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Adaptive correction on large ground-based telescopes is enabling a variety of novel studies that would be impossible at the limits of spatial and spectral resolution imposed by the Earth's turbulent atmosphere. Even relatively high-order systems, however, do not yield a perfect correction and as a result are compromised by a low-intensity halo of remnant light that is removed from the core of the point-spread function (PSF) and dispersed in the focal plane. Worse still, this halo is neither constant in time nor uniform in position but is concentrated in transient spots that move about as mutually coherent patches of phase over the telescope aperture happen to combine constructively in the image plane. These "speckles" in the PSF set limits on ground-based searches for faint companions to bright stars. We describe here simple properties of the physics of speckle formation that will affect the statistics of residual speckles in a fundamental way: at high correction, the secondary maxima of the static PSF will coherently amplify or "pin" the time-varying speckles, influencing their effective characteristic lifetimes and dramatically changing their spatial distribution. Furthermore, as a result of speckle pinning, temporal variations in the noncommon path errors that occur in practical adaptive optics systems will cause an additional gradual drift in the spatial distribution of speckles, as Airy rings shift. Speckle pinning may be exploited to suppress speckle noise by tailoring the PSF with static aberrations artificially injected via the deformable mirror so as to clear a region of the PSF of Airy rings and hence of pinned speckles, a technique that we term "speckle sweeping."

We describe the current performance of the Palomar 200 inch (5 m) adaptive optics system, which in December of 1998 achieved its first high order (241 actuators) lock on a natural guide star. In the K band (2.2 pm), the system has achieved Strehi ratios as high as 50% in the presence of 1.0 arcsecond seeing (0.5 tim). Predictions of the system's performance based on the analysis of real-time wavefront sensor telemetry data and a analysis based on a fitted Kolmogorov atmospheric model are shown to both agree with the observed science image performance. Performance predictions for various seeing conditions are presented and an analysis of the error budget is used to show which subsystems limit the performance of the AO system under various atmospheric conditions.

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Searches for faint companions to stars may use coronagraphs fed by adaptive optics (AO) systems of very high correction. Sensitivity will be limited by focal plane speckles from residual, uncorrected wave-front errors, so it is important to characterize remnant coronagraphic speckles. A general analysis is presented, illustrated with the classical Lyot coronagraph and the newer four quadrant phase mask scheme. Two kinds of remnant speckles, of distinct symmetry, occur that are closely analogous to those arising in direct imaging at high correction. Properties and typical intensity estimates are presented, which are useful for estimating speckle noise and false companion detection levels and reduction strategies. For realistic parameters describing some current groundbased observations, the novel antisymmetric "pinned" speckles, usually neglected even in direct imaging, are nonnegligible in narrowband short exposures. For parameters appropriate to a space-borne coronagraph on the Hubble Space Telescope, these anomalous speckles are in fact dominant close to the star and thus a potentially significant source of false companion detections.