The Science of ATST - National Solar Observatory

Advanced 

Technology 

Solar 

Telescope 

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 e

Inside front cover 

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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 

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). 

1 

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. 

3 

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 sunspots 

and other changing phenomena that can affect life on Earth. 

4 

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. 

5 

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. 

8 

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. 

10 

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. 

11 

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. 

13 

The ATST Team: ATST is being developed by a consortium led by the NSO and comprising the University of Chicago, 

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. 

17 

1/2 relative size 

Michigan State University 

AURA 

Advanced Technology Solar Telescope 

Collaborating Institutions 

National Solar Observatory 

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 

Princeton University 

Stanford University 

Montana State University 

California State University Northridge 

Harvard-Smithsonian Center for Astrophysics 

Colorado Research Associates 

Southwest Research Institution 

Air Force Research Laboratory 

Lockheed Martin 

NASA/Goddard Space Flight Center 

NASA/Marshall Space Flight Center 

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. 


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.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 

18 

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 Sunspot, 

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.

Cross-section 

of outer solar atmosphere 

Relative size of Earth 

12,723 km (7,908 mi) diameter 

Corona 

Millions of kilometers into space 

> 1 million K (1.8 million ˚F) 

Transition region 

~50-100 km (32-62 mi) deep, 

rapid heating 

Corona 

Anatomy of the Sun 

Transition region, 

Chromosphere, { Photosphere 

Core 

Flare 

Coronal 

mass 

ejection 

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, chromosphere, 

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. 

NASA/Marshall Space Flight Center 

Extended magnetic fi eld lines arcing 

through the chromosphere and into the 

corona can reconnect, causing an explosive 

fl are and/or releasing a coronal 

mass ejection (CME) into deep space. 

Before onset 

Confined eruption, ending 

Eruption onset 

Ejective eruption, middle 

Radiative zone 

Convective zone 

NSO/AURA/NSF 

NASA/Transition Region and Coronal Explorer (1), 

Bart De Pontieu, Swedish Vacuum Telescope (3-4) 

1 

2 

3 

4 

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. Coronal 

events are closely linked to magnetic activities 

in the lower atmosphere. 

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. 

Chromosphere 

~2,000-3,000 km (~1,243-1,864 mi) deep 

~6,000-10,000 K (~10,340-17,540 ˚F) 

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) 

NASA/ESA SOHO Consortium 

NSO/AURA/NSF 

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). 

Heights and temperatures are approximate and variable; colors, shapes 

and positions are illustrative.

The Need 

Since George Ellery Hale’s 1908 discovery that sunspots 

coincide with strong magnetic fields, we have become 

increasingly aware of the Sun’s magnetic fields as an complex 

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 science 

and technology, scientists have developed intriguing 

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 distant 

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 better 

measurements of magnetic fields and thermodynamics, 

and 

• Temporal — high cadence (frequent) images and spectra 

of rapidly developing events. 

Further, we must observe not just in familiar near-ultraviolet 

and visible light, but exploit the relatively unexplored 

infrared solar spectrum. We must see the faint solar 

corona in infrared, measure the polarization of sunlight 

with greater precision, and cover a large field 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 understanding 

the Sun requires that it be treated as a global 

system, in which critical processes occur on all scales, from 

the very small (

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 

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 

ATST specifications 

Aperture 4 m (13.1 ft) 

Configuration Gregorian, off-axis 

Mount Altitude-Azimuth (Alt-Az) 

Enclosure Ventilated corotating hybrid dome 

Field 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, 

0.3 @ = 500 nm 

Spectral 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 infrared 

Scattered light

Simulation 

0.”03 resolution 

(@550 nm, w/AO) 

0.1 

1” resolution 

(@550 nm, 

uncorrected) 

1 

ATST 

(2010) 

Scale (arc-seconds) 

10 

100 

300” field of view 

SDO 

(2005) 

(2004) 

STEREO 

SOLIS (2004) 

FASR: Frequency-Agile Solar 

Radiotelescope 

SDO: Solar Dynamics Observatory 

SOLIS: Synoptic Optical Long-term 

Investigations of the Sun 

STEREO: Solar-Terrestrial Relations 

Observatory 

FASR 

(2010) 

1000 

1,920” disk 

1920 

EUV UV VIS NIR FIR Thermal Radio 

nm 

10 eV 

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 arcseconds 

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 arcsecond 

or better). 

