Direct imaging of exoplanets relies on complex wavefront sensing and control architectures. In addition to fast adaptive optics systems, most of the future high-contrast imaging instruments will soon be equipped with focal plane wavefront sensing algorithms. These techniques use the science detector to estimate the static and quasi-static aberrations induced by optical manufacturing defects and system thermal variations. Pair-wise probing (PWP) has been the most widely used, especially for space-based application and will be tested at contrast levels of ∼1e-9 on-sky along with the future coronagraph instrument onboarding the Roman Space Telescope. This algorithm leans on phase diversities applied on the deformable mirror that are recorded in pairs. A minimum of two pairs of probes are required per bandwidth. An additional unprobed image is also recorded to verify the convergence rate of the correction. Before PWP, Borde & Traub proposed a similar algorithm that takes advantage of the unprobed image in the estimation process to get rid of the pair diversity requirement. In this work, we theoretically show that this latter technique should be more efficient than PWP when the convergence time is not limited by photon noise. We then present its performance and practical limitations on coronagraphic testbeds at JPL and exhibit a first on-sky control of non-common path aberrations with such method on VLT/SPHERE.
The spectroscopic study of mature giant planets and low mass planets (Neptune-like, Earth-like) requires instruments capable of achieving very high contrasts (10−10 − 10−11) at short angular separations. To achieve such high performance on a real instrument, many limitations must be overcome: complex component defects (coronagraph, deformable mirror), optical aberrations and scattering, mechanical vibrations and drifts, polarization effects, etc. To study the overall impact on a complete system representative of high contrast instruments, we have developed a test bench at Paris Observatory, called THD2. In this paper, we focus on the polarization effects that are present on the bench which creates differential aberrations between the two linear polarization states. We compare the recorded beam positions of the two polarization states with the predicted from the Goos-H¨anchen and Imbert-Fedorov effects, both of which cause spatial shifts and angular deviations of the beam, longitudinal and transverse respectively. Although these effects have already been studied in the literature from the optical and quantum mechanical points of view, their measurement and impact on a complete optical bench are rather rare, although they are crucial for high-contrast instruments. After describing the Goos-H¨anchen and Imbert-Fedorov effects and estimating their amplitude on the THD2 bench, we present the protocol we used to measure these effects of polarization on the light beam. We compare predictions and measurements and we conclude on the most limiting elements on our bench polarization-wise.
SAXO+ is a planned enhancement of the existing SAXO, the VLT/ SPHERE adaptive optics system, deployed on ESO’s Very Large Telescope. This upgrade is designed to significantly enhance the instrument’s capacity to detect and analyze young Jupiter-like planets. The pivotal addition in SAXO+ is a second-stage adaptive optics system featuring a dedicated near-infrared pyramid wavefront sensor and a second deformable mirror. This secondary stage is strategically integrated to address any residual wavefront errors persisting after the initial correction performed by the current primary AO loop, SAXO. However, several recent studies clearly showed that in good conditions, even in the current system SAXO, non-common path aberrations (NCPAs) are the limiting factor of the final normalized intensity in focal plane, which is the final metric for ground-based high-contrast instruments. This is likely to be even more so the case with the new AO system, with which the AO residuals will be minimized. Several techniques have already been extensively tested on SPHERE in internal source and/or on-sky and will be presented in this paper. However, the use of a new type of sensor for the second stage, a pyramid wavefront sensor, will likely complicate the correction of these aberrations. Using an end-to-end AO simulation tool, we conducted simulations to gauge the effect of measured SPHERE NCPAs in the coronagraphic image on the second loop system and their correction using focal plane wavefront sensing systems. We finally analyzed how the chosen position of SAXO+ in the beam will impact the evolution of the NCPAs in the new instrument.
Direct imaging is the primary technique currently used to detect young and warm exoplanets and understand their formation scenarios. The extreme flux ratio between an exoplanet and its host star requires the use of coronagraphs to attenuate the starlight and create high contrast images. However, their performance is limited by wavefront aberrations that cause stellar photons to leak through the coronagraph and on to the science detector preventing the observation of fainter extrasolar companions. The VLT/SPHERE instrument takes advantage of its efficient adaptive optics system to minimize dynamical aberrations to improve the image contrast. In good seeing conditions, the performance is limited by quasi-static aberrations caused by slowly varying aberrations and manufacturing defects in the optical components. The mitigation of these aberrations requires additional wavefront sensing and control algorithms to enhance the contrast performance of SPHERE. Dark hole algorithms initially developed for space-based application and recently performed on SPHERE calibration unit have shown significant improvement in contrast. This work presents a status update of dark hole algorithms applied on SPHERE and the results obtained during the on-sky tests performed on February 15th 2022.
SPHERE+ is a proposed upgrade of the SPHERE instrument on the ESO’s Very Large Telescope which aims at improving detection and characterization capabilities of young giant planets by means of a second-stage AO system, including dedicated wavefront sensor and deformable mirror to remove the residual wavefront errors left by the primary AO loop. This paper is focused on the numerical simulations of the second stage (SAXO+) and conclude on the impact of the main AO parameters used to build the design strategy.
SPHERE+ is a proposed upgrade of the SPHERE instrument at the VLT, which is intended to boost the current performances of detection and characterization for exoplanets and disks. SPHERE+ will also serve as a demonstrator for the future planet finder (PCS) of the European ELT. The main science drivers for SPHERE+ are 1/ to access the bulk of the young giant planet population down to the snow line (3 − 10 au), to bridge the gap with complementary techniques (radial velocity, astrometry); 2/ to observe fainter and redder targets in the youngest (1 − 10 Myr) associations compared to those observed with SPHERE to directly study the formation of giant planets in their birth environment; 3/ to improve the level of characterization of exoplanetary atmospheres by increasing the spectral resolution in order to break degeneracies in giant planet atmosphere models. Achieving these objectives requires to increase the bandwidth of the xAO system (from ~1 to 3 kHz) as well as the sensitivity in the infrared (2 to 3 mag). These features will be brought by a second stage AO system optimized in the infrared with a pyramid wavefront sensor. As a new science instrument, a medium resolution integral field spectrograph will provide a spectral resolution from 1000 to 5000 in the J and H bands. This paper gives an overview of the science drivers, requirements and key instrumental tradeoff that were done for SPHERE+ to reach the final selected baseline concept.
