A Statistical Framework for the Utilization of Simultaneous Pupil Plane and Focal Plane Telemetry for Exoplanet Imaging, Part I: Accounting for Aberrations in Multiple Planes
A new generation of telescopes with mirror diameters of 20 m or more, called extremely large telescopes (ELTs) has the potential to provide unprecedented imaging and spectroscopy of exo-planetary systems, if the difficulties in achieving the extremely high dynamic range required to differentiate the planetary signal from the star can be overcome to a sufficient degree. Fully utilizing the potential of ELTs for exoplanet imaging will likely require simultaneous and self-consistent determination of both the planetary image and the unknown aberrations in multiple planes of the optical system, using statistical inference based on the wavefront sensor and science camera data streams. This approach promises to overcome the most important systematic errors inherent in the various schemes based on differential imaging, such as ADI and SDI. This paper is the first in a series on this subject, in which a formalism is established for the exoplanet imaging problem, setting the stage for the statistical inference methods to follow in the future. Every effort has been made to be rigorous and complete, so that validity of approximations to be made later can be assessed. Here, the polarimetric image is expressed in terms of aberrations in the various planes of a polarizing telescope with an adaptive optics system. Further, it is shown that current methods that utilize focal plane sensing to correct the speckle field, e.g., electric field conjugation, rely on the tacit assumption that aberrations on multiple optical surfaces can be represented as aberration on a single optical surface, ultimately limiting their potential effectiveness for ground-based astronomy.
This report models the refractive index above a telescope site by an atmosphere with exponential decay of the refractive index (susceptibility) as a function of altitude. The air is represented as a spherical hull around the surface. We compute (i) the differential zenith angle -- the difference between the actual zenith angle at arrival of rays at the telescope and a hypothetical zenith angle without the atmosphere -- and (ii) the optical path length distribution of rays at arrival in the entrance pupil as a function of the distance to the pupil center. The key technique in this work is to ex…
The imaging and characterization of a larger range of exoplanets, down to young Jupiters and exo-Earths will require accessing very high contrasts at small angular separations with an increased robustness to aberrations, three constraints that drive current instrumentation development. This goal relies on efficient coronagraphs set up on extremely large diameter telescopes such as the Thirty Meter Telescope (TMT), the Giant Magellan Telescope (GMT), or the Extremely Large Telescope (ELT). However, they tend to be subject to specific aberrations that drastically deteriorate the coronagraph perf…
TianQin is a planned Chinese space-based gravitational wave (GW) observatory with a frequency band of 10-4 to 1Hz. Optical telescopes are essential for the delivery of the measurement beam to support a precise distance measurement between pairs of proof masses. As the design is driven by the interferometric displacement sensitivity requirements, the stability control of optical path length (OPL) is extremely important beyond the traditional requirement of diffraction-limited imaging quality. In a telescope system, the recurring tilt-to-length (TTL) coupling noise arises from the OPL variation …
A Statistical Framework for Utilization of Simultaneous Pupil Plane and Focal Plane Telemetry for Exoplanet Imaging, Part II: The Science Camera Image as a Function of the Wavefront Sensor Field
Telescope pupil fragmentation from spiders generates specific aberrations observed at various telescopes and expected on the large telescopes under construction. This so-called island effect induces differential pistons, tips and tilts on the pupil petals, deforming the instrumental PSF, and is one of the main limitations to the detection of exoplanets with high-contrast imaging. These aberrations have different origins such as the low-wind effect or petaling errors in the adaptive-optics reconstruction. In this paper, we propose to alleviate the impact of the aberrations induced by island eff…
We present early laboratory simulations and extensive on-sky tests validating of the performance of a shaped pupil coronagraph (SPC) behind an extreme-AO corrected beam of the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system. In tests with the SCExAO internal source/wavefront error simulator, the normalized intensity profile for the SPC degrades more slowly than for the Lyot coronagraph as low-order aberrations reduce the Strehl ratio from extremely high values (S.R. $\sim$ 0.93--0.99) to those characteristic of current ground-based extreme AO systems (S.R. $\sim$ 0.74--0.93) and t…