Getting the fine structures to see the big picture 

m 

1 THz 

mm 

1 GHz 

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 vortex 

flows, dissipation of magnetic fields, and magnetohydrodynamic 

(MHD) waves. Resolving these scales is of utmost importance 

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 terrestrial 

climate. Solar radiation received at Earth increases with solar 

activity (see page 5). Since the smallest magnetic elements contribute 

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 telescopes 

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 netuning 

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 cannot 

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. 

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

3

The Science 

Hale’s discovery in 1908 that magnetic fields permeate 

sunspots started a revolution that turned solar 

science into a field 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 flows are influenced by magnetic fields. 

Sunspots, it turns out, are the best-known manifestations 

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 flow of 

energy from the core into space. The outer region of the 

blanket is the convective zone where giant gas cells circulate 

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 fields that stretch 

and shear, disconnect and reconnect. 

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 fields loop out 

of the convective zone, into space and back to form an array 

of features, including sunspots, plages, filaments, and 

prominences. Magnetic fields 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 fields pierce the 

visible surface they often form darkened areas — sunspots 

— where the magnetic field 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 better-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 understood. 

Historical evidence indicates that changes in the 

(Earth’s own magnetic field has a strength of about 0.5 

gauss. A simple bar magnet has a field of about 3,000 gauss, 

but it drops sharply with distance, and is almost undetectable 

a few feet away. The magnetic fields 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 physical 

parameters, such as magnetic fi eld strength and direction, temperature 

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. 

Robert F. Stein, Michigan State University 

A series of images from a computational fl uid dynamics model depicts 

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. 

4

Sun and climate 

We think of the Sun as constant, but physical and historical records 

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 reversing 

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 coincidence. 

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 

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 

Sunspot Number 

200 

100 

Maunder Minimum 

1645-1715 

Little Ice Age (~1500-1850) 

Dalton 

Minumum 

1795-1820 

Pieter Bruegel the Elder, Kunsthistorisches Museum Wien, Vienna 

0 

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 

Greenland was green and colonized by the Vikings, as compared to 

today’s glacial island. 

(Climate is a general description of long-term environmental conditions 

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 terrestrial 

climate because neither is fully understood. Earth’s climate system 

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 

Solar Irradiance (W/m2) 

1369 

1368 

1367 

1366 

1365 

1364 

Total solar irradiance 

Annual sunspot number 

Cycle 21 Cycle 22 Cycle 23 

1363 

78 80 85 90 95 00 03 

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

150 

100 

by splitting oxygen molecules (O 2 ) into free atoms that recombine 

to form ozone (O 3 ). 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 sunspot 

numbers. We also know that geomagnetic storms, driven by 

solar activities, provide an additional, variable energy input through 

the polar caps. 

50 

0 

Sunspot (Wolf) number 

5

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 observe 

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 emergence 

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 motions 

and waves and determine how energy is transferred 

from turbulent gas motions to the magnetic field. 

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. 

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, advanced 

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 happening 

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. 

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 impact 

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 tubes 

Observations 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 interior 

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 laboratory 

Combining 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 dynamic 

magnetic phenomena observed on the Sun. Such tests offer 

insights into magnetic helicity and topology, triggering of instabilities, 

magnetic reconnection physics, conversion of magnetic energy 

into particle energy, importance of magnetic boundary conditions, 

and interactions between adjacent magnetic structures. 

6

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. 

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 enigmatic 

flux tube structures. 

Magnetic field generation and local dynamos 

To understand solar activity and solar variability, we 

need to understand how magnetic fields are generated and 

how they are destroyed. The 11-year sunspot cycle and the 

corresponding 22-year magnetic cycle are still shrouded 

in mystery. Global dynamo models that attempt to explain 

large-scale solar magnetic fields are based on 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 

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 

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. 

strong fields and weak fields interact? How are both generated? 

How do they disappear? Does the weak-field component 

have global importance and what is its significance for 

the solar cycle? ATST will measure distribution functions 

of field strength, field direction and flux tube sizes for comparison 

with theoretical models, and will observe plasma 

motions and relate them to the flux tube dynamics. 

Magnetic fields and mass flows 

The total magnetic field of sunspots is large enough 

to dominate the hydrodynamic behavior of the local gas, 

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. 

7

"Black" light 

~400-345 

Max. eye 

sensitivity 

~550 

Red laser 

pointer 

~650 

Incandescent 

lamp peak 

~999 

ATST and the many colors of the Sun 

Radiant heat 

300 

400 

~380 

500 

600 

700 

~770 

800 

900 

1,000 

(nm) 

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 

entire optical spectrum passed by Earth’s atmosphere. Within this are 

tens of thousands of emission and absorption lines, each telling a different 

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 yellow 

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.) 

