Adaptive Optics for Asq-onomy

• During the last decade optical astronomy has played an increasingly important role in our understanding of the universe, and many recent discoveries can be direcdy attributed to revolutionary improvements in telescope design. At least 10 optical systems with apertures exceeding 3.5 m are currently available to the scientific community, and several telescopes in the 8-to-l0-m class are now under construction. Although the light-gathering properties of these new telescopes are remarkable, at visible wavelengths their resolution is ultimately limited by phase distortions associated with atmospheric turbulence. Even under excellent seeing conditions, the imaging quality of these systems in the visible is seldom better than that obtainable from a 20-cm receiver. Recent changes in military security guidelines have now made it possible to apply technology developed for high-energy laser-beam control to the problem of turbulence compensation for astronomy. In particular, the combination of high-bandwidth adaptive optics and synthetic:-heacon sources offers the potential for near-difFraction-limited resolution at all optical wavelengths. This article investigates the principal design issues associated with the construction of adaptive optics systems for dim-target imaging, and develops quantitative performance estimates for a 4-m telescope.