The search for Earth-like exoplanets requires high-contrast and high-angular resolution instruments, which designs can be very complex: they need an adaptive optics system to compensate for the effect of the atmospheric turbulence on image quality and a coronagraph to reduce the starlight and enable the companion imaging. During the instrument design phase and the error budget process, studies of performance as a function of optical errors are needed and require multiple end-to-end numerical simulations of wavefront errors through the optical system. In particular, the detailed analysis of long-exposure images enables to evaluate the image quality (photon noise level, impact of optical aberrations and of adaptive optics residuals, etc.). Nowadays simulating one long but finite exposure image means drawing several thousands of random frozen phase screens, simulating the image associated with each of them after propagation through the imaging instrument, and averaging all the images. Such a process is time consuming, demands a great deal of computer resources, and limits the number of parametric optimization. We propose an alternative and innovative method to directly express the statistics of ground-based images for long but finite exposure times. It is based on an analytical model, which only requires the statistical properties of the atmospheric turbulence. Such a method can be applied to optimize the design of future instruments such as SPHERE+ (VLT) or the planetary camera and spectrograph (PCS - ELT) or any ground-based instrument.
Direct imaging is crucial to increase our knowledge on extrasolar planetary systems. It can detect long orbits planets that are inaccessible by other methods and it allows the spectroscopic characterization of exoplanet’s athmospheres. During the past fewyears, several giant planetswere detected by direct imaging methods. Yet, as exoplanets are 103 to 1010 fainter than their host star in visible and near-infrared wavelengths, direct imaging requires extremely high contrast imaging techniques, especially to detect low-mass and mature exoplanets. Coronagraphs are used to reject the diffracted light of an observed star and obtain images of its circumstellar environment. Nevertheless, coronagraphs are efficient only if the wavefront is flat because aberrated wavefronts induce speckles in the focal plane which mask exoplanet images. Thus, wavefront sensors associated to deformable mirrors are mandatory to correct speckles by reducing aberrations. To test coronagraph techniques and focal plane wavefront sensors at very high contrast level, we developed the THD2 bench in the optical wavelengths. On the THD2 bench, we routinely reach 108 raw contrast level inside the dark hole over broadbands but this level is not sufficient to detect low-mass exoplanets. At this level, it seems that many experimental factors can affect the contrast and understanding which one is limiting the final detection contrast will be useful to upgrade the THD2 bench and to develop the next generation of space-based instruments (LUVOIR, HabEx) aiming to reach 10-10 contrast level. We started a complete study of the instrumental limitations of the THD2 bench, focusing on scattering which could add intensity on the detector or polarization effects and residual laboratory turbulences. In this paper, we present the methods used to estimate the amount of scattered light that reaches the final detector on the THD2 bench.
KEYWORDS: Coronagraphy, Space telescopes, Signal to noise ratio, Planets, Point spread functions, Stars, Wavefronts, Telescopes, Optical instrument design, Adaptive optics
The Optimal Optical Coronagraph (OOC) Workshop at the Lorentz Center in September 2017 in Leiden, the Netherlands gathered a diverse group of 25 researchers working on exoplanet instrumentation to stimulate the emergence and sharing of new ideas. In this first installment of a series of three papers summarizing the outcomes of the OOC workshop, we present an overview of design methods and optical performance metrics developed for coronagraph instruments. The design and optimization of coronagraphs for future telescopes has progressed rapidly over the past several years in the context of space mission studies for Exo-C, WFIRST, HabEx, and LUVOIR as well as ground-based telescopes. Design tools have been developed at several institutions to optimize a variety of coronagraph mask types. We aim to give a broad overview of the approaches used, examples of their utility, and provide the optimization tools to the community. Though it is clear that the basic function of coronagraphs is to suppress starlight while maintaining light from off-axis sources, our community lacks a general set of standard performance metrics that apply to both detecting and characterizing exoplanets. The attendees of the OOC workshop agreed that it would benefit our community to clearly define quantities for comparing the performance of coronagraph designs and systems. Therefore, we also present a set of metrics that may be applied to theoretical designs, testbeds, and deployed instruments. We show how these quantities may be used to easily relate the basic properties of the optical instrument to the detection significance of the given point source in the presence of realistic noise.
KEYWORDS: Visibility, Calibration, Wavefronts, Point spread functions, Wavefront sensors, Signal to noise ratio, Modulation transfer functions, Speckle, Stars, Telescopes
Exoplanet detection and characterization through extreme adaptive optics (ExAO) is a key science goal of future extremely large telescopes. This achievement, however, will be limited in sensitivity by both quasi-static wavefront errors and residual AO-corrected atmospheric wavefront errors. A solution to both of these problems is to use the science camera of an ExAO system as a wavefront sensor to perform a fast measurement and correction method to remove these aberrations as soon as they are detected. We have developed the framework for one such method, using the self-coherent camera (SCC), to be applied to ground-based telescopes, called Fast Atmospheric SCC Technique (FAST; Gerard et al., submitted). Our FAST solution requires an optimally designed coronagraph (the SCC FPM) and post-processing algorithm and is in principle able to reach a“raw” contrast of a few times the photon noise limit, continually improving with integration time. In this paper, we present new ongoing work in exploring the manufacturing limitations of the SCC FPM as well as a general framework to implement and optimize a FAST deformable mirror control loop.
