A few years ago, a real break-through happened in observational astronomy: the un derstanding of the effect of atmospheric turbulence on the structure of stellar images, and of ways to overcome this dramatic degradation. This opened a route to diffraction-limited observations with large telescopes in the optical domain. Soon, the first applications of this new technique led to some outstanding astrophysical results, both at visible and infrared wavelengths. Yet, the potential of interferometric observations is not fully foreseeable as the first long-baseline arrays of large optical telescopes are being built or cOIIllnissioned right now. In this respect a comparison with the evolution of radio-astronomy is tempting. From a situation where, in spite of the construction of giant antennas, low angular resolution was prevailing, the introduction of long baseline and very long baseline interferometry and the rapid mastering of sophisticated image reconstruction techniques, have brought on a nearly routine basis high dynamic range images with milliarcseconds resolution. This, of course, has completely changed our views of the radio sky.

A JENAM 2002 Workshop, Porto, Portugal, 3-5 September 2002

Historically, the discovery of tools, or evidence that tools have been used, has been taken as proof of human activity; certainly the invention and spread of new tools has been a critical marker of human progress and has increased our ability to observe, measure, and understand the physical world. In astronomy the tools are telescopes and the optical and electronic instruments that support them. The use of the telescope by Galileo marked the beginning of a new and productive way to study and understand the universe in which we live. The effects of this new tool on what we can see, and how we see ourselves, are well known. However, after almost four centuries of developing ever more sensitive and subtle instruments as tools for astronomy, it might have been expected that only a few minor improvements would remain to be made, or that possibly the law of diminishing returns would have taken effect. On the contrary, the new instruments and ideas for new instruments described in this book make it clear that the rate of progress has not diminished, and that this subject is still as exciting and productive as ever. Instrumentation for Ground-Based Optical Astronomy was chosen as the theme for the Ninth Santa Cruz Summer Workshop in Astronomy and Astrophysics.