Solar telescope

The Swedish 1-Meter Solar Telescope on the Canary Island of La Palma.
Monika Rüger, Munich

Tracking the Sun

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

As the central star of our solar system the sun plays a special role in astronomy. No other star is as close to the planet Earth as the sun. No other celestial body appears to us so bright and so hot. Special solar telescopes have to be constructed to observe the sun, and they are fundamentally different from other telescopes. At present, the most powerful is the Swedish 1-m Solar Telescope (SST) located on La Palma, one of the Canary Islands. The Institute for Solar Physics of the Royal Swedish Academy of Sciences has been operating it since 2002. With this telescope it was possible for the first time ever to detect structures of just 75 kilometers in size on the sun’s surface.

Sunspots with fascinating details

Sunspots represent darker, cooler areas on the sun’s surface. They occur primarily in times of increased solar activity and can grow to diameters as large as planets. The center of a sunspot, the umbra, is surrounded by a brighter outer part called the penumbra. This penumbra consists of long filaments that are wrapped around the center like hair. With the help of the Swedish Solar Telescope, astronomers discovered for the first time that many of these filaments have a dark inner core. According to Professor Göran Scharmer, Director of the Swedish Institute for Solar Physics, these cores are between 150 and 180 kilometers wide and possibly originate from magnetic flux tubes that transport hot plasma. However, scientists are still unclear about their exact explanation.

Mountains several hundred kilometers high

The sun, the object of thousands of years of observation and research, still holds many secrets.
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.
Using an oblique angle, spectacular images have also been obtained of the areas at the solar limb. Due to the large observation angle to the sun’s surface, objects can be viewed there from a different perspective. The images show for the first time ever three- dimensional structures of the sun’s surface: for example, mountains and valleys of hot plasma with differences in height of up to 450 kilometers. The sunspots appear as dark cavities surrounded by mountains. Scientists used to see them only as darker areas that could not be differentiated as flat, deep or high. Furthermore, many bright spots called faculae, which extend above the sun’s surface, can also be observed. These faculae are hotter than their surroundings and release an especially large amount of energy.

Special features of solar telescope constructions

SCHOTT manufactured two flat adaptive mirrors with a diameter of 1.4 meters from “ZERODUR®” glass ceramic for the main optical system of the solar telescope.
The many new discoveries are even more surprising considering that the Swedish Solar Telescope is not even the largest. However, it has a highly advanced design with adaptive optics and thus produces sharper and more detailed images than all other solar telescopes.

With adaptive optics the flickering of the image caused by differences in density of the Earth’s atmosphere is corrected by a selective deformation of a mirror. This principle is widely established in large modern night-time telescopes, but not in solar telescopes. The reason for this is that no other stars can be used as reference points with a solar telescope. Instead one has to make use of fine structures on the sun’s surface, and this requires far more computation capacity. The Swedish Solar Telescope is the first one to be designed from the beginning with adaptive optics. Its adaptive mirror corrects the image 1,000 times per second.

Another fundamental difference between night-time telescopes and solar telescopes is the fact that the latter suffer not only under the influence of the Earth’s atmosphere, but also under the sun’s extreme heat. Both the optical system and the air inside the telescope heat up. For the optical element it is possible to solve this problem by choosing materials that have a very low thermal expansion, for example the lenses are made from fused silica and the mirrors from “ZERODUR®” glass ceramic. The heated up air that causes the image to become blurred – a phenomenon that can be seen above hot asphalt – can be avoided by evacuating the telescope, thus the name vacuum telescope. In the case of the Swedish Solar Telescope, this problem is solved in a particularly ingenious way. Instead of the usual flat vacuum window, a 1-meter lens made from fused silica takes care of this function as well, thus eliminating an optical component. Together with two 1.4-meter flat deflecting mirrors made from “ZERODUR®” glass ceramic, the 1-meter lens forms the main optical system. It is movable as a whole in order to track the sun during its orbit. Like an arm it extends 17 meters above the tower, which contains the other optical components and the detectors.

Future projects

At present the Swedish Solar Telescope is the largest in Europe and the second largest in the world after the McMath-Pierce Telescope in Arizona. However, three new projects are planned or are already under construction. On the island of Tenerife the 1.5-meter solar telescope GREGOR of the Kiepenheuer Institute for Solar Physics (KIS) in Freiburg, Germany, will start operations in 2004. A solar telescope with a 1.7-meter mirror made from “ZERODUR®” is set to be built at the Big Bear Solar Observatory (BBSO) in California by the year 2006. SCHOTT has already supplied the mirror blank for this project. And a 4-meter telescope (ATST = Advanced Technology Solar Telescope), which will probably be the basis for the next generation of solar telescopes, is currently under consideration. Due to their size, none of these future observatories can be constructed as a vacuum telescope. Their design will instead require open mirror systems with a special cooling method.