While radial velocity and transit techniques are efficient to probe exoplanets with short orbits, the study of long-orbit planets requires direct imaging and coronagraphic techniques. However, the coronagraph must deal with planets that are 104 to 1010 fainter than their hosting star at a fraction of arcsecond, requiring efficient coronagraphs at short angular separation. Phase masks proved to be a good solution in monochromatic or limited spectral bandwidth but expansion to broadband requires complex phase achromatization. Solutions use photonic crystals, subwavelength grating or liquid crystal polymers but their manufacturing remains complex. An easier solution is to use photolithography and reactive ion etching and to optimize the azimuthal phase distribution like achieved in the six-level phase mask (SLPM) coronagraph (Hou et al. 2014). We present here the laboratory results of two SLPM coronagraphs enabling high-contrast imaging in wide-band. The SLPM is split in six sectors with three different depths producing three levels of optical path difference and yielding to uniform phase shifts of 0, π or 2π at the specified wavelength. Using six sectors instead of four sectors enables to mitigate the chromatic effects of the SLPM compared to the FQPM (Four-Quadrant Phase Mask) while keeping the manufacturing easy. Following theoretical developments achieved by University of Shanghai and based on our previous experience to fabricate FQPM components, we have manufactured SLPM components by reactive ion etching at Paris Observatory and we have tested it onto the THD2 facility at LESIA. The THD2 bench was built to study and compare high-contrast imaging techniques in the context of exoplanet imaging. The bench allows reducing the starlight below a 10−8 contrast level in visible/near-infrared. In this paper, we show that the SLPM is easy to fabricate at low cost and is easy to implement with a unique focal plane mask and no need of pupil apodization. Detection of a planet can be achieved at small inner working angle down to 1 λ/D. The on-axis attenuation of the best SLPM component reaches 2 × 10−5 at λ = 800 nm and is better than 10−4 in intensity over a 10% spectral bandwidth. Along the diagonal transition, we show that the off-axis transmission is attenuated by less than 3% over a 10% bandwidth and will need to be calibrated. Any etching imperfections can affect the SLPM performance, by lowering the on-axis attenuation and by changing the optimal wavelength. Despite few nanometers of uncertainty for etching the depths, we show that this first component can provide a high-contrast attenuation in laboratory
New space missions dedicated to exoplanet imaging will rely on coronagraphs to address the high contrast between the stars and their environments. In order to avoid the image of planets to be lost in post-coronagraphic starlight residuals, high precision wavefront sensing and control is a key element to these missions. We present recent results of simultaneous post-coronagraphic phase and amplitude sensing obtained on the THD bench using the coronagraphic phase diversity. We also present results of simulation studies on the non-linear dark hole technique to assess the main limitations of this technique. Finally, we present a first experimental validation of its principle and corroborate expectations on its speed of convergence. These results suggest that the non-linear dark hole is a good candidate for wave-front control for future space-based exoplanet imaging missions, where fast techniques to produce deep dark holes are of paramount importance.
In context of wavefront control for high-contrast coronagraphy, we present the spectral characterization of speckles generated by deformable mirrors (DMs) at various distances downstream of the pupil plane. We have measured the focal-plane electric field at 600 to 690nm of speckles generated with an in- and out-of-pupil DM on a highcontrast testbed using a self-coherent camera. These measurements are analyzed analytically and are compared to numerical end-to-end simulations of the setup. This will aid in the design of wavefront correction algorithms with multiple DMs.
High-contrast imaging (HCI) techniques appear like the best solutions to directly characterize the atmosphere of large orbit planets and planetary environments. In the last 20 years, different HCI solutions have been proposed based on coronagraphs. Some of them have been characterized in the laboratory or even on the sky. The optimized performance of these coronagraphs requires a perfect wavefront unreachable without active control of the complete electrical field (phase and amplitude) at the entrance of the instrument. While the correction of the phase aberrations is straight forward using deformable mirrors (DM), correcting amplitude defects is complex and still under study at the laboratory level. The next generation of HCI instrument either for ground-based (PCS instrument for ELT) or space-based (LUVOIR, HabEx) telescopes will require a practical and operational solution for amplitude corrections. The implementation of a DM located at a finite distance from the pupil is a simple solution that has been chosen by most of the projects. There have been only a few investigations on the optimization of the mirror positions for dedicated optical designs. In this paper, we give an intuitive approach that helps defining the best deformable mirror position in an instrument. Then, we describe its application to the THD2 and the performance in the laboratory that reaches a contrast level below 10-8 at distance larger than 6 λ/D.
The Optimal Optical Coronagraph (OOC) Workshop held at the Lorentz Center in September 2017 in Leiden, the Netherlands, gathered a diverse group of 25 researchers working on exoplanet instrumentation to stimulate the emergence and sharing of new ideas. In this second installment of a series of three papers summarizing the outcomes of the OOC workshop, we present an overview of common path wavefront sensing/control and Coherent Differential Imaging techniques, highlight the latest results, and expose their relative strengths and weaknesses. We layout critical milestones for the field with the aim of enhancing future ground/space based high contrast imaging platforms. Techniques like these will help to bridge the daunting contrast gap required to image a terrestrial planet in the zone where it can retain liquid water, in reflected light around a G type star from space.
The Optimal Optical CoronagraphWorkshop at the Lorentz Center in September 2017 in Leiden, the Netherlands gathered a diverse group of 30 researchers working on exoplanet instrumentation to stimulate the emergence and sharing of new ideas. This contribution is the final part of a series of three papers summarizing the outcomes of the workshop, and presents an overview of novel optical technologies and systems that are implemented or considered for high-contrast imaging instruments on both ground-based and space telescopes. The overall objective of high contrast instruments is to provide direct observations and characterizations of exoplanets at contrast levels as extreme as 10-10. We list shortcomings of current technologies, and identify opportunities and development paths for new technologies that enable quantum leaps in performance. Specifically, we discuss the design and manufacturing of key components like advanced deformable mirrors and coronagraphic optics, and their amalgamation in "adaptive coronagraph" systems. Moreover, we discuss highly integrated system designs that combine contrast-enhancing techniques and characterization techniques (like high-resolution spectroscopy) while minimizing the overall complexity. Finally, we explore extreme implementations using all-photonics solutions for ground-based telescopes and dedicated huge apertures for space telescopes.
