SCHOTT Info 98 | SCHOTT AG

A Step Closer to the Mystery of the Universe

Modern telescopes differ widely in design depending on what they are to be used for. “ZERODUR®” is very frequently used as the material for mirror blanks.

Whereas telescopes at one time were pure observation instruments, through which astronomers used to view the night sky for hours on end, nowadays they are serving increasingly as measuring instruments. Instead of the human eye, CCD chips (Charge-Coupled Device, a semiconductor material component for processing electrical and optical signals) are used as receivers and data collected is stored and processed in computers. Modern astronomy is also no longer restricted to visible light. The entire bandwidth of electromagnetic radiation in the universe can be investigated with different types of telescopes.

VLT

The fourth 8.2 meter telescope mirror receiving its fine polishing treatment at the French company REOSC.

The VLT is located on the 2660 meter high Cerro Paranal in the Atacama Desert in Chile.

The “Very Large Telescope” (VLT) on the 2660 meter high Cerro Paranal in Chile is currently the largest and most powerful telescope array in the world. It has four main mirrors made of “ZERODUR®,” each with a diameter of 8.2 meters – the largest ever cast in a single piece. The four telescopes are at present still being operated independent of each other, but they are already delivering images from the visible and near infrared spectrum in a sensational resolution. When they are coupled together to make a giant telescope with an effective mirror diameter of 16 meters, it should be possible in theory to watch an astronaut walking on the moon. In practice, however, the VLT is set to explore dim objects in the universe and, for example, collect information for the first time about planet systems of other stars.

GTC

With a diameter of 10.4 meters and a maximum size of 11.3 meters the GTC is one of the world’s largest telescopes. The primary mirror is composed of 36 “ZERODUR®” glass ceramic hexagons.

The Gran Telescopio Canarias (GTC) is currently being built to explore the heavens of the northern hemisphere. Its location on La Palma in the Canary Islands is considered to be one of the best in the world for observations in the visible and near infrared spectrum, for there is very little atmospheric disturbance in that area. The GTC’s giant 10.4 meter diameter main mirror was not cast in one piece, but is made up of 36 hexagonal “ZERODUR®” segments. A sophisticated system of supports, drives and sensors ensures that the individual segments are exactly aligned in relation to each other while astronomic measurements are being carried out. It is planned to start operating with a temporary main mirror consisting of 8 to 10 segments at the end of 2002 with the whole system being ready by 2004.

Chandra

The Chandra observatory circles the Earth in an elliptical orbit at a distance between 10,000 and 140,000 kilometers and is thus well away from radiation belts that could interfere with its operations.

The Chandra X-ray telescope was transported into space by the space shuttle in 1999. Since then it has circled the Earth in a highly elliptical orbit from which it carries out measurements in the invisible X-ray spectrum. It can be used to investigate the extremely hot gas in a cluster of galaxies for example, which can give indications about the size of black holes.

When X-rays strike matter head on, they penetrate deep inside or even pass right through it. For that reason, special cylindrical “ZERODUR®” mirrors were developed for Chandra which are arranged so that the X-rays are reflected at a grazing incidence and are then reflected onto the detectors.

SOFIA

SOFIA airborne telescope: the optical instrument is installed in the Boeing 747’s fuselage.

A honeycomb structure has been milled out of the reverse side of the “ZERODUR®” mirror blank, reducing its weight from four tonnes to 850 kilograms.

SOFIA stands for “Stratospheric Observatory for Infrared Astronomy” and it is basically a jumbo jet (Boeing 747) with a telescope on board. As the majority of the infrared radiation being investigated does not penetrate the Earth’s atmosphere, SOFIA’s scientific observation flights are expected to take place from 2004 at a height of about 13 km. The conditions found there are rough, i.e. low pressure, turbulent winds and a temperature of about –60°C. The “ZERODUR®” mirror substrate can withstand all this very well, even though its reverse side consists only of a thin honeycomb structure. By producing it in this way it was possible to reduce its weight from an original figure of 4 tonnes to 850 kg.