Speckle noise and the detection of faint companions

Speckles dominate shot noise within the halo of adaptively corrected bright star images and, consequently, impose severe limits on ground-based attempts to directly detect planets around nearby stars. The effect is orders of magnitude greater than conventional photon noise. It depends on the dwell time of the speckle pattern, the brightness of the star, and the fraction (1 - S) of residual light in the halo (S being the Strehl ratio of the image). These predictions agree well with limits found using the Canada-France-Hawaii Telescope adaptive optics bonnette. The limiting brightness for detection is proportional to (1 - S)/S, emphasizing the need for large Strehl ratios. Strategies to reduce speckle noise are proposed; the encouraging results of a test are presented.