S4EI (Spectral Sampling with Slicer for Stellar and Extragalactical Instrumentation) is a new concept for extending Multichannel Subtractive Double Pass (ie S4I - Spectral Sampling with Slicer for Solar Instrumentation) to night-time astronomy. The Multichannel Subtractive Double Pass (MSDP) spectrographs have been widely used in solar spectroscopy because of their ability to provide an excellent compromise between field of view and the spatial and spectral resolutions. Compared with other spectrographs, MSDP can deliver simultaneous monochromatic images without any time-scanning requirements (as the standard Fabry-Perot), with limited loss of flux. Spatial resolution is the same as for an Imager given by the telescope: it can be very high. It is based on new generation reflecting plane image slicers working with large apertures specific to night-time telescopes. The resulting design could be potentially very attractive and innovative for different domains of astronomy, e.g., the simultaneous spatial mapping of accurately flux-calibrated emission lines between OH sky lines in extragalactic astronomy or the simultaneous imaging of stars, exoplanets and interstellar medium. The determination of physical and chemical properties of galaxies needs to observe several emission lines at different wavelengths. The combination of these lines gives access to the distribution in dust, star formation rate, metallicity, the kinematics or even to the electron density of the gas in the galaxies. The spatial resolution of MSDP allows, like the 3D or integral field spectrographs the construction of spatial distribution maps. The advantage of S4EI is that by measuring simultaneously the different lines, the relative errors of the flux calibration between the different wavelengths of the lines are potentially limited by the uncertainty of the calibration source used, which is expected to significantly reduce the associated errors and thus increase the precision and accuracy of estimates.
The final performance of current and future instruments dedicated to exoplanet detection and characterization
is limited by intensity residuals in the scientific image plane, which originate in uncorrected optical aberrations.
In order to reach very high contrasts, these aberrations needs to be compensated for. We have proposed a focalplane
wave-font sensor called COFFEE (for COronagraphic Focal-plane wave-Front Estimation for Exoplanet
detection), which consists in an extension of conventional phase diversity to a coronagraphic system. In this
communication, we study the extension of COFFEE to the joint estimation of the phase and the amplitude in
the context of space-based coronagraphic instruments: we optimize the diversity phase in order to minimize the
reconstruction error; we also propose and optimize a novel low-amplitude high-frequency diversity that should
allow the phase-diverse images to still be used for science. Lastly, we perform a first experimental validation of
COFFEE in the very high, space-like contrast conditions of the THD bench and show that COFFEE is able to
distinguish between phase and amplitude aberrations.
The major source of noise in high-contrast imaging is the presence of slowly evolving speckles that do not average with time. The temporal stability of the point-spread-function (PSF) is therefore critical to reach a high contrast with extreme adaptive optics (XAO) instruments. Understanding on which timescales the PSF evolves and what are the critical parameters driving the speckle variability allow to design an optimal observing strategy and data reduction technique to calibrate instrumental aberrations and reveal faint astrophysical sources. We have obtained a series of 52 min, AO-corrected, coronagraphically occulted, high-cadence (1.6Hz), H-band images of the star HR 3484 with the SPHERE (Spectro-Polarimeter High-contrast Exoplanet REsearch1) instrument on the VLT. This is a unique data set from an XAO instrument to study its stability on timescales as short as one second and as long as several tens of minutes. We find different temporal regimes of decorrelation. We show that residuals from the atmospheric turbulence induce a fast, partial decorrelation of the PSF over a few seconds, before a transition to a regime with a linear decorrelation with time, at a rate of several tens parts per million per second (ppm/s). We analyze the spatial dependence of this decorrelation within the well-corrected radius of the adaptive optics system and show that the linear decorrelation is faster at short separations. Last, we investigate the influence of the distance to the meridian on the decorrelation.
The goal of a coronagraph is to reduce the flux of a bright object (e.g. a star) in order to distinguish its faint neighborhood (e.g. exoplanets and disks). In this context, we proposed one coronagraph that uses a four quadrant phase mask (FQPM). Since 2000, we fabricated several monochromatic FQPM working in visible and near-infrared light at the Paris Observatory. We have developed systematic procedures for fabrication and characterization of the phase masks. Visual inspections with an optical microscope are performed for every component and a coronagraphic performance measurement based on inclination of the component is done on a dedicated bench that is set up in a clean room. This procedure gives a quick feedback on the quality and performance of the component. Depending on the results, images of the central transition can be recorded with an electron microscope to understand the limitations of the fabrication process. This procedure allowed us to understand the influence of various parameters such as the width of the transitions between the quadrants, the alignment of the transitions or the step depth. Based on these results, we modified the mask design and the fabrication process to improve our success rate to nearly 100% when building a FQPM for any given optimal wavelength in visible or near-infrared. Moreover, we improved the performance of the components, reaching attenuations of more than 20,000 on the central peak in raw images for most coronagraphs. The best of these components are now used on the THD bench, an optical/NIR bench developed for the study of high contrast imaging techniques, reaching 10-8 contrast level routinely.
Direct imaging of exoplanets is very attractive but challenging and specific instruments like Sphere (VLT) or GPI (Gemini) are required to provide contrasts up to 16-17 magnitudes at a fraction of arcsec. To reach higher contrasts and detect fainter exoplanets, more-achromatic coronagraphs and a more-accurate wavefront control are needed. We already demontrated contrasts of ~10-8 at ~4 λ/D at 635nm using a four quadrant phase mask and a self-coherent camera on our THD bench in laboratory. In this paper, we list the different techniques that were tested on the THD bench in monochromatic and polychromatic lights. Then, we present the upgraded version of the THD bench that includes several deformable mirrors for correcting phase and amplitude simultaneously and obtain a field-of-view covering the complete 360 degrees arouns the star with contrasts down to ~10-8 -10-9.
