REMNANT SPECKLES IN A HIGHLY CORRECTED CORONAGRAPH

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 ground-based 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. Subject headings: instrumentation: adaptive optics — techniques: high angular resolution — techniques: image processing — turbulence 1