Continuum H core H wing 629.4 629.6 629.8 630.0 

nm 

Sample spectral lines of interest 

630.2 630.4 630.6 

K.S. Balasubramaniam, NSO/AURA/NSF 

393.3682 & 396.8492, Calcium II K & H: Deep 

violet (right) emitted and absorbed by calcium 

ions in the chromosphere. Useful for monitoring 

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 D 1 , D 2 : Orange-yellow double line emitted 

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. 

1074.6 & 1079.8, Iron (Fe XIII): Yields highly sensitive measurements 

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 because 

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). Observations 

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. 

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

A yardstick for light 

For 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 preference 

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). 

8

a regime very different from that of the rest of the solar 

photosphere. Earth’s magnetic field 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 field. 

Numerical simulations and theoretical models predict dynamic 

phenomena, such as oscillatory convection in the 

strong-field regions of sunspot umbrae, flows that move at 

the speed of sound along penumbral filaments, and oscillations 

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-resolution 

(better than 0.1 arc-seconds) vector polarimetry combined 

with high sensitivity and low-scattering optics are 

required. Understanding the interaction of magnetic flux 

and mass flows is crucial for our understanding of the behavior 

of magnetic fields on scales ranging from planetary 

magnetospheres to clusters of galaxies. Sunspots allow us 

to test those theories in a regime where magnetic fields 

dominate mass flows. 

spectra near 4,700 nm show surprisingly cool clouds that 

appear to occupy much of the lower chromosphere. Apparently 

only a small fraction of the volume is filled with 

hot gas, contrary to classical static models that exhibit a 

sharp temperature rise in those layers. The observed spectra 

can be explained by a new class of dynamic models of 

the solar atmosphere. However, the numerical simulations 

indicate that the temperature structures occur on spatial 

scales that cannot be resolved with current solar infrared 

telescopes. A test of the recent models requires a large-aperture 

solar telescope that provides access to the thermal 

infrared. 

Another mystery comes from spicules, forests of hot 

jets that penetrate from the photosphere through the chromosphere 

and then fade in a few minutes. These are believed 

to be an MHD phenomenon that is not understood 

nor adequately modeled. Combined with extreme ultraviolet 

observations from space, ATST will allow us to resolve 

their structure and dynamics and understand their nature 

and roles. 

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 

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 interior. 

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. 

Magnetic fields 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 fields 

rooted at the 0.1-arc-second scale in the photosphere. However, 

neither observation nor theory has clearly identified 

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 forcing 

of the coronal fields by photospheric motions, but also 

to measure the coronal magnetic field strength itself. This is 

important for developing and testing models of flares and 

coronal mass ejections, which propel magnetic field and 

plasma into interplanetary space and induce geomagnetic 

disturbances. In particular, precise measurements of the 

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 characteristics, 

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 

endanger astronauts and satellites. Coronal mass ejections 

(CMEs) produce interplanetary blast waves that also accelerate 

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 satellites, 

endanger the health of air crews crossing the polar 

cap and astronauts in orbit, and have other impacts on human 

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 magnetic 

instabilities, and the origins of atmospheric heating 

events. These models will provide a much better capability 

for understanding and predicting when and where activity 

will occur on the Sun and for predicting the level of enhanced 

emissions expected from these events. These in turn 

will let us refine solar-terrestrial models to include critical 

solar data instead of the proxies that are used now. 

K.Strassmeier, Vienna, NOAO/AURA/NSF 

Looking beyond the Sun 

Solar 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 Universe. 

Detailed observations of flux tubes are crucial for our understanding 

not only of the activity and heating of the outer atmospheres 

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 astrophysical 

systems but can be observed only on the Sun. 

Solar flares serve as a prototype for many high-energy phenomena 

and particle acceleration mechanisms in the Universe. 

The cause of the Sun’s million-degree corona must be understood 

before we can confidently interpret EUV and X-ray emissions from 

distant stars. Our scientific understanding of these solar phenomena 

is limited by our inability to resolve them at spatial scales less 

than a few tenths of an arc-second. The study of solar plasmamagnetic 

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. 

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. 

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. 

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 measurements 

to planetary astronomy. 

10

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 Institute 

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 defined 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, spatial, 

and temporal resolution. It also will serve as the focal 

point for systematic campaigns involving other groundbased 

and orbiting observatories. ATST will become a major 

component of the solar facilities operated by the NSO 

and available to U.S. and international solar astronomers. 

