low-order AO24 system used 24 subapertures and 97 actuators, and corrected the mirror 1,200 times a second (1,200 Hertz, Hz). Soon after its first demonstration it was improved and placed into routine operation on the Dunn. In 2003, the same team demonstrated the more advanced high-order AO76 version with 76 subapertures and 97 actuators at 2,500 Hz.

This demonstrated the feasibility of scaling the Shack-Hartmann concept upwards by several orders of magnitude to meet ATST’s requirements, an anticipated 1,000 subapertures. AO technology is improving spatial resolution of existing solar telescopes, and allows them to operate close to their diffraction limits. Using AO, the ATST team anticipates providing resolutions down to 0.03 arc-second in green light (550 nm). This is equivalent to 1/64,000th the apparent diameter of the Sun.

Thomas Rimmele & Kit Richards, NSO/AURA/NSF

Uncorrected and corrected images show the striking improvement possible with the NSO’s adaptive optics system.

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where it intercepts most of the light and allows only light from the area being studied to pass to the secondary mir-ror. To keep ATST’s optical path clear, the secondary mir-ror is offset from the center of the primary. For example, the points that stars have in many astronomical photos are caused by light diffracted by the secondary mirror sup-ports. Such unwanted light would spoil ATST observations of the faint corona as well as complicate the heat rejection problem. Thus, the primary mirror actually is an off-axis segment of a virtual 12-meter mirror. The primary will be a 100-mm (4 in) thin mirror mounted on active supports to maintain the appropriate parabolic curve as loads on the mirror shift during Sun tracking. The secondary mirror will focus light downward through a series of relay mirrors that eventually conduct the light into the science gallery below the main structure.

Building the telescope without a barrel or other shroud 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 optics will transmit the image from the secondary mirror to 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 sci-ence 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 on-site fine-tuning, an important consideration). Alterna-tively, scientists can use complete instruments or instru-ment modules that will be added to ATST over time as part of the observatory’s capabilities.

Current plans call for enclosing ATST with a new hybrid enclosure that draws features from other existing

designs. The slim contours of the dome were selected to minimize dome heating by sunlight while allowing con-trolled flow of ambient air for cooling. It will also allow a full range of pointing angles for ATST operations.

Science instrumentsNo telescope produces science without instruments. This

is particularly true for solar telescopes where we to observe the same object with a large variety of different instruments. However, cost constraints may severely limit the number of instruments that will be available in the beginning of ATST’s operations. ATST will use a new modular instrument ap-proach, where a variety of instruments can be assembled from a limited number of common optical modules. ATST also will be designed so the science community can develop and install its own instruments to meet specific needs.

S. Kasiviwanathan, NSO/AURA/NSF

Magnetic fields polarize light as it is emitted by a hot gas. From measurements of plane (Q, U) and circular (V) polarizations, as well as intensity (I) — the Stokes parameters — can let scientists calculate the direction and strength of the magnetic field. This im-age, covering an area about 1/50th the diameter of the Sun, was taken with the Diffraction-Limited Spectropolarimeter at the Dunn Solar Telescope at Sacramento Peak. More powerful versions of the DLSP will be employed by ATST.

U V

I Q

NSO/AURA/NSF

Preliminary development schedule for ATST anticipates “first light” and operations in 2010 assuming an authority to proceed in 2005.

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013

Scientific and Technical Advisory Groups

High-order AO development AO76 demonstration

Select finalists Final selection

Concept & design

Technology development

Mirror production

Construction

Integration Integration

Startprogram

CoDR PDR CDR AO Adaptive OpticsCDR Critical Design ReviewCoDR Concept Design ReviewPDR Preliminary Design Review

Note: Schedule subject to change

Site surveys

OperationsFirst light

Schedule extension if mirror is procured at start of construction

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The ATST will support up to nine instruments in three areas, the upper and lower Coude platforms, and the Gre-gorian focus. The Coude platforms will form the lower lev-els of the large rotating structure supporting the telescope proper. Each will have four instrument ports. The upper Coude platform is slightly smaller than the lower platform and thus will accommodate a slightly shorter instrument. The Gregorian instrument bench will be located on the op-tic support structure directly below the elevation axis, just above the Coude levels. In addition, instruments design-ers will have to meet other constraints, such as a 1.6-meter width for floor supports, and handling allowances for doors.

Agreements for instrument design development have been established with some of the co-principal investigator

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 dif-fraction-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-sphere.

