Speckle temporal stability in XAO coronagraphic images

¹ UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble UMR 5274, 38041 Grenoble, France
e-mail: patrice.martinez@obs.ujf-grenoble.fr
² European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany

Received: 15 November 2011
Accepted: 19 March 2012

Abstract

Context. The major source of noise limiting high-contrast imaging is caused by quasi-static speckles. Speckle noise originates from wavefront errors caused by various independent sources, and evolves on different timescales depending on their nature. An understanding of how quasi-static speckles originate from instrumental errors is paramount to the search for faint stellar companions. Instrumental speckles average to form a fixed pattern, which can be calibrated to a certain extent, but their temporal evolution ultimately limits this possibility.

Aims. This study focuses on the laboratory evidence and characterization of the quasi-static pinned speckle phenomenon. Specifically, we examine the coherent amplification of the static speckle contribution to the noise variance in the scientific image, through its interaction with quasi-static speckles.

Methods. The analysis of a time series of adaptively corrected, coronagraphic images recorded in the laboratory enables the characterization of the temporal stability of the residual speckle pattern in both direct and differential coronagraphic images.

Results. We estimate that spoiled and rapidly evolving quasi-static speckles present in the system at the angstrom/nanometer level affect the stability of the static speckle noise in the final image after the coronagraph. The temporal evolution of the quasi-static wavefront error exhibits a linear power law, which can be used to first order to model quasi-static speckle evolution in high-contrast imaging instruments.