Research Note

Speckle temporal stability in XAO coronagraphic images

II. Refine model for quasi-static speckle temporal evolution for VLT/SPHERE

¹ Laboratoire Lagrange, UMR 7293, Université de Nice Sophia-Antipolis, CNRS, Observatoire de la Côte dÁzur, Bd. de l’Observatoire, 06304 Nice, France
e-mail: patrice.martinez@oca.eu
² European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
³ UJF – Grenoble 1/CNRS – INSU, Institut de Planétologie et d’Astrophysique de Grenoble UMR 5274, 38041 Grenoble, France
⁴ Laboratoire d’Astrophysique de Marseille, UMR 7326, CNRS, Université de Provence, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
⁵ Office National d’Études et de Recherches Aérospatiales (ONERA), Optics Department, BP 72, 92322 Chatillon Cedex, France

Received: 29 November 2012
Accepted: 4 March 2013

Abstract

Context. Observing sequences have shown that the major noise source limitation in high-contrast imaging is the presence of quasi-static speckles. The timescale on which quasi-static speckles evolve is determined by various factors, mechanical or thermal deformations, among others.

Aims. Understanding these time-variable instrumental speckles and, especially, their interaction with other aberrations, referred to as the pinning effect, is paramount for the search for faint stellar companions. The temporal evolution of quasi-static speckles is, for instance, required for quantifying the gain expected when using angular differential imaging (ADI) and to determining the interval on which speckle nulling techniques must be carried out.

Methods. Following an early analysis of a time series of adaptively corrected, coronagraphic images obtained in a laboratory condition with the high-order test bench (HOT) at ESO Headquarters, we confirm our results with new measurements carried out with the SPHERE instrument during its final test phase in Europe. The analysis of the residual speckle pattern in both direct and differential coronagraphic images enables the characterization of the temporal stability of quasi-static speckles. Data were obtained in a thermally actively controlled environment reproducing realistic conditions encountered at the telescope.

Results. The temporal evolution of the quasi-static wavefront error exhibits a linear power law, which can be used to model quasi-static speckle evolution in the context of forthcoming high-contrast imaging instruments, with implications for instrumentation (design, observing strategies, data reduction). Such a model can be used for instance to derive the timescale on which non-common path aberrations must be sensed and corrected. We found in our data that quasi-static wavefront error increases with ~0.7 Å per minute.