penumbra. This penumbra consists of longfilaments that are wrapped around the cen-ter like hair. With the help of the SwedishSolar Telescope, astronomers discovered forthe first time that many of these filamentshave a dark inner core. According to Profes-sor Göran Scharmer, Director of theSwedish Institute for Solar Physics, thesecores are between 150 and 180 kilometerswide and possibly originate from magneticflux tubes that transport hot plasma. How-ever, scientists are still unclear about theirexact explanation.

Mountains several hundredkilometers high

Using an oblique angle, spectacular imageshave also been obtained of the areas at thesolar limb. Due to the large observation an-gle to the sun’s surface, objects can beviewed there from a different perspective.

The images show for thefirst time ever three-dimensional structures ofthe sun’s surface: for ex-ample, mountains andvalleys of hot plasmawith differences inheight of up to 450 kilo-meters. The sunspots ap-pear as dark cavities sur-rounded by mountains.Scientists used to seethem only as darker ar-eas that could not bedifferentiated as flat,deep or high. Further-more, many bright spotscalled faculae, which ex-tend above the sun’s sur-

face, can also be observed. These faculaeare hotter than their surroundings and re-lease an especially large amount of energy.

Special features of solar telescope constructions

The many new discoveries are even moresurprising considering that the Swedish So-lar Telescope is not even the largest. Howev-er, it has a highly advanced design withadaptive optics and thus produces sharperand more detailed images than all other so-lar telescopes.

With adaptive optics the flickering of theimage caused by differences in density ofthe Earth’s atmosphere is corrected by a se-lective deformation of a mirror. This princi-ple is widely established in large modernnight-time telescopes, but not in solar tele-scopes. The reason for this is that no otherstars can be used as reference points with asolar telescope. Instead one has to make useof fine structures on the sun’s surface, andthis requires far more computation capacity.The Swedish Solar Telescope is the first oneto be designed from the beginning withadaptive optics. Its adaptive mirror correctsthe image 1,000 times per second.

Another fundamental difference betweennight-time telescopes and solar telescopes isthe fact that the latter suffer not only underthe influence of the Earth’s atmosphere, butalso under the sun’s extreme heat. Both theoptical system and the air inside the tele-

Monika Rüger

Munich

With the help of a new solar telescopeastronomers have discovered properties of the sun’s surface that have been unknown up to now.

Tracking the Sun

�As the central star of our solar systemthe sun plays a special role in astronomy.No other star is as close to the planet Earthas the sun. No other celestial body appearsto us so bright and so hot. Special solar tele-scopes have to be constructed to observethe sun, and they are fundamentally differ-ent from other telescopes. At present, themost powerful is the Swedish 1-m SolarTelescope (SST) located on La Palma, one ofthe Canary Islands. The Institute for SolarPhysics of the Royal Swedish Academy ofSciences has been operating it since 2002.With this telescope it was possible for thefirst time ever to detect structures of just 75kilometers in size on the sun’s surface.

Sunspots with fascinating details

Sunspots represent darker, cooler areas onthe sun’s surface. They occur primarily intimes of increased solar activity and cangrow to diameters as large as planets. Thecenter of a sunspot, the umbra, is surround-ed by a brighter outer part called the

The sun, the object of thousands ofyears of observation and research,still holds many secrets.

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The source of all life

The sun is our nearest star. Its distance to Earth is some 150 million kilome-ters. Although it is considered the source of all terrestrial life, our sun is justaverage in size compared with the billions of other stars in the universe. Stars like the sun are giant balls of gas in a hydrostatic equilibrium, whichmeans that their inward-directed gravitational forces are balanced by theoutward-directed gas and radiation pressure. The temperature and pressureare so high at the center that nuclear fusion occurs, in which hydrogenatoms are fused to helium. The energy released in the process is transportedto the sun’s surface and radiated into space. Sunspots can hinder this energy transmission because of their strong mag-netic fields. This is why they are cooler and darker than their surroundings.The frequency of sunspots and thus also the sun’s activity fluctuate overabout an 11-year cycle.

Facts about the sunAge: 4.6 billion yearsDiameter: 1,392,000 kmMass: 2 x 1030 kg

Temperature:at the surface: 5,750 Kin sunspots: 4,000 Kat the center: 15 million K

Chemical composition:of the surface: 73 % hydrogen,25 % helium, 2 % other elementsat the center: 35 % hydrogen, 63 % helium, 2 % other elements

The Swedish 1-Meter SolarTelescope on the CanaryIsland of La Palma.