Second-generation exoplanet imagers using extreme adaptive optics and coronagraphy have demonstrated their great potential for studying close circumstellar environments and for detecting new companions and helping to understand their physical properties. However, at very small angular separation, their performance in contrast is limited by several factors: diffraction by the complex telescope pupil not perfectly canceled by the coronagraph, residual dynamic wavefront errors, chromatic wavefront errors, and wavefront errors resulting from noncommon path aberrations (NCPAs). In a previous work, …
Polarization aberrations in next-generation Giant Segmented Mirror Telescopes (GSMTs). II. Influence of segment-to-segment coating variations on high-contrast imaging and polarimetry
Focal-plane wavefront sensing with moderately broadband light using a short multi-mode fiber
Instituto de Astrofísica de Canarias (IAC)（加那利天体物理研究所）；Universidad de La Laguna (ULL)（拉古纳大学）；University of Hawai’i at Manoa Institute for Astronomy（夏威夷大学马诺阿分校天文研究所）
The Low Wind Effect (LWE) refers to a phenomenon that occurs when the wind speed inside a telescope dome drops below $3$m/s creating a temperature gradient near the telescope spider. This produces phase discontinuities in the pupil plane that are not detected by traditional Adaptive Optics (AO) systems such as the pyramid wavefront sensor or the Shack-Hartmann. Considering the pupil as divided in 4 quadrants by regular spiders, the phase discontinuities correspond to piston, tip and tilt aberrations in each quadrant of the pupil. Uncorrected, it strongly decreases the ability of high contrast …
We present an enhanced version of the Zernike wavefront sensor, that simultaneously measures phase and amplitude aberrations. The 'vector-Zernike' wavefront sensor consists of a patterned liquid-crystal mask, which imposes $\pm π/2$ phase on the point spread function core through the achromatic geometric phase acting with opposite sign on opposite circular polarizations. After splitting circular polarization, the ensuing pupil intensity images are used to reconstruct the phase and the amplitude of the incoming wavefront. We demonstrate reconstruction of the complex wavefront with monochromatic…
GRAVITY acquisition camera implements four optical functions to track multiple beams of Very Large Telescope Interferometer (VLTI): a) pupil tracker: a $2 \times 2$ lenslet images four pupil reference lasers mounted on the spiders of telescope secondary mirror; b) field tracker: images science object; c) pupil imager: reimages telescope pupil; d) aberration tracker: images a Shack-Hartmann. The estimation of beam stabilization parameters from the acquisition camera detector image is carried out, for every 0.7 s, with a dedicated data reduction software. The measured parameters are used in: a) …
Atmospheric turbulence and precise measurement of the astrometric baseline vector between any two telescopes are two major challenges in implementing phase referenced interferometric astrometry and imaging. They limit the performance of a fibre-fed interferometer by degrading the instrument sensitivity and astrometric measurements precision and by introducing image reconstruction errors due to inaccurate phases. A multiple beam acquisition and guiding camera was built to meet these challenges for a recently commissioned four beam combiner instrument, GRAVITY, at the ESO Very Large Telescope In…
Imaging faint objects, such as exoplanets or disks, around nearby stars is extremely challenging because host star images are dominated by the telescope diffraction pattern. Using a coronagraph is an efficient solution for removing diffraction but requires an incoming wavefront with good quality to maximize starlight rejection. On the ground, the most advanced exoplanet imagers use extreme adaptive optics (ExAO) systems that are based on a deformable mirror (DM) with a large number of actuators to efficiently compensate for high-order aberrations and provide diffraction-limited images. While s…
Context. Since 1995 and the first discovery of an exoplanet orbiting a main-sequence star, 4000 exoplanets have been discovered using several techniques. However, only a few of these exoplanets were detected through direct imaging. Indeed, the imaging of circumstellar environments requires high-contrast imaging facilities and accurate control of wavefront aberrations. Ground-based planet imagers such as VLT/SPHERE or Gemini/GPI have already demonstrated great performance. However, their limit of detection is hampered by suboptimal correction of aberrations unseen by adaptive optics (AO). Aims.…
The Space Coronagraph Optical Bench (SCoOB): 11. Modeling and correction of chromatic aberrations
Steward Observatory, University of Arizona（亚利桑那大学斯图尔德天文台）；James C. Wyant College of Optical Sciences, University of Arizona（亚利桑那大学詹姆斯·C·怀安特光学科学学院）；Department of Physics, University of California, Santa Barbara（加州大学圣塔芭芭拉分校物理系）