Detecting light reflected from exoplanets by direct imaging is the next major milestone in the search for, and characterization of, an Earth twin. Due to the high-risk and cost associated with satellites and limitations imposed by the atmosphere for ground-based instruments, we propose a bottom-up approach to reach that ultimate goal with an endeavor named MAPLE. MAPLE first project is a stratospheric balloon experiment called MAPLE-50. MAPLE-50 consists of a 50 cm diameter off-axis telescope working in the near-UV. The advantages of the near-UV are a small inner working angle and an improved contrast for blue planets. Along with the sophisticated tracking system to mitigate balloon pointing errors, MAPLE-50 will have a deformable mirror, a vortex coronograph, and a self-coherent camera as a focal plane wavefront-sensor which employs an Electron Multiplying CCD (EMCCD) as the science detector. The EMCCD will allow photon counting at kHz rates, thereby closely tracking telescope and instrument-bench-induced aberrations as they evolve with time. In addition, the EMCCD will acquire the science data with almost no read noise penalty. To mitigate risk and lower costs, MAPLE-50 will at first have a single optical channel with a minimum of moving parts. The goal is to reach a few times 109 contrast in 25 h worth of flying time, allowing direct detection of Jovians around the nearest stars. Once the 50 cm infrastructure has been validated, the telescope diameter will then be increased to a 1.5 m diameter (MAPLE-150) to reach 1010 contrast and have the capability to image another Earth.
We measure the long-term systematic component of the astrometric error in the GeMS MCAO system as a function of field radius and Ks magnitude. The experiment uses two epochs of observations of NGC 1851 separated by one month. The systematic component is estimated for each of three field of view cases (15'' radius, 30'' radius, and full field) and each of three distortion correction schemes: 8 DOF/chip + local distortion correction (LDC), 8 DOF/chip with no LDC, and 4 DOF/chip with no LDC. For bright, unsaturated stars with 13 < Ks < 16, the systematic component is < 0.2, 0.3, and 0.4 mas, respectively, for the 15'' radius, 30'' radius, and full field cases, provided that an 8 DOF/chip distortion correction with LDC (for the full-field case) is used to correct distortions. An 8 DOF/chip distortion-correction model always outperforms a 4 DOF/chip model, at all field positions and magnitudes and for all field-of-view cases, indicating the presence of high-order distortion changes. Given the order of the models needed to correct these distortions (~8 DOF/chip or 32 degrees of freedom total), it is expected that at least 25 stars per square arcminute would be needed to keep systematic errors at less than 0.3 milliarcseconds for multi-year programs. We also estimate the short-term astrometric precision of the newly upgraded Shane AO system with undithered M92 observations. Using a 6-parameter linear transformation to register images, the system delivers ~0.3 mas astrometric error over short-term observations of 2-3 minutes.
Direct imaging of exoplanets is very attractive but challenging. It requires high angular resolution and very high-contrast imaging. One solution is the use of coronagraphs behind the adaptive optics of large telescopes. Unfortunately, optics of space telescope and ground telescope introduce quasi-static aberrations which strongly limit the quality of the final image and a dedicated stage of adaptive optics is required. We proposed a self- coherent camera (SCC) in 2006 and we obtained contrast levels of (approximately equal to) 2 10-8 at a few λ0=D at 638 nm in laboratory. In this paper, we explain how to achromatize the SCC. We present laboratory performance in wide spectral band (approximately equal to) 5 - 10 % bandpass.
We present in this paper an overview of the high contrast imaging modes of the wide-field imager MICADO. MICADO is a near-IR camera for the European Extremely Large Telescope (E-ELT), featuring a wide field (75”), spectroscopic and coronagraphic capabilities. It has been chosen by ESO as one of the two first-light instruments. MICADO will be optimized for the multi-conjugate adaptive optics module MAORY and will also work in SCAO mode. This SCAO mode will provide MICADO with a high-level, on-axis correction, making use of the M4 adaptive mirror in the telescope.
After presenting the scientific interest for high contrast imaging modes in MICADO, we describe the technical choices we are studying. We present the hypotheses chosen for our simulation tools and contrast as well as planet detection performance derived from this tool.
KEYWORDS: Sensors, Gemini Planet Imager, Calibration, Electroluminescent displays, Electrons, Gemini Observatory, Iterated function systems, Point spread functions, Signal to noise ratio, Data modeling
The Gemini Planet Imager is a newly commissioned facility instrument designed to measure the near-infrared spectra of young extrasolar planets in the solar neighborhood and obtain imaging polarimetry of circumstellar disks. GPI’s science instrument is an integral field spectrograph that utilizes a HAWAII-2RG detector with a SIDECAR ASIC readout system. This paper describes the detector characterization and calibrations performed by the GPI Data Reduction Pipeline to compensate for effects including bad/hot/cold pixels, persistence, non- linearity, vibration induced microphonics and correlated read noise.
We present in this paper an overview of the single-conjugate adaptive optics (SCAO) module of the wide-field imager MICADO. MICADO is a near-IR camera for the European ELT, featuring a wide field (75"), spectroscopic and coronagraphic capabilities. It has been chosen by ESO as one of the two first-light instruments. MICADO will be optimized for the multi-conjugate adaptive optics module MAORY and will also work in SCAO mode. This SCAO mode will provide MICADO with a high-level, on-axis correction, making use of the M4 adaptive mirror in the telescope. We present first the current design of the different subsystems of the SCAO module (namely the optical relay interfacing MICADO to the telescope in its SCAO mode, the wavefront sensor, the real-time computer and the high contrast imaging). We then present the adaptive optics and coronagraphic simulations. The following section is devoted to the presentation of the project organization. We end with the conclusions and perspectives of the project.
