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 attemp ts to directly detect planets around nearby stars. The effect is orders of magnitude greater than conventional photon noise. It depends on the dwell t ime 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-Ha waii Telescope adaptive optics bonnette. The limiting brightness for detect ion is proportional to (1 - S)/S, emphasizing the need for large Strehl rat ios. Strategies to reduce speckle noise are proposed; the encouraging resul ts of a test are presented.