Optical components

The VLT on Cerro Paranal in Chile is currently the world’s most powerful telescope.
The Centaurus A galaxy photo- graphed by the VLT instrument FORS2.
Monika Rueger, Science Journalist, Munich, Germany

“Eyes” for a Telescope

It is not enough for a telescope to possess a large reflecting surface. Only when the image quality of the astronomical instruments and the optical components are at their best is it possible to make new discoveries about the cosmos.

Telescopes were at one time purely observational instruments through which astronomers studied the night sky for hours on end. Today they primarily serve as measuring devices. The light emitted from stars is carefully registered and analyzed with the help of various astronomical instruments. CCD (Charge Coupled Device) chips, which are far more sensitive than the human eye, are used as receivers.

How does a modern telescope function, and what can be “seen” through it? First of all it has to “capture” as much light as possible from space. This is the task of the telescope’s main mirror, and the larger the mirror surface, the better the results. With the help of another mirror, the collected light is united in a focus. This is where the astronomical instruments are positioned to analyze the starlight. A spectrograph is used to investigate the spectral composition of the light that is emitted from a star or reflects from a planet. Just as fingerprints are individual, every element displays its own typical spectrum. This is how information on the chemical composition of cosmic objects can be obtained and conclusions drawn as to their properties and age.

FORS – Instruments for the Very Large Telescope

The VLT instrument FORS1 is flanged on to a support element of the main mirror. The imaging optics, made of special glasses from SCHOTT, are located in the yellow cylinder. The silver and red boxes contain the electronic components and the servomotors for the control system.
The Very Large Telescope (VLT) in Chile, currently the world’s largest and most powerful telescope, has four main mirrors made of “ZERODUR®” glass ceramic from SCHOTT, each with a surface of over 50 square meters and a diameter of 8.2 meters. The VLT can focus light interchangeably with twelve different astronomical instruments. Two of these are practically identical – the FORS1 and FORS2 (Focal Reducer / low dispersion Spectrograph). These enable the device, which weighs more than two tons, to take pictures and carry out measurements (e.g. spectroscopy and polarimetry).

The imaging optics of the FORS instrument consists of a total of 17 lenses. This optical system has two main functions. It must decrease the size of the image, so that it fits on the 5 x 5 cm surface of the CCD chip (hence the name focal reducer). In addition, it must correct any image distortions caused by the telescope mirror, as the mirror system in itself can only provide good image quality in the proximity of the optical axis.