The clear nights on top of the 2,426 meter high Roque de los Muchachos, the highest peak on La Palma, are ideal for viewing the galaxy with the Gran Telescopio Canarias. Photo: Nik Szymanek
Jürgen Breier

Hunters of the Galaxies

The world’s largest telescope is located on the Canary Island of La Palma. Astronomers are hoping it will help them find a second earth.

They say the future is written in the stars. But, this also applies to the past. For thousands of years, celestial bodies could only be viewed with the naked eye, until Galileo Galilei became the first person to use a telescope to make astronomical observations in 1609. Today, his successors are using more and more powerful eyes on the sky to explore the depths of the universe. They hope to solve the riddle of where stars and planets come from and, thus, be able to understand our own cosmic past.

Paradise for stellar researchers

The Gran Telescopio Canarias (GTC) is considered to be the new star among the world’s spies on the sky. It is located on top of La Palma’s highest peak, the 2,426 meter high Roque de los Muchachos. Thanks to the clear nights, both this Canary Isle and its sister Tenerife are considered to be ideal locations for observing the universe. In fact, some 20 large telescopes are already in operation on these islands. ”The viewing quality here is incredible, thanks to the transparent atmosphere above the clouds and the trade winds that result in an even laminar air flow, without turbulences,” explains Pedro Alvarez, Director of the GTC.
Source: SCHOTT
With a height of 41 meters and a weight of 500 tons, the gtc has a primary mirror with a 10.4 meter diameter and is, thus, the largest optical telescope in the world. Following seven years of construction, it saw its so-called ”first light” in July of 2007. The hatch opened for the first time ever and starlight fell onto the mirror that had not even been completely mounted at the time. Astronomers from all over the world and guests of honor, headed by the Spanish Crown Prince Felipe, witnessed this exciting moment. The nearly 130 million euro galactic hunting station is operated by the Instituto de Astrofísica de Canarias (IAC). In addition, the large telescope located on La Palma also reinforces the importance of the Canary Islands as an European northern observatory antipole to the European southern observatory. If everything continues to run as planned, researchers will be able to begin their work at the end of 2008.
Photo: Miguel Briganti, IAC
Primary mirror weighs 18 tons

As a modern large telescope, the Gran Telescopio Canarias is equipped with both active and adaptive optics. With the active optics, the mirror is supported from behind by alignment elements at hundreds of positions. When the mirror is tilted, their lengths are readjusted to ensure that they retain their perfect hyperbolic shape. This allows for even the slightest of geometric irregularities in the entire system to be corrected. After all, perfect alignment is needed to achieve high imaging quality. The technology used in adaptive optics helps ensure that the images produced by the large telescope are not blurred. The air above the telescope never stops moving and the striae this causes result in haziness. The larger the telescope, the more apparent this becomes. The primary mirror of the large telescope on La Palma comprises 36 hexagonal segments. Each of these has a diagonal length of 1.9 meters, is 8.5 centimeters thick and weighs 500 kilograms. Together, they form a surface of 75 square meters and have a weight of around 18 tons.

SCHOTT in Mainz received the order to produce these mirror substrate segments, following initial contacts that took place back in 1994. The substrates are made of the Zerodur® glass ceramic. The coefficient of thermal expansion of this material developed by SCHOTT is practically zero. This means that hardly any change in the dimensions can be observed, even when temperatures shift considerably. Therefore, this material is ideal for use as a substrate for telescopes and satellite optics. The high demands of astronomers, not only with respect to the internal quality of the material, but also the accuracy of the geometry that results from processing, required the use of modern measurement technology. Here, SCHOTT relied on a mobile 3D laser tracker system that precisely measures the segments even down to hundredth of millimeters. ”Projected onto a surface the size of Manhattan, the maximum deviation would only be a couple of centimeters,” notes Volker Wittmer, the project manager at SCHOTT, in referring to this high precision work. It took about eight months for the glass specialists to produce each substrate. From January 2000 on, up to three segments left the plant each month, safely packaged in special containers, for further polishing by a French company based near Paris. Afterwards, an extremely thin reflective aluminum coating was vapor-deposited onto the segments. Due to the effects of the climate, this coating has to be replaced at regular intervals. For this reason, the customer ordered six additional modules that ensure quick replacement and, thus, minimize telescope downtime.
The GTC is currently considered to be the best instrument available for researching the sky over the northern hemisphere. Its performance is comparable to the Keck Telescope on Hawaii and the Very Large Telescope operated by the European Southern Observatory on top of the Cerro Paranal in Chile. The new Spanish observatory will not only increase the number of eight to ten-meter telescopes found in the world today, it will also combine a large collector surface with excellent image quality. On the one hand, observations will focus on distant objects. Here, the objective is to obtain new information on where galaxies come from and how they developed, but also to learn more about the origin of the universe. On the other hand, it will be possible to examine faint objects, such as planets near distant suns, more closely.

”It would be fantastic if this telescope could help us to discover a planet that resembles our own,” concludes the Director of the GTC, Pedro Alvarez.
A giant among telescopes: The GTC is 41 meters high, weighs 500 tons and has a primary mirror 10.4 meters in diameter (far left). This consists of 36 hexagonal segments (upper left) that form a surface of 75 square meters that weighs approximately 18 tons. gtc Director Pedro Alvarez (lower left) is hoping to obtain new insights on distant galaxies and the origin of the universe. In July of 2007, the hatch opened for the first time to provide a view of the galaxy. Photos: (on the bottom left) GTC, (on the right) Ángel L. Aldai