Telescope design 

The need to provide solar observers with greatly improved 

images, spectra, and data in turn leads to demanding 

observing requirements that drive ATST’s design. These 

include: 

• A planned 300x300 arc-seconds field 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 

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 

11

Secondary 

mirror 

assembly 

Secondary mirror 

(70 cm [27.6 in.])) 

Baffle 

Field 

stop 

Light from Sun 

Primary mirror 

assembly 

Heat stop 

assembly 

System 

interconnects 

Pier and 

Coude 

Feed optics 

Feed optics 

Deformable 

mirror 

Azimuth cable wrap 

Mark Warner, NSO/AURA/NSF 

To Coude labs 

(instruments) 

Primary mirror 

(4 meters [13.1 ft]) 

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 compensate 

for atmospheric blurring. 

• Temporal resolution to reveal events spanning a few 

seconds (e.g., fast movies rather than long time exposures). 

• Spectral resolution to enable precision, sensitive measurements 

of magnetic activities in the solar atmosphere 

from near ultraviolet into the thermal infrared. 

• Sufficient light flux and control of scattered light to reveal 

faint structures and activities in the corona. 

• Sufficient light flux to allow high-resolution polarimetry, 

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 

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-meter 

(13.1-ft) primary mirror. This design differs from familiar 

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 instruments. 

Most nighttime telescopes have similar arrangements. 

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 

12

Basic concept for Shack-Hartmann 

Adaptive Optics System 

Deformable mirror 

Uncorrected 

light 

Actuators 

Correlation & 

Reconstruction 

Digital Signal 

Processors 

Beam 

splitter 

Corrected 

light 

CCD camera 

Shack-Hartmann 

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) 

technology that allows the telescope to operate near its diffraction 

limit, the theoretical best that a telescope can deliver. Diffractionlimited 

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 “seeing 

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 dimmer 

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 measure 

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 sophisticated 

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 development 

since the 1990s. 

In the Shack-Hartmann AO design, small lenses divide the 

incoming image into multiple images, each using a different subaperture 

of the image of a sunspot or other feature. Each subap- 

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 mirror 

by as little as a few wavelengths of light — or even fractions of 

wavelengths — in response to varying electric signals. A tilt/tip mirror 

in the light path recenters the image in case of major distortions. 

The result is a striking stabilization of the image and removal of blurring 

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. 

13

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

Start 

program 

Scientific and Technical Advisory Groups 

Concept & design CoDR PDR CDR AO Adaptive Optics 

CDR Critical Design Review 

Site surveys Select finalists 

Final selection 

CoDR Concept Design Review 

Technology development 

PDR Preliminary Design Review 

High-order AO development 

Note: Schedule subject to change 

AO76 demonstration 

Mirror production 

Construction 

Integration 

First light 

Schedule extension if 

mirror is procured at 

start of construction 

Integration 

Operations 

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 

from the area being studied to pass to the secondary mirror. 

To keep ATST’s optical path clear, the secondary mirror 

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 supports. 

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. 

designs. The slim contours of the dome were selected to 

minimize dome heating by sunlight while allowing controlled 

flow of ambient air for cooling. It will also allow a 

full range of pointing angles for ATST operations. 

Science instruments 

No 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 approach, 

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. 

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. 

U 

V 

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 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 

on-site fine-tuning, an important consideration). Alternatively, 

scientists can use complete instruments or instrument 

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 

I 

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 image, 

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. 

Q 

14

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. 

ATST maximum allowable instrumentation sizes 

Location 

Upper Coude 2 

Lower Coude 

Gregorian bench 

L x W x H 1 

cm (in) 

500 x 240 x 240 

(196 x 94 x 94) 

450 x 240 x 240 

(177 x 94 x 94) 

250 x 210 x 40 

(98 x 83 x 16) 

Mass 

kg (lbs) 

3,000 

(6,600) 

3,000 

(6,600) 

1,000 

(2,200) 

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. 

The ATST will support up to nine instruments in three 

areas, the upper and lower Coude platforms, and the Gregorian 

focus. The Coude platforms will form the lower levels 

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 optic 

support structure directly below the elevation axis, just 

above the Coude levels. In addition, instruments designers 

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 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 atmosphere. 

Broadband filter system (Lockheed Martin). This first-light 

instrument will be used to commission the telescope 

and provide background images supporting other investigations. 

Visible tunable filter/polarimeter (NSO and NASA/ 

Marshall Space Flight Center and the Kiepenheuer-Institut 

für Sonnenphysik ). Make two-dimensional vector 

magnetograms and spectral images. 

