May 09, 2012
Nighttime is the astronomer’s day. After all, anyone wishing to see the stars must wait until it is dark. There is one exception to this rule, however: the sun. Close up, it can be used to study not only the properties of a typical star, but also its relationship with the planets. Observing eruptions of matter, sunspots or even the solar magnetic field in detail requires telescopes with a large aperture, such as Gregor, which will be inaugurated in Tenerife in the coming days. With a mirror diameter of 1.5 metres, the telescope will be able to show structures on the sun on spatial scales as small as 70 kilometres, making it one of the three most powerful instruments in the world.
Text: Helmut Hornung
Tenerife is not just a popular destination for holidaymakers. Astronomers have also long since discovered the appeal of the Canary Island. Observation conditions are ideal on the plateau at the foot of the 3,718-metre-high Teide volcano. A consortium of researchers from the Kiepenheuer Institute for Solar Physics, the Leibniz Institute for Astrophysics Potsdam, the Institute for Astrophysics Goettingen, the Max Planck Institute for Solar System Research and other international partners have been constructing the Gregor solar telescope there since 2000.
The name is not an acronym; the telescope is named after James Gregory (1638 to 1675). During the 17th century, the Scottish mathematician and astronomer developed a telescope in which a secondary concave mirror directed the reflected light from the primary parabolic mirror through a tiny hole in the primary mirror on to the eye-lens and thus into the eye. This optical principle is also used in the new telescope in Tenerife.
However, it is unlikely that any scientist will use GREGOR to ‘look’ directly at the sun; this will be done using electronic detectors, such as spectrographs, polarimeters, interferometers and cameras. The high-tech ‘eye in the sky’ also differs in other ways from James Gregory’s design. The telescope has a completely open structure to prevent air turbulence in the optical path. It is housed in a building with a retractable dome.
The 1.5-metre primary mirror is made from the heat-resistant glass ceramic Zerodur and is actively cooled to prevent the surface of the mirror from heating up as a result of the absorbed sunlight. Engineers manufactured two additional mirrors from silicon carbide (Cesic).
Thanks to its adaptive lens, GREGOR delivers particularly sharp images: a complex system of actuators and mirrors helps the system to compensate for the schlieren within the earth’s atmosphere, which constantly distorts the image of the sun similar to the way in which swirling air masses shimmer over an asphalt road in the summer heat.
A rotating fold mirror deflects the bundled beam generated by the adaptive optics system to the various instruments. Their purpose is to measure various physical solar parameters with an unprecedented level of precision, in particular the sun’s magnetic field, and in doing so reveal small structures down to a scale of 70 kilometres – an astounding resolution capacity given that the sun is located approximately 150 million kilometres from the earth.