Speckle temporal stability in XAO coronagraphic images

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.

Adaptive pupil masking for quasi-static speckle suppression

“…The noise attenuation found here is comparable with the attenuation from the high‐order test bench with ADI (Martinez et al ). The advantage of this technique over ADI is that it does not rely on PSF subtraction and is therefore insensitive to any changes in the quasi‐static pattern.…”

“…ADI is more generic, but if the speckles evolve during the observation the suppression provided by the technique reduces dramatically. The temporal decorrelation time‐scale of these quasi‐static speckles is an important factor when estimating the performance of image subtraction techniques which have proved themselves to be efficient at Strehl ratios of the order of 20–40 per cent (Martinez et al ). At a high Strehl ratio (>80 per cent), the quasi‐static speckles will become even more dominant as the PSF halo is reduced further.…”

“…In this high Strehl regime, the speckle coherence time‐scale is unknown. Martinez et al () show that the quasi‐static speckles become unstable over a time‐scale of a few seconds on a laboratory XAO test bench. It is thought that the evolution of the speckle pattern was primarily caused by temperature fluctuations and so, on a dedicated instrument, this could be more controlled.…”

Calibration of quasi-static aberrations in exoplanet direct-imaging instruments with a Zernike phase-mask sensor

“…These wavefront errors can be split into two contributions: the longtimescale aberrations that are due to the optical surface errors or misalignments in the instrument optical train and the slowly varying instrumental aberrations that are caused by thermal or opto-mechanical deformations as well as moving optics such as atmospheric dispersion correctors (e.g. Macintosh et al 2005;Martinez et al 2012Martinez et al , 2013. They lead to static and quasi-static speckles in the coronagraphic images, which represent critical limitations for the detection and observation of older or lighter gaseous planets at smaller separations.…”

Speckle temporal stability in XAO coronagraphic images

“…Residual speckles with decorrelation times faster than the time needed to obtain the ADI reference image cannot then be removed, while quasi-static speckles associated with larger timescales can usually be subtracted. In this context, several authors have investigated the decorrelation timescale of quasi-static residuals in the particular context of ADI but at moderate 20−40% Strehl levels (Marois et al 2006;Lafrenière et al 2007;Hinkley et al 2007), while in a former paper (Martinez et al 2012, hereafter Paper I), we explored the realm of very high Strehl ratios (extreme adaptive optic systems, XAO). In Paper I, we analyzed a time series of adaptively corrected, coronagraphic images recorded in the laboratory with the high-order test bench (HOT), a versatile highcontrast imaging, adaptive optics bench developed at ESO.…”