Technology for Telescopes

Since its invention 400 years ago, the astronomical telescope has evolved from a small, manually pointed device for visual observations to a large, sophisticated, computer-controlled instrument with full digital output. Throughout this development, two properties have been particularly important: the light-collecting power, or diameter of the telescope's mirror (allowing for the detection of fainter and more distant objects), and the image sharpness, or angular resolution (allowing smaller and fainter objects to be seen).

The European Southern Observatory (ESO), as a worldwide leader in astronomy, has developed several advanced technologies that have enabled the construction of ever larger telescope mirrors, while maintaining optical accuracy.

ESO has developed the technique of active optics, which is now in use in most modern medium-sized and large telescopes. It preserves optimal image quality by pairing a flexible mirror with actuators that actively adjust the mirror's shape during observations.

The bigger a mirror, the greater its theoretical resolution, but even at the best sites for astronomy, large, ground-based telescopes observing at visible wavelengths cannot achieve an image sharpness better than telescopes with a 20- to 40-cm diameter, due to distortions introduced by atmospheric turbulence. For a 4-metre telescope, atmospheric distortion degrades the resolution by more than an order of magnitude compared with what is theoretically possible, and the intensity of light at the centre of the star's image is lowered by a factor of 100 or more. One of the principal reasons for launching the NASA/ESA Hubble Space Telescope was to avoid this image smearing. The effects of the atmosphere can be compensated for in some modern telescopes by the adaptive optics technique. ESO's VLT has also led the way in adaptive optics, which has revolutionised ground-based astronomy.

Combining the light collected by two or more telescopes in a technique known as interferometry can boost the resolution beyond what a single telescope can accomplish. ESO has been a pioneer in this field with the Very Large Telescope Interferometer (VLTI) at Paranal.

In addition to atmospheric turbulence, the telescopes themselves can introduce errors into astronomical observations. Manufacturing errors and irregularities in equipment, ranging from mirrors to structural components, can mar views of the cosmos. Over the years, engineers have made a series of improvements to minimise wear-and-tear errors caused by the mechanical movement of the telescope and heat damage. Mirror figuring and polishing have improved, along with the design of stiffer support structures and mirrors to reduce deformations. Low expansion glass has also reduced mirror distortions when temperatures vary. To reduce the small, but noticeable, turbulence inside the telescope dome, heat loss from motors and electronic equipment is curtailed during the night, and the dome that shields the telescope from the wind is cooled during the day.