A new exoplanet finding algorithm called TLOCI (for Template LOCI) is presented to subtract high-contrast imaging PSFs by “maximizing a companion signal-to-noise ratio”. TLOCI uses an input spectrum and template PSFs to optimize the reference image least-squares coefficients to minimize the flux contamination via self-subtraction of any specific planet in the image, while trying to maximize, at the same time, the noise subtraction. The new algorithm has been developed using on sky Gemini Planet Imager data and has achieved impressive contrast.
Direct imaging of exoplanet systems requires the use of coronagraphs to reach high contrast levels (10-8 to 10-11) at small angular separations (0.100). However, the performance of these devices is drastically limited by aberrations (in phase or in amplitude, introduced either by atmosphere or by the optics). Coronagraphs must therefore be combined with extreme adaptive optic systems, composed of a focal plane wavefront sensor and of a high order deformable mirror. These adaptive optic systems must reach a residual error in the corrected wavefront of less than 0.1 nm (RMS) with a rate of 1 kHz. In addition, the surface defects of the deformable mirror, inherent from the fabrication process, must be limited in order to avoid the introduction of amplitude aberrations. An experimental high contrast bench has been developed at the Paris Observatory (LESIA). This bench includes a Boston Micromachine deformable mirror composed of 1024 actuators. For a precise analysis of its surface and performance, we characterized this mirror on the interferometric bench developed since 2004 at the Marseille Observatory (LAM). In this paper, we present this interferometric bench as well as the results of the analysis. This will include a precise surface characterization and a description of the behavior of the actuators, on a 10 by 10 actuator range (behavior of a single actuator, study of the cross-talk between neighbor actuators, influence of a stuck actuator) and on full mirror scale (general surface shape).
MICADO is a near-IR camera for the European ELT, featuring an extended field (75” diameter) for imaging, and also spectrographic and high contrast imaging capabilities. It has been chosen by ESO as one of the two first-light instruments. Although it is ultimately aimed at being fed by the MCAO module called MAORY, MICADO will come with an internal SCAO system that will be complementary to it and will deliver a high performance on axis correction, suitable for coronagraphic and pupil masking applications. The basis of the pupil masking approach is to ensure the stability of the optical transfer function, even in the case of residual errors after AO correction (due to non common path errors and quasi-static aberrations). Preliminary designs of pupil masks are presented. Trade-offs and technical choices, especially regarding redundancy and pupil tracking, are explained.
Multichannel Subtractive Double Pass (MSDP) spectrographs have been widely used in solar spectroscopy because of
their ability to provide an excellent compromise between field of view and spatial and spectral resolutions. Compared
with other types of spectrographs, MSDP can deliver simultaneous monochromatic images at higher spatial and spectral
resolutions without any time-scanning requirement (as with Fabry-Perot spectrographs), and with limited loss of flux.
These performances are obtained thanks to a double pass through the dispersive element. Recent advances with VPH
(Volume phase holographic) Grisms as well as with image slicers now make MSDP potentially sensitive to much smaller
fluxes. We present S4EI (Spectral Sampling with Slicer for Stellar and Extragalactical Instrumentation), which is a new
concept for extending MSDP to night-time astronomy. It is based on new generation reflecting plane image slicers
working with large apertures specific to night-time telescopes. The resulting design could be potentially very attractive
and innovative for different domains of astronomy, e.g., the simultaneous spatial mapping of accurately flux-calibrated
emission lines between OH sky lines in extragalactic astronomy or the simultaneous imaging of stars, exoplanets and
interstellar medium. We present different possible MSDP/S4EI configurations for these science cases and expected
performances on telescopes such as the VLT.
Direct detection is a very promising field in exoplanet science. It allows the detection of companions with large separation and allows their spectral analysis. A few planets have already been detected and are under spectral analysis. But the full spectral characterization of smaller and colder planets requires higher contrast levels over large spectral bandwidths. Coronagraphs can be used to reach these contrasts, but their efficiency is limited by wavefront aberrations. These deformations induce speckles, star lights leaks, in the focal plane after the coronagraph. The wavefront aberrations should be estimated directly in the science image to avoid usual limitations by differential aberrations in classical adaptive optics. In this context, we introduce the Self- Coherent Camera (SCC). The SCC uses the coherence of the star light to produce a spatial modulation of the speckles in the focal plane and estimate the associated electric complex field. Controlling the wavefront with a deformable mirror, high contrasts have already been reached in monochromatic light with this technique. The performance of the current version of the SCC is limited when widening the spectral bandwidth. We will present a theoretical analysis of these issues and their possible solution. Finally, we will present test bench performance in polychromatic light.
Direct imaging of exoplanets is very challenging because the planet is 104 to 1010 fainter than the star at a separation of a fraction of arcsec. Several coronagraphs have been proposed to reduce the contrast ratio but their performance strongly depends on the level of phase and amplitude aberrations that induce speckles in the science image. An active control of the aberrations and a posteriori calibration are thus required to reach very high contrasts. Classical adaptive optics are not sufficient for this purpose because of non-common path aberrations. Our team proposed a self-coherent camera that spatially modulates the speckles in the science image. It is then possible to both actively control a deformable mirror and calibrate the residuals a posteriori. The current paper is an overview of the developments we have been working on for 7 years. We present the principle of the self-coherent camera, laboratory performance obtained in monochromatic light, and upgrades of the technique to make it achromatic.
