Introduction

Astronomical telescopes are devices which collect as much radiation from astronomical (stellar) objects and put it in an as sharp (small) an image as possible. Both collecting area and angular resolution play a role. The relative merit of these two functions has changed over the years in optical astronomy, with the angular resolution initially dominating and then, as the atmospheric seeing limit was reached, the collecting area becoming the most important factor. Therefore it is the habit these days to express the quality of a telescope by its (collecting) diameter rather than by its angular resolution. With the introduction of techniques which overcome the limits set by atmospheric seeing, the emphasis is changing back to angular resolution. This time, however, it is set by the diffraction limit of the telescope so that both angular resolution and collecting power of a telescope will be determined by its diameter. Both telescope functions will therefore go hand-in-hand.

Although image selection and various speckle image reconstruction techniques have been successful in giving diffraction limited images (see, e.g., the paper by Oskar von der Lühe in the First Canary Island Winter School, 1989), the most powerful and promising technique for all astronomical applications is the one using adaptive optics. That is because, for an unresolved image, it puts most of the collected photons in an as small an image as possible which benefits both in discriminating against the sky background, in doing high spectral and spatial resolution spectroscopy and in doing interferometric imaging with telescope arrays.