Island effect (IE) aberrations are induced by differential pistons, tips, and tilts between neighboring pupil segments on ground-based telescopes, which severely limit the observations of circumstellar environments on the recently deployed exoplanet imagers (e.g., VLT/SPHERE, Gemini/GPI, Subaru/SCExAO) during the best observing conditions. Caused by air temperature gradients at the level of the telescope spiders, these aberrations were recently diagnosed with success on VLT/SPHERE, but so far no complete calibration has been performed to overcome this issue. We propose closed-loop focal plane …
The Habitable Exoplanet Imaging Mission (HabEx) concept requires an optical coronagraph that provides deep starlight suppression over a broad spectral bandwidth, high throughput for point sources at small angular separation, and insensitivity to temporally-varying, low-order aberrations. Vortex coronagraphs are a promising solution that perform optimally on off-axis, monolithic telescopes and may also be designed for segmented telescopes with minor losses in performance. We describe the key advantages of vortex coronagraphs on off-axis telescopes: 1) Unwanted diffraction due to aberrations is …
Future large segmented space telescopes and their coronagraphic instruments are expected to provide the resolution and sensitivity to observe Earth-like planets with a 10^10 contrast ratio at less than 100 mas from their host star. Advanced coronagraphs and wavefront control methods will enable the generation of high-contrast dark holes in the image of an observed star. However, drifts in the optical path of the system will lead to pointing errors and other critical low-order aberrations that will prevent maintenance of this contrast. To measure and correct for these errors, we explore the use…
Field of View (FoV) and contrast limitations of stellar interferometers have been the scope of numerous publications for more than thirty years. Recently, this topic regained some interest since long-baseline terrestrial interferometers or space borne nulling interferometers are envisioned for detecting and characterizing extra-solar planets orbiting in the habitable zone of their parent star. This goal supposes to achieving sufficient contrast ratio in the high angular frequency domain, thus on the whole interferometer FoV. In this paper are reviewed some of the contrast and FoV limiting fact…
Imaging exo-Earths is an exciting but challenging task because of the 10^-10 contrast ratio between these planets and their host star at separations narrower than 100 mas. Large segmented aperture space telescopes enable the sensitivity needed to observe a large number of planets. Combined with coronagraphs with wavefront control, they present a promising avenue to generate a high-contrast region in the image of an observed star. Another key aspect is the required stability in telescope pointing, focusing, and co-phasing of the segments of the telescope primary mirror for long-exposure observa…
High-contrast imager for complex aperture telescopes (HiCAT): 11. System-level demonstration of the Apodized Pupil Lyot Coronagraph with a segmented aperture in air
One of the major difficulties limiting ground-based direct imaging of exoplanets with adaptive optics is quasi-static speckles in the science camera (SC) that obscure the planetary image. These speckles are caused by aberrations, called non-common path aberrations (NCPA), that are not corrected in the adaptive optics loop, and all attempts to subtract them in post-processing have been problematic. The method of Frazin (2013) (F13) uses simultaneous millisecond telemetry from wavefront sensor (WFS) and the SC to estimate the both the NCPA and the exoplanet image in a self-consistent manner. Rod…
HiCAT is a high-contrast imaging testbed designed to provide complete solutions in wavefront sensing, control and starlight suppression with complex aperture telescopes. The pupil geometry of such observatories includes primary mirror segmentation, central obstruction, and spider vanes, which make the direct imaging of habitable worlds very challenging. The testbed alignment was completed in the summer of 2014, exceeding specifications with a total wavefront error of 12nm rms over a 18mm pupil. The installation of two deformable mirrors for wavefront control is to be completed in the winter of…
JWST telemetry combined with active coronagraphy: raw contrast predictions for exoplanet imaging with the Habitable Worlds Observatory
Space Telescope Science Institute（太空望远镜科学研究所）；University of California Santa Cruz（加州大学圣克鲁兹分校）；Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange（蔚蓝海岸大学，蔚蓝海岸天文台，法国国家科学研究中心，拉格朗日实验室）；Max Planck Institute for Astronomy（马克斯·普朗克天文学研究所）
Instrumental aberrations strongly limit high-contrast imaging of exoplanets, especially when they produce quasistatic speckles in the science images. With the help of recent advances in deep learning, we have developed in previous works an approach that applies convolutional neural networks (CNN) to estimate pupil-plane phase aberrations from point spread functions (PSF). In this work we take a step further by incorporating into the deep learning architecture the physical simulation of the optical propagation occurring inside the instrument. This is achieved with an autoencoder architecture, w…
