Speckle lifetime in XAO coronagraphic images: temporal evolution of SPHERE coronagraphic images

Abstract

The major source of noise in high-contrast imaging is the presence of slowly evolving speckles that do not average with time. The temporal stability of the point-spread-function (PSF) is therefore critical to reach a high contrast with extreme adaptive optics (XAO) instruments. Understanding on which timescales the PSF evolves and what are the critical parameters driving the speckle variability allow to design an optimal observing strategy and data reduction technique to calibrate instrumental aberrations and reveal faint astrophysical sources. We have obtained a series of 52 min, AO-corrected, coronagraphically occulted, high-cadence (1.6Hz), H-band images of the star HR 3484 with the SPHERE (Spectro-Polarimeter High-contrast Exoplanet REsearch 1 ) instrument on the VLT. This is a unique data set from an XAO instrument to study its stability on timescales as short as one second and as long as several tens of minutes. We find di↵erent temporal regimes of decorrelation. We show that residuals from the atmospheric turbulence induce a fast, partial decorrelation of the PSF over a few seconds, before a transition to a regime with a linear decorrelation with time, at a rate of several tens parts per million per second (ppm/s). We analyze the spatial dependence of this decorrelation within the well-corrected radius of the adaptive optics system and show that the linear decorrelation is faster at short separations. Last, we investigate the influence of the distance to the meridian on the decorrelation.

Azimuthal asymmetries in the debris disk around HD 61005

“…5. Overall, this suggests quite extreme forward scattering in the disk around HD 61005 -a result that remains model-dependent since the PCA attenuation is estimated from our results (see also Milli et al 2016, for a similar discussion for the disk around HR 4796 A). Nonetheless, the strong peak of scattering for small angles (down to ∼10…”

“…For the SED, we need a whole range of sizes (from µm-to mm-sized grains), but the emission is actually dominated by the smaller grains (discussed further in this section). Numerous studies of debris disks around different stars (e.g., Rodigas et al 2015;Lebreton et al 2012;Milli et al 2015Milli et al , 2016 already reported and discussed similar issues in reconciling the modeling results of different kind of observations. Throughout this paper, for their computational merits, we have either used the Mie theory or the HG prescription while both of them may not be accurate descriptions of the nature of the dust grains.…”

Speckle statistics in adaptive optics images at visible wavelengths

“…1 Although current high contrast imaging instruments and coronagraphs make all use of sophisticated and high performance AO systems, 2 small optical imperfections yield quasi-static speckles and residual stray light, that can still represent a severe limitation to the achievement of the high contrast needed to detect faint companions. 3,4,5,6 For these reasons, a deep understanding of the speckle variability and statistics is of fundamental importance for the optimization of post-facto techniques aimed at increasing the image contrast, 7,8,9 like angular differential imaging (ADI), locally optimized combination of images (LOCI), or principal component analysis (PCA) 10,11,12,13 . In addition, it has been demonstrated that speckle intensity statistics represents a powerful tool for speckle discrimination and, therefore, for their post-facto suppression.…”

Speckle statistics in adaptive optics images at visible wavelengths

“…[21][22][23][24] However, these statistical studies have mostly focused on intensity fluctuations in specific locations of the focal plane, using data sequences with a limited temporal cadence (>40 to 50 ms), longer than the typical atmospheric timescales (5 to 10 ms). Recently, other authors 9 have investigated the speckle lifetime in the H-band by exploiting a 1.6-Hz cadence set of ExAO images.…”

“…1 Although current high contrast imaging instruments and coronagraphs make use of sophisticated and high performance AO systems, 2 small optical imperfections yield quasistatic speckles and residual stray light, which still represents a severe limitation on achieving the high contrast needed to detect faint companions. [3][4][5][6] For these reasons, a deep understanding of the speckle variability and statistics is of fundamental importance for the optimization of postfacto techniques aimed at increasing the image contrast, [7][8][9] such as angular differential imaging (ADI), locally optimized combination of images, and principal component analysis. [10][11][12][13] In addition, it has been demonstrated that speckle intensity statistics represents a powerful tool for speckle discrimination and, therefore, for their postfacto suppression.…”