The Gemini Planet Imager (GPI) is a new facility instrument for the Gemini Observatory designed to detect
and characterize planets and debris disks orbiting nearby stars; its science camera is a near infrared integral
field spectrograph. We have developed a data pipeline for this instrument, which will be made publicly available
to the community. The GPI data reduction pipeline (DRP) incorporates all necessary image reduction and
calibration steps for high contrast imaging in both the spectral and polarimetric modes, including datacube
generation, wavelength solution, astrometric and photometric calibrations, and speckle suppression via ADI and
SSDI algorithms. It is implemented in IDL as a flexible modular system, and includes both command line and
graphical interface tools including a customized viewer for GPI datacubes.
This GPI data reduction pipeline is currently working very well, and is in use daily processing data during
the instrument’s ongoing integration and test period at UC Santa Cruz. Here we summarize the results from
recent pipeline tests, and present reductions of instrument test data taken with GPI. We will continue to refine
and improve these tools throughout the rest of GPI’s testing and commissioning, and they will be released to the
community, including both IDL source code and compiled versions that can be used without an IDL license.
The Self-Coherent Camera is dedicated to the direct detection of exoplanets. This instrument can be used as a focal plane wavefront sensor to measure static aberrations that induce speckles on the detector, which prevents the detection of faint companions. The Self-Coherent Camera creates a reference beam in the Lyot stop pupil plane in order to spatially modulate the speckle pattern with Fizeau fringes. We can then estimate for wavefront aberrations upstream of the coronagraphic mask and correct for them using a deformable mirror. Currently, the Self-Coherent Camera is combined with a deformable mirror located in the pupil plane upstream of a Four-Quadrant Phase Mask Coronagraph. In this paper, we present the formalism that explains how the Self-Coherent Camera encodes speckles and how we estimate the wavefront aberrations directly from the science image. We present numerical simulation results on speckle suppression in the focal plane. Then, we give experimental results on wavefront correction on our optical bench using a 32x32 actuators deformable mirror. We show that we can improve the contrast in the focal plane by a factor of more than 100 in the PSF wings up to 12/λD.
Direct imaging and low-resolution spectroscopy of extrasolar planets are exciting but challenging scientific applications of coronagraphy. While the angular separation is well within the reach of actual telescope in the
near IR or visible, the planet-star contrast (from 10−6 to 10−10) requires wavefront quality and stability hard to reach even with a well-polished space telescope. Several solutions have been proposed to tackle the speckle noise introduced by the residual optical defects. While some concepts rely only on active wavefront correction
using deformable mirror, other techniques are based on post-processing and subtract a reference image recorded
sometimes simultaneously with the science image. One interesting solution is to choose a concept that allows
both active correction and post-processing of high contrast coronagraphic images. This is the case of the Self
Coherent Camera (SCC), which has been proposed for the project of space coronagraph SPICES and for the
ground-based planet finder EPICS studied for the European Extremely Large Telescope. After recalling the SCC
principle, we present both monochromatic and modest bandwidth (2%) experimental results of Dark Hole in the
focal plane using a SCC. Example of a post-processing result with SCC is also given to emphasize the interest
of combining it with active correction.
Direct imaging of exoplanets requires the detection of very faint objects orbiting close to very bright stars. In this context, the SPICES mission was proposed to the European Space Agency for planet characterization at visible wavelength. SPICES is a 1.5m space telescope which uses a coronagraph to strongly attenuate the central source. However, small optical aberrations, which appear even in space telescopes, dramatically decrease coronagraph performance. To reduce these aberrations, we want to estimate, directly on the coronagraphic image, the electric field, and, with the help of a deformable mirror, correct the wavefront upstream of the coronagraph. We propose an instrument, the Self-Coherent Camera (SCC) for this purpose. By adding a small "reference hole" into the Lyot stop, located after the coronagraph, we can produce interferences in the focal plane, using the coherence of the stellar light. We developed algorithms to decode the information contained in these Fizeau fringes and retrieve an estimation of the field in the focal plane. After briefly recalling the SCC principle, we will present the results of a study, based on both experiment and numerical simulation, analyzing the impact of the size of the reference hole.
Several small space coronagraphs have been proposed to characterize cold exoplanets in reflected light. Studies
have mainly focused on technical feasibility because of the huge star/planet
flux ratio to achieve in the close-in
stellar environment (108-1010 at 0.2"). However, the main interest of such instruments, the analysis of planet
properties, has remained highly unexplored so far. We performed numerical simulations to assess the ability
of a small space coronagraph to retrieve spectra of mature Jupiters, Neptunes and super-Earths under realistic
assumptions. We describe our assumptions: exoplanetary atmosphere models, instrument numerical simulation
and observing conditions. Then, we define a criterion and use it to determine the required exposure times to
measure several planet parameters from their spectra (separation, metallicity, cloud and surface coverages) for
particular cases. Finally, we attempt to define a parameter space of the potential targets. In the case of a
solar-type star, we show that a small coronagraph can characterize the spectral properties of a 2-AU Jupiter up
to 10 pc and the cloud and surface coverage of super-Earths in the habitable zone for a few stars within 4-5 pc.
Potentially, SPICES could perform analysis of a hypothetical Earth-size planet around α Cen A and B.
The study of the physico-chemical properties of wide-separated exoplanets (> 1 AU) is a major goal of high-contrast
imaging techniques. SPICES (Spectro-Polarimetric Imaging and Characterization of Exoplanetary
Systems) is a project of space coronagraph dedicated to the spectro-polarimetric analysis of gas and ice giant
planets, super-Earths and circumstellar disks in visible light at a spectral resolution of about 40. After recalling
the science cases of the mission, we describe the optical design and the critical subsystems of the instrument.
We then discuss the SPICES performance that we derived from numerical simulations.
To characterize orbits and atmospheres of exoplanets with large orbits (≥ a few AU), direct imaging is nowadays
the sole way. From space, this involves high contrast imaging techniques as coronagraphy, differential imaging or
wavefront control. Several methods exist or are under development and several small (~1.5m) space telescope
missions are proposed. One of them is See-coast (super-Earth explorer coronagraphic off-axis space telescope)
which will be proposed to the next ESA Cosmic Vision call. It will provide polarimetric and spectral characterization
of giant gazeous planets and possibly Super-Earths in visible light. In this paper, we first detail science
cases of this mission. We then describe the foreseen telescope design and its instrumentation. We finally derive
performance for a particular instrumental configuration from numerical simulation and we show how See-coast
can retrieve planet spectra.