Coronagraph instruments on future space telescopes will enable the direct detection and characterization of Earth-like exoplanets around Sun-like stars for the first time. The quest for the optimal optical coronagraph designs has made rapid progress in recent years thanks to the Segmented Coronagraph Design and Analysis (SCDA) initiative led by the Exoplanet Exploration Program at NASA's Jet Propulsion Laboratory. As a result, several types of high-performance designs have emerged that make use of dual deformable mirrors to (1) correct for optical aberrations and (2) suppress diffracted starli…
Optimal Design of Shaped Pupil Lyot Coronagraph for China's Large Segmented Telescope
College of Future Technology, Peking University（北京大学未来技术学院）；Academy for Advanced Interdisciplinary Studies, Peking University（北京大学前沿交叉学科研究院）；National Biomedical Imaging Center, Peking University（北京大学国家生物医学成像中心）；Department of Astronomy, Xiamen University（厦门大学天文学系）；State Key Laboratory of Dark Matter Physics, Tsung-Dao Lee Institute & School of Physics and Astronomy, Shanghai Jiao Tong University（上海交通大学物理与天文学院李政道研究所暗物质物理全国重点实验室）；Department of Astronomy, School of Physics, Peking University（北京大学物理学院天文学系）；The Kavli Institute for Astronomy and Astrophysics, Peking University（北京大学科维理天文与天体物理研究所）；CAS Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台光学天文重点实验室）
The GRAVITY instrument on the ESO VLTI pioneers the field of high-precision near-infrared interferometry by providing astrometry at the $10 - 100\,μ$as level. Measurements at such high precision crucially depend on the control of systematic effects. Here, we investigate how aberrations introduced by small optical imperfections along the path from the telescope to the detector affect the astrometry. We develop an analytical model that describes the impact of such aberrations on the measurement of complex visibilities. Our formalism accounts for pupil-plane and focal-plane aberrations, as well a…
The performance of high-contrast imaging instruments is limited by wavefront errors, in particular by non-common path aberrations (NCPAs). Focal-plane wavefront sensing (FPWFS) is appropriate to handle NCPAs because it measures the aberration where it matters the most, that is to say at the science focal plane. Phase retrieval from focal-plane images results, nonetheless, in a sign ambiguity for even modes of the pupil-plane phase. The phase diversity methods currently used to solve the sign ambiguity tend to reduce the science duty cycle, that is, the fraction of observing time dedicated to s…
Directly imaging exoplanets is challenging because quasi-static phase aberrations in the pupil plane (speckles) can mimic the signal of a companion at small angular separations. Kernel phase, which is a generalization of closure phase (known from sparse aperture masking), is independent of pupil plane phase noise to second order and allows for a robust calibration of full pupil, extreme adaptive optics observations. We applied kernel phase combined with a principal component based calibration process to a suitable but not optimal, high cadence, pupil stabilized L' band ($3.8~μ\text{m}$) data s…
The leading difficulty in achieving the contrast necessary to directly image exoplanets and associated structures (eg. protoplanetary disks) at wavelengths ranging from the visible to the infrared are quasi-static speckles, and they are hard to distinguish from planets at the necessary level of precision. The source of the quasi-static speckles is hardware aberrations that are not compensated by the adaptive optics system. These aberrations are called non-common path aberrations (NCPA). In 2013, Frazin showed how, in principle, simultaneous millisecond (ms) telemetry from the wavefront sensor …
Caractérisation et intégration de composants d'optique intégrée sur l'interféromètre fibré FIRST au télescope Subaru pour l'étude des protoplanètes en accrétion
Oncoming exoplanet spectro-imagers like the Planetary Camera and Spectrograph (PCS) for the Extremely Large Telescope (ELT) will aim for a new class of exoplanets, including Earth-like planets evolving around M dwarfs i.e., closer than 0.1'' with contrasts around 10^-8. This can be achieved with coronagraphs to modulate the wavefront. Classical coronagraphs are not optimal: 1) they impose a planetary photon loss, which is particularly problematic when the instrument includes a high spectral-resolution spectrograph, 2) some aberrations such as the missing segments of the ELT are dynamic and not…
Directly imaging and characterizing Earth-like exoplanets is a tremendously difficult instrumental challenge. Present coronagraphic systems have yet to achieve the required $10^{-10}$ broadband contrast in a laboratory environment, but promising progress towards this goal continues. A new approach to starlight suppression is the use of a single-mode fiber behind a coronagraph. By using deformable mirrors to create a mismatch between incoming starlight and the fiber mode, a single-mode fiber can be turned into an integral part of the starlight suppression system. In this paper, we present simul…
