Abstract
The major noise source limiting high-contrast imaging is due to the presence of quasi-static speckles. Speckle noise originates from wavefront errors caused by various independent sources, and it evolves on different timescales pending to their nature. An understanding of quasi-static speckles originating from instrumental errors is paramount for the search of faint stellar companions. Instrumental speckles average to a fixed pattern, which can be calibrated to a certain extent, but their temporal evolution ultimately limit this possibility. 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. 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. We estimate that spoiled and fast-evolving quasi-static speckles present in the system at the angstrom/nanometer level are affecting the stability of the static speckle noise in the final image after the coronagraph. The temporal evolution of the quasi-static wavefront error exhibits linear power law, which can be used in first order to model quasi-static speckle evolution in high-contrast imaging instruments.
Highlights
The direct detection of planets around nearby stars is challenging owing to the required flux contrast and angular separation of the companion from its bright parent star
A mask reproducing the footprint of the Very Large Telescope (VLT) pupil is installed on a 60-element bimorph deformable mirror (DM), which is held by a tip-tilt mount and employed to correct for static aberrations
A typical hour-long at different orientations (ADI) observing sequence provides a partial self-calibration of the residuals after a rotation of ∼1λ/D at a given angular separation, which generally takes less than a few minutes (e.g., 5 to 7 min at 1 on a 8-m class telescope for stars near the meridian in H-band), though it depends on wavelength
Summary
The direct detection of planets around nearby stars is challenging owing to the required flux contrast and angular separation of the companion from its bright parent star. Optical quality (polishing) or misalignment errors generate static speckles that constitute a deterministic contribution to the noise variance, and thereby long-timescale wavefront errors. Though it corresponds to an additional perturbation, a deterministic contribution can be calibrated. Quasi-static speckles correspond to slowly varying instrumental wavefront aberrations (amplitude and phase errors) present in the system, which dominate the companion signal These speckles have various causes, among others mechanical or thermal deformations, or Fresnel effect, highly chromatic speckles originating from phase-to-amplitude conversion due to out-of-pupil plane optics (Marois et al 2006b), and evolve on a shorter timescale than long-lived aberrations
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