Broadband filter system (Lockheed Martin). This first-light instrument will be used to commission the telescope and provide background images supporting other in-vestigations.

Visible tunable filter/polarimeter (NSO and NASA/Marshall Space Flight Center and the Kiepenheuer-In-stitut für Sonnenphysik ). Make two-dimensional vec-tor magnetograms and spectral images.

Thermal IR instrument (NASA/Goddard Space Flight Cen-ter). The GSFC solar group is developing designs for cameras and spectrographs that will exploit ATST’s capabilities in the thermal infrared.

In addition, the ATST control system will incorporate the Virtual Instrument concept. A Virtual Instrument is a col-lection of components operating concurrently. Typically the majority of a Virtual Instrument’s components will be on the same optical bench, although a Virtual Instrument may be spread across multiple benches, share a bench with other instruments, or even include off-site components. This will allow greater flexibility for adapting instruments to explore new results in a multi-bench environment.

Site selectionA telescope with as much promise and capability as

the ATST must be placed at a location that provides the best possible conditions within Earth’s atmosphere. A leading selection criterion is high altitude to place the telescope above much of the atmosphere (always the first step in reducing the blurring problem), hence the choice of mountains in each case. Other criteria include minimal cloud cover, many continuous hours of sunshine, excel-lent average seeing conditions (including many continu-ous hours of excellent seeing), good infrared transparency,

NSO/AURA/NSF

The two-level structure beneath the ATST will provide eight locations — four on each level — for large, specialized science instruments that will rotate with the telescope structure. In addition, a smaller ninth position is available at the Gregorian focus.

L x W x H1

cm (in) Location

Upper Coude2

Lower Coude

Gregorian bench

Masskg (lbs)

500 x 240 x 240(196 x 94 x 94)

3,000(6,600)

250 x 210 x 40(98 x 83 x 16)

1,000(2,200)

450 x 240 x 240(177 x 94 x 94)

3,000(6,600)

1 Equipment must pass through 2.5 m-square doors, so allowances must be made for handling gear. Also, equipment legs can be separated by no more than the 1.6 m (63 in) width of the reinforced flooring.

2 The upper Coude platform is narrower than the lower platform, thus reducing instrument length by 50 cm.

ATST maximum allowable instrumentation sizes

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16 17

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 of seeing distortions),

• A Shadow Band Array and Ranger to estimate turbu-lence several hundred meters above the site,

• A coronal photometer to determine sky brightness that would compete with the faint corona, and

• Dust and water vapor monitors to measure dust ac-cumulation on optics and humidity that may impact infrared observations.Economic and other factors also will be considered. Fi-

nal site selection is scheduled for mid- to late 2004.

Facilities and science instituteAdjacent to the ATST building will be facilities to op-

erate and maintain the ATST (including periodic mirror recoating), plus offices, computers, and housing for per-

manent staff and visiting scientists. A small visitor center will be included and sized to match access and anticipated visitors.

When design and development of the ATST are well under way, the National Solar Observatory will seek part-ners to develop and build a solar physics research institute to complement the telescope facility. Although plans are tentative, the NSO anticipates relocating its headquarters at or near a university with a world-class solar physics or astrophysics program to nurture ATST’s growth in the com-ing decades.

NSO/AURA/NSF

Each candidate ATST site is equipped with a semi-automated ob-servatory, like this one at Big Bear Lake, CA, to record seeing condi-tions over a period of at least a year.

Observatorio Roque de Los Muchachos, La Palma,

Canary Islands, Spain

Observatorio Astronomico Nacional, San Pedro Martir, Baja

California, Mexico

Panguitch Lake, Utah

NSO/Sacramento Peak, New Mexico

Mees Solar Observatory, Haleakala, Hawaii

Big Bear Solar Observatory,Big Bear Lake, California

Candidate sites for ATST

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.

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1/2 relative size

Advanced Technology Solar TelescopeCollaborating Institutions

National Solar ObservatoryHigh Altitude ObservatoryNew Jersey Institute of TechnologyUniversity of HawaiiUniversity of ChicagoUniversity of RochesterUniversity of ColoradoUniversity of California Los AngelesUniversity of California San DiegoCalifornia Institute of TechnologyPrinceton UniversityStanford UniversityMontana State UniversityMichigan State UniversityCalifornia State University NorthridgeHarvard-Smithsonian Center for AstrophysicsColorado Research AssociatesSouthwest Research InstitutionAir Force Research LaboratoryLockheed MartinNASA/Goddard Space Flight CenterNASA/Marshall Space Flight Center

