Active minimization of non-common path aberrations in long-exposure imaging of exoplanetary systems

Abstract

Context. Spectroscopy of exoplanets is very challenging because of the high star-planet contrast. A technical difficulty in the design of imaging instruments is the noncommon path aberrations (NCPAs) between the adaptive optics (AO) sensing and the science camera, which induce planet-resembling stellar speckles in the coronagraphic science images. In an observing sequence of several long exposures, quickly evolving NCPAs average out and leave behind an AO halo that adds photon noise to the planet detection. Static NCPA can be calibrated a posteriori using differential imaging techniques. However, NCPAs that evolve during the observing sequence do not average out and cannot be calibrated a posteriori. These quasi-static NCPAs are one of the main limitations of the current direct imaging instruments such as SPHERE, GPI, and SCExAO. Aims. Our aim is to actively minimize the quasi-static speckles induced in long-exposure images. To do so, we need to measure the quasi-static speckle field above the AO halo. Methods. The self-coherent camera (SCC) is a proven technique which measures the speckle complex field in the coronagraphic science images. It is routinely used on the THD2 bench to reach contrast levels of <10^{-8} in the range 5-12 λ/D in space-related conditions. To test the SCC in ground conditions on THD2, we optically simulated the residual aberrations measured behind the SPHERE/VLT AO system under good observing conditions. Results. We demonstrate in the laboratory that the SCC can minimize the quasi-static speckle intensity in the science images down to a limitation set by the AO halo residuals. The SCC reaches 1σ raw contrast levels below 10^{-6} in the region 5-12 λ/D at 783.25 nm in our experiments.

10 pages, 11 figures, accepted for publication to A&A

References in corpus (5)

• The Gemini Planet Imager: First Light
• Theoretical Limits on Extrasolar Terrestrial Planet Detection with Coronagraphs
• On-sky speckle nulling demonstration at small angular separation with SCExAO
• The coronagraphic Modal Wavefront Sensor: a hybrid focal-plane sensor for the high-contrast imaging of circumstellar environments
• Speckle suppression and companion detection using coherent differential imaging

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• The spectrally modulated self-coherent camera (SM-SCC): Increasing throughput for focal-plane wavefront sensing
• First on-sky demonstration of spatial Linear Dark Field Control with the vector-Apodizing Phase Plate at Subaru/SCExAO
• The polarization-encoded self-coherent camera
• Polarization effects on high contrast imaging: measurements on THD2 Bench