Solar telescope

In this H-Alpha image from the Big Bear Solar Observatory of New Jersey Institute of Tech- nology, details of the surface of the sun, such as turbulences, currents and spots, can be seen. (Photo: Big Bear Solar Observatory)
Gerhard Samulat

Watchful Eye on the Sun

Researchers at the »Big Bear Solar Observatory« (bbso) in California are looking to obtain new insights on the activities of our most important heavenly body.

The sun means light, warmth and life. It dictates our rhythm each day and night, during summer and winter. For as long as mankind can remember, scholars have been following the path of the fireball in the firmament. Using more and more sophisticated instruments, researchers all over the world are trying to unravel the sun’s remaining secrets, both with satellites and using telescopes here on earth. Big Bear Solar Observatory (BBSO) not only owns the newest solar observation device, with a 1.7 meter Zerodur® mirror substrate from SCHOTT, this is also the world’s largest. The observatory is located on a man-made tongue of land in California that stretches into a lake 2,000 meters high in the San Bernardino Mountains, east of Los Angeles.
The ideal location for the high-tech observatory: Big Bear Lake in the San Bernardino Mountains. (Photo: Big Bear Solar Observatory/M. Vincent)
From here, astronomers enjoy a view of the seething mass in outer space, located roughly 150 million kilometers away from the earth, with all of its swirls, currents and spots. Some are so large that they would take up our entire planet. And they often cause magnetic storms that can even reach earth. This is when they make the sky glow over the poles. But, severe attacks can also cause damage to satellites, for example, interrupt communication links or take down power lines. To avoid being surprised by unforeseen occurrences like these, astronomers at the BBSO are now keeping a close eye on the sun with their reflector telescope. Technologically speaking, this is a difficult task. Whereas observing the sky at night usually means having to capture every slight beam of light, solar telescopes block out most of the light or focus mainly on select wavelengths within the visible to infrared spectral range, which reduces the heat load on the downstream instrumentation. However, the primary mirror must take the brunt of the sunlight. There is a heat stop at the focus of the primary mirror, which passes light from a small part of the sun and rejects the rest. This makes the heat load tolerable on the next few small mirrors. The rejected heat at the focal point of the primary mirror can get even hotter than an electric iron. This is why powerful ventilators constantly cool off the instruments inside the observatory.
The primary mirror for the new solar telescope was polished at the University of Arizona’s Steward Observatory. The meniscus has a diameter of 1.7 meters and is 100 millimeters thick. (Photo: Steward Observatory)
Despite the enormous thermal stress, the primary mirror is not permitted to move even a thousandth of a millimeter on its own. Any unevenness would detract from the clarity of the images. This is why the operators decided to rely on Zerodur® glass ceramic from SCHOTT. Its coefficient of thermal expansion is nearly zero, even at strong variations in temperature. The observatory is able to recognize structures on the glistening surface of the sun that are only 50 kilometers in size. This is like trying to judge a cherry pit at a distance of thirty kilometers. The astronomers are now hoping to be able to use their new instrument to arrive at a better type of weather forecast for the solar system, in order to make sure that solar storms no longer cause any more damage here on earth.
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