Progress in ground-based optical telescopes

For several decades, the 5 metre Palomar telescope has set a practical limit to the size of ground-based optical telescopes only exceeded by the Russian 6 metre telescope. Using new technologies to produce either large monolithic mirrors or large mosaic of smaller elements, astronomers in Europe, the United States and Japan are developing a new generation of telescopes. Not only are these new telescopes much bigger in size, offering considerably more collecting power, but they also provide a much better angular resolution. To achieve this they rely heavily on computer control techniques and advanced modelling. Even more progress in angular resolution is expected when modern techniques such as adaptive optics and interferometry, which are being implemented on most of the new telescopes, are fully operational. This article evaluates the impact of new technologies on the development of large optical telescopes, in particular on the realization of their optics. The key mirror technologies are reviewed and a particular project, the ESO VLT is reviewed in some more detail as an illustration.