In the framework of Extremely Large Telescope (ELT), several instruments are considered for the characterization of
extrasolar planets. Since the performance of such an instrument is limited by wavefront errors, the use of extreme
Adaptive Optic (AO) systems is mandatory. Studies for future planet finder instruments such as SPHERE/VLT or
GPI/GEMINI show that one limitation of the performance for a planet finder is the differential aberrations that are not
measured by the wavefront sensor, which is physically separated from the common optics by a beam splitter. These
defects create a field of residual speckles in the focal plane that need to be calibrated to separate the planet signal from
the speckle noise [1].
We propose here to simultaneously estimate these aberrations and also detect the planet directly from the final science
image. To do so, we propose to couple the foreseen extreme high speed AO of an ELT planet Finder with a low speed
Self-Coherent Camera (SCC [2-5]). The SCC which is based on the principle of light coherence can both estimate the
wavefront errors and reduce speckle noise by calibration. After recalling the principle of the SCC, we present simulation
results of the SCC performance in the context of EPICS.
In the framework of exoplanet direct imaging, a few coronagraphs have been proposed to overcome the large flux ratio
that exists between the star and its planet. However, there are very few solutions that gather in the same time broad band
achromaticity, a small inner working angle (shortest angular distance for planet detection), a good throughput for the
planet light, and a mature technical feasibility. Here, we propose to use a combination of chromatic Four Quadrant Phase
Mask coronagraphs to achromatize the dephasing of this well-studied monochromatic coronagraph. After describing the
principle of the technique, we present preliminary results for a compact prototype. Contrast larger than 10000 are
reached with more than 250 nm of spectral bandwidth in the visible. Stability over time and effect of the filtering is also
discussed.
Extreme adaptive optics and coronagraphy are mandatory for direct imaging of exoplanets. Quasi-static aberrations
limit the instrument performance producing speckle noise in the focal plane. We propose a Self-Coherent
Camera (SCC) to both control a deformable mirror that actively compensates wavefront error, and calibrate the
speckle noise. We create a reference beam to spatially modulate the coronagraphic speckle pattern with Fizeau
fringes. In a first step, we are able to extract wavefront aberrations from the science image and correct for them
using a deformable mirror. In a second step, we apply a post-processing algorithm to discriminate the companion
image from the residual speckle field.
To validate the instrumental concept, we developed a high contrast imaging bench in visible light. We associated
a SCC to a four quadrant phase mask coronagraph and a deformable mirror (DM) with a high number
of actuators (32x32 Boston Michromachines MEMS). We will present this bench and show first experimental
results of focal plane wavefront sensing and high contrast imaging. The measurements are compared to numerical
simulations.
Detecting light from faint companions or protoplanetary disks lying close to their host star is a demanding task
since these objects are often hidden in the overwhelming star light. A lot of coronagraphs have been proposed to
reduce that stellar light and thus, achieve very high contrast imaging, which would enable to take spectra of the
faint objects and characterize them. However, coronagraph performance is limited by residual wavefront errors
of the incoming beam which create residual speckles in the focal plane image of the central star. Correction or
calibration of the wavefront are then necessary to overcome that limitation. We propose to use a Self-Coherent
Camera (SCC, Baudoz et al. 2006). The SCC is one of the techniques proposed for EPICS, the futur planet
finder of the European Extremely Large Telescope but can also be studied in a space telescope context. The
instrument is based on the incoherence between stellar and companion lights. It works in two steps. We first
estimate wavefront errors to be corrected by a deformable mirror and then, we apply a post-processing algorithm
to achieve very high contrast imaging.
A large number of coronagraphs have been proposed to overcome the ratio that exists between the star and its planet.
The planet finder of the Extremely Large Telescope, which is called EPICS, will certainly need a more efficient
coronagraph than the ones that have been developed so far. We propose to use a combination of chromatic Four
Quadrant Phase Mask coronagraph to achromatize the dephasing of the device while maintaining a high rejection
performance. After describing this multi-stage FQPM coronagraph, we show preliminary results of a study on its
capabilities in the framework of the EPICS instrument, the planet finder of the European Extremely Large Telescope.
Eventually, we present laboratory tests of a rough prototype of a multi-stage four-quadrant phase mask. On one hand, we
deduce from our laboratory data that a detection at the 10-10 level is feasible in monochromatic light. On the other hand,
we show the detection of a laboratory companion fainter than 10-8 with a spectral bandwidth larger than 20%.
Performance of wave-front control and coronagraphy are very critical for high contrast imaging on an ELT. Quasi-static
aberrations will have a dramatic impact on the detection performance for the most demanding science objectives of
EPICS, the direct imaging of exo-planets. The contribution of these systematic errors will have to be significantly lower
than the overall residual halo level due to turbulence correction residual after coronagraphy. We present preliminary
results of the strategy to measure wave front errors after the coronagraph with the goal of minimizing common path
errors for the correction of static speckles. The development of two novel post-coronagraphic wave-front sensors is
given.
Amplitude apodization of a telescope's pupil can be used to reduce the diffraction rings (Airy rings) in the PSF to allow high contrast imaging. Rather than achieving this apodization by selectively removing light at the edges of the pupil, we propose to produce the desired apodized pupil by redistributing the pupil's light. This lossless apodization concept can yield a high contrast PSF which allows the efficient detection of Earth-sized planets around stars at ~10pc with a 2m visible telescope in space. We review the current status of a JPL-funded study of this concept for the Terrestrial Planet Finder (TPF) mission, including a lab experiment and extensive computer simulations.
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