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scope heat up. For the optical element it ispossible to solve this problem by choosingmaterials that have a very low thermal ex-pansion, for example the lenses are madefrom fused silica and the mirrors from “Ze-rodur” glass ceramic. The heated up air thatcauses the image to become blurred – aphenomenon that can be seen above hotasphalt – can be avoided by evacuating thetelescope, thus the name vacuum telescope.

In the case of the Swedish Solar Telescope,this problem is solved in a particularly inge-nious way. Instead of the usual flat vacuumwindow, a 1-meter lens made from fused sil-ica takes care of this function as well, thuseliminating an optical component. Togetherwith two 1.4-meter flat deflecting mirrorsmade from “Zerodur” glass ceramic, the 1-meter lens forms the main optical system. Itis movable as a whole in order to track the

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One task of the new telescope is to examine the origin of the dark cores that occur in some of the thin filaments surrounding the centers of the sunspots.

SCHOTT manufactured two flatadaptive mirrors with a diameter of 1.4 meters from “Zerodur” glassceramic for the main optical system of the solar telescope.

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The Royal Swedish Academy of Sciences isprobably best known throughout the world forthe Nobel Prize, but it also operates sevenscientific institutes, including one for solarphysics. The Director of the Institute, ProfessorGöran Scharmer, created the optical and mechan-ical design of the Swedish Solar Telescope andplayed a major role in the development of theadaptive optics of the telescope.

Professor Scharmer, why is Sweden operating a solar telescope onLa Palma?

Prof. Scharmer: It is a long story. Sweden already had a solar tele-scope on Capri, Italy, in the 1950s. Around 1970 Swedish scientists to-gether with British and Spanish astronomers began to look for a loca-tion with better conditions. They decided on La Palma, which is clearlyone of the best sites for solar telescopes in the world. The Swedish Ob-servatory thus moved from Capri to La Palma. A 50-centimeter tele-scope, the predecessor of the 1-meter telescope, was built there in the1980s. Plans for the new 1-meter telescope began in 1995, as it be-came clear that adaptive optics for a solar telescope could be realized.

The Swedish 1-meter telescope currently supplies the best imagesin the world. Who is allowed to use it for research purposes?

Prof. Scharmer: Since it is a Swedish telescope, it is obviously main-ly used by astronomers of the Swedish Institute for Solar Physics. How-ever, we have two partners: the Institute for Astrophysics in Oslo, Nor-way and the U.S. company Lockheed Martin. Each shared 10 percent ofthe costs and is therefore allowed 10 percent of the observation time.There was also a grant from the European Union so that a small part ofthe observation time has been dedicated to other European astrono-mers.

What investigations are you planning next?

Prof. Scharmer: We plan to further investigate the magnetic field ofthe sun, and particularly the sunspots. There are still many unansweredquestions here. For example, what exactly is the explanation for thedark cores in the filaments of sunspots?

Can you make conclusions about the effects on theEarth’s climate from your results?

Prof. Scharmer: No, not really up to now. But this maybe possible in 10 to 20 years.�

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SST: Sights set on sunspots

sun during its orbit. Like an arm it extends 17meters above the tower, which contains theother optical components and the detectors.

Future projects

At present the Swedish Solar Telescope is thelargest in Europe and the second largest in theworld after the McMath-Pierce Telescope in Ari-zona. However, three new projects are plannedor are already under construction. On the islandof Tenerife the 1.5-meter solar telescope GRE-GOR of the Kiepenheuer Institute for SolarPhysics (KIS) in Freiburg, Germany, will start op-erations in 2004. A solar telescope with a 1.7-meter mirror made from “Zerodur” is set to bebuilt at the Big Bear Solar Observatory (BBSO)in California by the year 2006. SCHOTT has al-ready supplied the mirror blank for this project.And a 4-meter telescope (ATST = AdvancedTechnology Solar Telescope), which will proba-bly be the basis for the next generation of solartelescopes, is currently under consideration.Due to their size, none of these future observa-tories can be constructed as a vacuum tele-scope. Their design will instead require openmirror systems with a special cooling method.�

www.solarphysics.kva.sewww.schott.com/optics_devices

The Swedish solarresearcher Professor GöranSchwarmer photographedon the platform of the 17-meter telescope tower.

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