Speckle temporal stability in XAO coronagraphic images II. Refine model for quasi-static speckle temporal evolution for VLT/SPHERE

TL;DR

This paper refines a model for the temporal evolution of quasi-static speckles in high-contrast imaging, crucial for improving detection of faint companions by understanding speckle stability and evolution in VLT/SPHERE observations.

Contribution

The paper introduces an improved model for the linear power law evolution of quasi-static speckles, validated with laboratory and on-sky data, aiding in instrument design and observation strategies.

Findings

Quasi-static wavefront error increases by ~0.7 Å per minute.

The speckle evolution follows a linear power law.

Model implications for speckle nulling and aberration correction timing.

Abstract

Observing sequences have shown that the major noise source limitation in high-contrast imaging is due to the presence of quasi-static speckles. The timescale on which quasi-static speckles evolve, is determined by various factors, among others mechanical or thermal deformations. Understanding of these time-variable instrumental speckles, and especially their interaction with other aberrations, referred to as the pinning effect, is paramount for the search of faint stellar companions. The temporal evolution of quasi-static speckles is for instance required for a quantification of the gain expected when using angular differential imaging (ADI), and to determine the interval on which speckle nulling techniques must be carried out. 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. The temporal evolution of the quasi-static wavefront error exhibits 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 angstrom per minute.