ATST Team

International partnerships under development

Management Stephen Keil National Solar Observatory, Sunspot; Principal investigator Thomas Rimmele* National Solar Observatory, Sunspot; Project scientist Jim Oschmann National Solar Observatory, Tucson; Project manager

Co-principal investigators (lead) Tim Brown, David Elmore, Philip Judge, Michael Knoelker, Bruce Lites, Steve Tomczyk High Altitude Observatory Carsten Denker, Philip Goode, Haimin Wang New Jersey Institute of Technology Jacques Beckers, Fausto Cattaneo, Robert Rosner University of Chicago Roy Coulter, Jeffrey Kuhn, Haosheng Lin University of Hawaii

NSO science team K.S. Balasubramaniam, Alexei Pevtsov, Han Uitenbroek National Solar Observatory, Sunspot Mark Giamapapa, Jack Harvey, Frank Hill, Christoph Keller, Matthew Penn National Solar Observatory, Tucson

Scientific collaborators Nathan Dalrymple, Richard Radick Air Force Research Laboratory Paul Bellan California Institute of Technology Cristina Cadavid, Gary Chapman, Stephen Walton California State University, Northridge Kimberly Leka Colorado Research Adriaan van Ballegooijen, Ruth Esser, Peter Nisenson, John Raymond Harvard-Smithsonian Center for Astrophysics Hector Socas-Navarro High Altitude Observatory Manolo Collados Vera Instituto de Astrofisica de Canarias Michael Sigwarth Kiepenheuer-Institut für Sonnenphysik Thomas Berger, Theodore Tarbell, Alan Title Lockheed Martin Solar and Astrophysics Laboratory Robert Stein Michigan State University Dana Longcope Montana State University Donald Jennings NASA Goddard Space Flight Center John Davis, Allen Gary, Ron Moore 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)

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Understanding the coupling between the interior and surface. Coupling between surface and interior processes that lead to irradiance variations and the build-up of solar activity.

Understanding the coupling of the surface and the envelope: transient events. Mechanisms of coronal heating, flares, and coronal mass ejections which lead to effects on space weather and the terrestrial atmosphere.

Exploring the unknown. Fundamental plasma and magnetic field processes on the Sun in both their astrophysical and laboratory context.

The Advanced Technology Solar Telescope will incorporate the current capabilities of the Dunn and the McMath-Pierce telescopes while providing an unprecedented extension in spatial, temporal, and spectral resolution. As a necessary prelude to the ATST and as indispensable facilities for current research in solar physics, NSO will operate the McMath-Pierce and the Dunn Solar telescopes through first light at the ATST.

The mission of the National Solar Observatory is to advance our understanding of the Sun as an astronomical object and as the dominant external influence on Earth and by providing forefront ob-servational opportunities to the research community. The NSO has two principal sites located at Sunspot, NM, and Kitt Peak, AZ. The Sunspot and Kitt Peak sites represent separate origins through, re-spectively, the U.S. Air Force and the National Science Foundation. These were combined under one organization in the 1970s. Today NSO comprises world-class telescopes, the Dunn Solar Telescope at Sunspot and the McMath-Pierce Solar Telescope at Kitt Peak, and the Global Oscillation Network Group (GONG).

NSO’s mission includes the operation of cutting edge facili-ties, the continued development of advanced instrumentation both in-house and through partnerships, conducting solar research, and educational and public outreach.

NSO accomplishes this mission through:• Leadership for the development of new ground-based facilities

that support the scientific objectives of the solar and solar-ter-restrial 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 gradu-ate students, helping develop classroom activities, working with teachers, and mentoring high school students; and

• Innovative staff research.

The broad research goals of NSO include:Understanding the mechanisms generating solar cycles. Mecha-

nisms driving the surface and interior dynamo and the creation and destruction of magnetic fields on both global and local scales.

National Solar Observatory

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).

NSO/AURA/NSF

NSO also sponsors and manages the Global Oscillation Network Group (GONG) which studies the internal structure and dynamics of the Sun by helioseismology — the measurement of acoustic waves that penetrate the solar interior — by using six stations around the world, such as the one atop Tiede, Canary Islands (left). In addition, NSO is commissioning the Solar Optical Long-term Investigations of the Sun (SOLIS) facility (right) that will operate atop Kitt Peak and eventually be co-located with ATST.

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Inside back coverleft blank

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National Solar ObservatorySunspot, NM 88349-0062http://atst.nso.edu/

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