Extreme Adaptive Optics Guyon, Olivier
Annual review of astronomy and astrophysics,
09/2018, Letnik:
56, Številka:
1
Journal Article
Recenzirano
Over the last two decades, several thousand exoplanets have been identified, and their study has become a high scientific priority. Direct imaging of nearby exoplanets and the circumstellar disks in ...which they form and evolve is challenging due to the high contrast ratio and small angular separation relative to the central star. Exoplanets are typically within 1 arcsec of, and between 4 and 10 orders of magnitude fainter than, the stars they orbit. To meet these challenges, ground-based telescopes must be equipped with extreme adaptive optics (ExAO) systems optimized to acquire high-contrast images of the immediate surrounding of nearby bright stars. Current ExAO systems have the sensitivity to image thermal emission from young massive planets in near-IR, while future systems deployed on Giant Segmented Mirror Telescopes will image starlight reflected by lower-mass rocky planets. Thanks to rapid progress in optical coronagraphy, wavefront control, and data analysis techniques, direct imaging and spectroscopic characterization of habitable exoplanets will be within reach of the next generation of large ground-based telescopes.
The effects of photon noise, aliasing, wave front chromaticity, and scintillation on the point-spread function (PSF) contrast achievable with ground-based adaptive optics (AO) are evaluated for ...different wave front sensing schemes. I show that a wave front sensor (WFS) based on the Zernike phase contrast technique offers the best sensitivity to photon noise at all spatial frequencies, while the Shack-Hartmann WFS is significantly less sensitive. In AO systems performing wave front sensing in the visible and scientific imaging in the near-IR, the PSF contrast limit is set by the scintillation chromaticity induced by Fresnel propagation through the atmosphere. On an 8 m telescope, the PSF contrast is then limited to 10 super(-4) to 10 super(-5) in the central arcsecond. Wave front sensing and scientific imaging should therefore be done at the same wavelength, in which case, on bright sources, PSF contrasts between 10 super(-6) and 10 super(-7) can be achieved within 1" on an 8 m telescope in optical/near-IR. The impact of atmospheric turbulence parameters (seeing, wind speed, turbulence profile) on the PSF contrast is quantified. I show that a focal plane wave front sensing scheme offers unique advantages, and I discuss how to implement it. Coronagraphic options are also briefly discussed.
A new wavefront sensing approach, derived from the successful curvature wavefront sensing concept but using a nonlinear phase retrieval wavefront reconstruction scheme, is described. The nonlinear ...curvature wavefront sensor (nlCWFS) approaches the theoretical sensitivity limit imposed by fundamental physics by taking full advantage of wavefront spatial coherence in the pupil plane. Interference speckles formed by natural starlight encode wavefront aberrations with the sensitivity set by the telescope’s diffraction limitλ/D
λ
/
D
rather than the seeing limit of more conventional linear wavefront sensors (WFSs). Closed-loop adaptive optics simulations show that with an nlCWFS, a 100 nm rms wavefront error can be reached on an 8 m telescope on an
m
V
= 13
m
V
=
13
natural guide star. The nlCWFS technique is best suited for high precision adaptive optics on bright natural guide stars. It is therefore an attractive technique to consider for direct imaging of exoplanets and disks around nearby stars, where achieved performance is set by wavefront control accuracy and exquisite control of low-order aberrations is essential for high contrast coronagraphic imaging. Performance gains derived from simulations are shown, and approaches for high speed reconstruction algorithms are briefly discussed.
Wavefront sensing and control are important for enabling one of the key advantages of using large apertures, namely higher angular resolution. Pyramid wavefront sensors are becoming commonplace in ...new instrument designs owing to their superior sensitivity. However, one remaining roadblock to their widespread use is the fabrication of the pyramidal optic. This complex optic is challenging to fabricate due to the pyramid tip, where four planes need to intersect at a single point. Thus far, only a handful of these have been produced due to the low yields and long lead times. To address this, we present an alternative implementation of the pyramid wavefront sensor which relies instead on two roof prisms. Such prisms are easy and inexpensive to source. We demonstrate the successful operation of the roof prism pyramid wavefront sensor on an 8 m class telescope, at visible and near-infrared wavelengths, for the first time using a SAPHIRA HgCdTe detector without modulation for a laboratory demonstration, and elucidate how this sensor can be used more widely on wavefront control test benches and instruments.
