Abstract
Direct detection and spectroscopy of exoplanets requires high-contrast imaging. For habitable exoplanets in particular, located at a small angular separation from the host star, it is crucial to employ small inner working angle (IWA) coronagraphs that efficiently suppress starlight. These coronagraphs, in turn, require careful control of the wavefront that directly impacts their performance. For ground-based telescopes, atmospheric refraction is also an important factor, since it results in a smearing of the point-spread function (PSF), that can no longer be efficiently suppressed by the coronagraph. Traditionally, atmospheric refraction is compensated for by an atmospheric dispersion compensator (ADC). ADC control relies on an a priori model of the atmosphere whose parameters are solely based on the pointing of the telescope, which can result in imperfect compensation. For a high-contrast instrument like the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system, which employs very small IWA coronagraphs, refraction-induced smearing of the PSF has to be less than 1 mas in the science band for optimum performance. In this paper, we present the first on-sky measurement and correction of residual atmospheric dispersion. Atmospheric dispersion is measured from the science image directly, using an adaptive grid of artificially introduced speckles as a diagnostic to feedback to the telescope’s ADC. With our current setup, we were able to reduce the initial residual atmospheric dispersion from 18.8 mas to 4.2 in broadband light (y- to H-band) and to 1.4 mas in the H-band only. This work is particularly relevant to the upcoming extremely large telescopes (ELTs) that will require fine control of their ADC to reach their full high-contrast imaging potential.
Highlights
Several thousand exoplanets have far been discovered using indirect methods, such as transit and radial velocity but very few using direct imaging9
A potentially significant source of coronagraphic leakage comes from atmospheric refraction once the turbulence has been accounted for, which results in an elongated point spread function (PSF) that limits the achievable angular resolution and coronagraphic contrast
Subaru Coronagraphic Extreme Adaptive Optics (SCExAO)’s deformable mirror (DM) was used, which has a total of 2000 actuators, with 45 actuators across the pupil
Summary
Several thousand exoplanets have far been discovered using indirect methods, such as transit and radial velocity but very few using direct imaging. A potentially significant source of coronagraphic leakage comes from atmospheric refraction once the turbulence has been accounted for, which results in an elongated point spread function (PSF) that limits the achievable angular resolution and coronagraphic contrast. This effect can be devastating if the coronagraph operates at a small inner working angle (IWA). The science requirement for SCExAO is a spread in the PSF of ∼ 1/50th of the diffraction-limit, which translates to < 1 mas in H-band At this level the system will no longer be limited by residual atmospheric dispersion, instead the finite stellar angular size will become the dominant term contributing to stellar leakage.
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