SCHOTT solutions no. 1/2009 > Astronomie

An impressive view: The Chinese Xinglong Observatory is located about two hours by car to the northeast of Beijing, near the city of Nanshuangdong. Photo: LAMOST

Bright Spots with LAMOST

The world’s largest Schmidt
Telescope is based in China
and sets new standards for spectroscopic scanning operations on the sky – thanks to a primary mirror made of Zerodur® segments.

Dr. Jakob Staude

In October of 2008, the Chinese Academy of Sciences put a highly efficient instrument into operation at the Xinglong Observatory northeast of Beijing: the Large Sky Area Multi-Object Fiber Spectroscopic Telescope or LAMOST for short. As a Schmidt Telescope, it is not used for visual imaging, but for spectroscopic scanning of the sky at night. Questions on observational cosmology, the galactic structure and stellar astrophysics are now to be explored based on the light spectra that are captured. With the development of this device, new ground was broken that has resulted in a unique combination of light sensitivity, field size and optical quality. The design is extremely unusual: the corrective optics (= corrector) that corrects the spherical aberration (= image distortion) of the primary mirror, is not a lens, as in conventional Schmidt Telescopes, but rather a movable mirror. This tracks the apparent movement of the area of the sky that is observed. Here, exposure times of up to one and a half hours are possible. The corrector directs the light that has been collected inside a huge field of view (five degrees) onto a fixed primary mirror with a usable aperture of four meters. From here, the light is reflected to a focal plane 20 meters away, where it is bundled, fed into 4,000 glass fibers and simultaneously guided to 16 spectrographs. These are capable of measuring light wavelengths in the region of 370 to 900 nanometers – slightly more than the visible light spectrum – and offer spectral resolution of 0.25 to one nanometer. This results in a level of efficiency for spectroscopic scanning operations that has never been achieved in the past.
The primary mirror of the telescope consists of 37 hexagonal 1.1 meter Zerodur® mirror segments. Photo: LAMOST
The corrector consists of 24 hexagonal 1.1 meter mirror segments, whose shape constantly changes as they trace in order to compensate for deviations from the required shape. Active optics are just as involved in this as they are in the focusing of the 37 hexagonal, spherical shaped segments of the 6.7 by 6.0 meter primary mirror onto the focal plane. The positions of the glass fibers inside the focal plane are also adjusted using active optics. This complex optical system is essentially what is new about LAMOST. The Zerodur® material used for the primary mirror segments, on the other hand, is truly a classic. SCHOTT developed this glass ceramic with thermal expansion of nearly zero more than 40 years ago and already used it to build the telescopes at the Calar Alto Observatory back in the 1970s. Since then, the manufacturing process has constantly been improved. The excellent material properties of Zerodur® continue to set the standard for mirror substrates in astronomical telescopes.

One thing is for sure: LAMOST will extend and strengthen the success story of major scanning operations on the sky, such as the Sloan Digital Sky Survey (SDSS). The many millions of optical spectra that will be acquired with LAMOST will create a new foundation for observational cosmology, researching the origin and development of galaxies, and the structure of the Milky Way system, but also for stellar astrophysics.