Thermal IR instrument (NASA/Goddard Space Flight Center). 

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 collection 

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 selection 

A 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, excellent 

average seeing conditions (including many continuous 

hours of excellent seeing), good infrared transparency, 

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, Canary 

Islands, Spain, and 

• Panguitch Lake, Utah. 

Sites are evaluated with identical sets of instruments, 

including: 

• Solar Dual Image Motion Monitor to measure differential 

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 turbulence 

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 accumulation 

on optics and humidity that may impact 

infrared observations. 

Economic and other factors also will be considered. Final 

site selection is scheduled for mid- to late 2004. 

NSO/AURA/NSF 

Each candidate ATST site is equipped with a semi-automated observatory, 

like this one at Big Bear Lake, CA, to record seeing conditions 

over a period of at least a year. 

Facilities and science institute 

Adjacent to the ATST building will be facilities to operate 

and maintain the ATST (including periodic mirror 

recoating), plus offices, computers, and housing for permanent 

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 partners 

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 coming 

decades. 

 

Candidate sites for ATST 

Panguitch Lake, Utah 

Big Bear Solar Observatory, 

Big Bear Lake, California 

Mees Solar Observatory, 

Haleakala, Hawaii 

Observatorio Astronomico 

Nacional, San Pedro Martir, Baja 

California, Mexico 

NSO/Sacramento Peak, New 

Mexico 

Observatorio Roque de Los 

Muchachos, La Palma, 

Canary Islands, Spain 

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

ATST Team 

International partnerships 

under development 

1/2 relative size 

Management 

Stephen Keil National Solar Observatory, Sunspot; Principal investigator 

Advanced Thomas Technology Rimmele* National Solar Telescope 

Observatory, Sunspot; Project scientist 

Collaborating Jim Institutions Oschmann National Solar Observatory, Tucson; Project manager 

Co-principal investigators (lead) 


Tim Brown, David Elmore, Philip Judge, 

Michael Knoelker, 

High 

Bruce 

Altitude 

Lites, Steve 

Observatory 

Tomczyk High Altitude Observatory 

Carsten Denker, New Philip Jersey Goode, Institute Haimin Wang of Technology New Jersey Institute of Technology 

Jacques Beckers, Fausto University Cattaneo, of Hawaii Robert Rosner University of Chicago 

Roy Coulter, University Jeffrey Kuhn, of Chicago Haosheng Lin University of Hawaii 

University of Rochester 

University of Colorado NSO science team 

K.S. Balasubramaniam, Alexei Pevtsov, Han Uitenbroek National Solar Observatory, Sunspot 

University of California Los Angeles 

Mark Giamapapa, Jack Harvey, Frank Hill, Christoph Keller, Matthew Penn National Solar Observatory, Tucson 

University of California San Diego 

California Institute Scientific of Technology collaborators 

Nathan Princeton Dalrymple, University Richard Radick Air Force Research Laboratory 

Stanford University Paul Bellan California Institute of Technology 

Cristina Cadavid, Gary Montana Chapman, State Stephen University Walton California State University, Northridge 

Michigan State Kimberly University Leka Colorado Research 

Adriaan van Ballegooijen, Ruth Esser, Peter 

California 

Nisenson, 

State 

John 

University 

Raymond 

Northridge 


Hector Socas-Navarro High Altitude Observatory 


Manolo Collados Vera Instituto de Astrofisica de Canarias 

Colorado Research Michael Sigwarth Associates Kiepenheuer-Institut für Sonnenphysik 

Thomas Berger, Southwest Theodore Research Tarbell, Alan Institution Title Lockheed Martin Solar and Astrophysics Laboratory 

Air Force Research Robert Laboratory Stein Michigan State University 

Lockheed Martin Dana Longcope Montana State University 

NASA/Goddard Donald Space Jennings Flight NASA Center Goddard Space Flight Center 

John NASA/Marshall Davis, Allen Gary, Space Ron Moore Flight NASA Center 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) 

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). 


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 observational 

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, respectively, 

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 facilities, 

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-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 

• Innovative staff research. 

The broad research goals of NSO include: 

Understanding the mechanisms generating solar cycles. Mechanisms 

driving the surface and interior dynamo and the creation 

and destruction of magnetic fields on both global and local 

scales. 

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. 

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. 

18

Inside back cover 

left blank 

19


Sunspot, NM 88349-0062 

http://atst.nso.edu/ 

1/04

The Science of ATST - National Solar Observatory

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