ABSTRACT
The characterization of exoplanets is critical to understanding planet diversity and formation, their atmospheric composition, and the potential for life. This endeavour is greatly enhanced ...when light from the planet can be spatially separated from that of the host star. One potential method is nulling interferometry, where the contaminating starlight is removed via destructive interference. The GLINT instrument is a photonic nulling interferometer with novel capabilities that has now been demonstrated in on-sky testing. The instrument fragments the telescope pupil into sub-apertures that are injected into waveguides within a single-mode photonic chip. Here, all requisite beam splitting, routing, and recombination are performed using integrated photonic components. We describe the design, construction, and laboratory testing of our GLINT pathfinder instrument. We then demonstrate the efficacy of this method on sky at the Subaru Telescope, achieving a null-depth precision on sky of ∼10−4 and successfully determining the angular diameter of stars (via their null-depth measurements) to milliarcsecond accuracy. A statistical method for analysing such data is described, along with an outline of the next steps required to deploy this technique for cutting-edge science.
Abstract Recent high-sensitivity observations reveal that accreting giant planets embedded in their parental circumstellar disks can emit H α at their final formation stages. While the origin of this ...emission is not yet determined, magnetospheric accretion is currently the most plausible hypothesis. In order to test this hypothesis further, we develop a simplified but physics-based model and apply it to our observations taken toward HD 163296 with Subaru/SCExAO+VAMPIRES. We specify under which conditions embedded giant planets can undergo magnetospheric accretion and emit hydrogen lines. We find that when the stellar accretion rates are high, magnetospheric accretion becomes energetic enough to self-regulate the resulting emission. On the other hand, when massive planets are embedded in disks with low accretion rates, earlier formation histories determine whether magnetospheric accretion occurs. We explore two different origins for the hydrogen emission lines (magnetospheric accretion flow heated by accretion-related processes versus planetary surfaces via accretion shock). The corresponding relationships between the accretion and line luminosities dictate that the emission from accretion flow achieves higher line flux than that from accretion shock, and the flux decreases with increasing wavelengths (i.e., from H α to Pa β and up to Br γ ). Our observations do not detect any point-like source emitting H α , and they are used to derive the 5 σ detection limit. The observations are therefore not sensitive enough, and a reliable examination of our model becomes possible when the observational sensitivity is improved by a factor of 10 or more. Multi-band observations increase the possibility of efficiently detecting embedded giant planets and carefully determining the origin of the hydrogen emission lines.
Abstract We present a multiwavelength (1.16–2.37 μ m) view of the debris disk around BD+45 ° 598, using the Subaru Coronagraphic Extreme Adaptive Optics system paired with the Coronagraphic High ...Angular Resolution Imaging Spectrograph. With an assumed age of 23 Myr, this source allows us to study the early evolution of debris disks and search for forming planets. We fit a scattered light model to our disk using a differential evolution algorithm, and constrain its geometry. We find the disk to have a peak density radius of R 0 = 109.6 au, an inclination of i = 88.1°, and position angle PA = 111.0°. While we do not detect a substellar companion in the disk, our calculated contrast limits indicate sensitivity to planets as small as ∼10 M Jup at a projected separation of 12 au of the star, and as small as ∼4 M Jup beyond 38 au. When measuring intensity as a function of wavelength, the disk color constrains the minimum dust grain size within a range of ∼0.13–1.01 μ m.
Photometric and astrometric calibration of high-contrast images is essential for the characterization of companions at small angular separation from their stellar host. The main challenge to ...performing accurate relative photometry and astrometry of high-contrast companions with respect to the host star is that the central starlight cannot be directly used as a reference, as it is either blocked by a coronagraphic mask or saturating the detector. Our approach is to add fiducial incoherent faint copies of the host star in the image plane and alternate the pattern of these copies between exposures. Subtracting two frames with different calibration patterns removes measurement bias due to static and slowly varying incoherent speckle halo components, while ensuring that calibration references are inserted on each frame. Each calibration pattern is achieved by high-speed modulation of a pupil-plane deformable mirror to ensure incoherence. We implemented the technique on-sky on the Subaru Coronagraphic Extreme Adaptive Optics instrument with speckles which were of the order of 103 times fainter than the central host. The achieved relative photometric and astrometric measurement precisions for 10 s exposure were respectively 5% and 20 milliarcsecond. We also demonstrate, over a 540 s measurement span, that residual photometric and astrometric errors are uncorrelated in time, indicating that residual noise averages as the inverse square root of the number of exposures in longer time-series data sets.
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