The future Habitable Worlds Observatory aims to characterize the atmospheres of rocky exoplanets around solar-type stars. The Vector Vortex Coronagraph (VVC) is a main candidate to reach the required contrast of 10−10. However, the VVC requires polarization filtering and every observing band requires a different VVC. The triple-grating vector vortex coronagraph (tgVVC) aims to mitigate these limitations by combining multiple gratings that minimize the polarization leakage over a large spectral bandwidth. In this paper, we present laboratory results of a tgVVC prototype using the In-Air Coronagraphic Testbed (IACT) facility at NASA’s Jet Propulsion Laboratory and the Space Coronagraph Optical Bench (SCoOB) at the University of Arizona Space Astrophysics Lab (UASAL). We study the coronagraphic performance with polarization filtering at 633 nm and reach a similar average contrast of 2 × 10−8 between 3-18 λ/D at the IACT, and 6 × 10−8 between 3-14 λ/D at SCoOB. We explore the limitations of the tgVVC by comparing the testbed results. We report on other manufacturing errors and ways to mitigate their impact.
Coronagraphic instruments provide a great chance of enabling high contrast spectroscopy for the pursuit of finding a habitable world. Future space telescope coronagraph instruments require high performing focal plane masks in combination with precise wavefront sensing and control techniques to achieve dark holes for planet detection. Several wavefront control algorithms have been developed in recent years that might vary in performance depending on the coronagraph they are paired with. This study compares three model-free and model-based algorithms when coupled with either a Vector (VVC) or a Scalar (SVC) Vortex Coronagraph mask in the same laboratory conditions: Pairwise Probing with Electric Field Conjugation, the Self-Coherent Camera with Electric Field Conjugation, and Implicit Electric Field Conjugation. We present experimental results from the In-Air Coronagraph Testbed (IACT) at JPL in narrowband and broadband light, comparing the pros and cons of each of these wavefront sensing and control algorithms with respect to their potential for future space telescopes.
In order to directly image Earth-like exoplanets (exoEarths) orbiting Sun-like stars, the Habitable Worlds Observatory coronagraph instrument(s) will be required to suppress the starlight to raw contrasts of approximately 10−10 . Coronagraphs use active methods of Wavefront Sensing and Control (WFSC) such as Pairwise Probing (PWP) and Electric Field Conjugation (EFC) to create regions of high contrast in the science camera image, called dark holes. Due to the low flux of these exoEarths, long exposure times are required to spectrally characterize them. During these long exposures, the optical system will drift resulting in degradation of the contrast over time. To prevent such contrast drift, a WFSC algorithm running in parallel to the science acquisition can stabilize the contrast in the dark hole. However, PWP cannot be reused to efficiently stabilize the contrast since it relies on strong temporal modulation of the intensity in the image plane that would interrupt the science acquisition. Conversely, spectral Linear Dark Field Control (LDFC) takes advantage of the linear relationship between the change in intensity of the post-coronagraph out-of-band image and small changes in wavefront to preserve the dark hole region during science exposures. In this paper, we show experimental results that demonstrate spectral LDFC stabilizes the contrast to levels of a few 10−9 on a Lyot coronagraph testbed which is housed in a vacuum chamber. Promising results show that spectral LDFC is able to correct for disturbances that degrade the contrast by more than 100×. To our knowledge, this is the first experimental demonstration of spectral LDFC and the first demonstration of spatial or spectral LDFC on a vacuum coronagraph testbed and at contrast levels less than 10−8 .
Space-based stellar coronagraph instruments aim to directly image exoplanets that are a fraction of an arcsecond separated from and 10 billion times fainter than their host star. To achieve this, one or more deformable mirrors (DMs) are used in concert with coronagraph masks to control the wavefront and minimize diffracted starlight in a region of the image known as the “dark zone” or “dark hole (DH).” The DMs must have a high number of actuators (50 to 96 across) to allow for DHs that are large enough to image a range of desired exoplanet separations. In addition, the surfaces of the DMs must be controlled at the picometer level to enable the required contrast. Any defect in the mechanical structure of the DMs or electronic system could significantly impact the scientific potential of the mission. Thus NASA’s Exoplanet Exploration Program procured two 50 × 50 microelectromechanical DMs manufactured by Boston Micromachines Corporation to test their robustness to the vibrational environment that the DMs will be exposed to during launch. The DMs were subjected to a battery of functional and high-contrast imaging tests before and after exposure to flight-like random vibrations. The DMs did not show any significant functional nor performance degradation at 10 − 8 contrast levels.
Of the over 5000 exoplanets that have been detected, only about a dozen have ever been directly imaged. Earth-like exoplanets are on the order of 10 billion times fainter than their host star in visible and near-infrared, requiring a coronagraph instrument to block primary starlight and allow for the imaging of nearby orbiting planets. In the pursuit of direct imaging of exoplanets, scalar vortex coronagraphs (SVCs) are an attractive alternative to vector vortex coronagraphs (VVCs). VVCs have demonstrated 2 × 10 − 9 raw contrast in broadband light but have several limitations due to their polarization properties. SVCs imprint the same phase ramp as VVCs on the incoming light and do not require polarization splitting, but they are inherently chromatic. Discretized phase ramp patterns such as a wrapped staircase help reduce SVC chromaticity and simulations show it outperforms a chromatic classical vortex in broadband light. We designed, fabricated, and tested a wrapped staircase SVC, and here we present the broadband characterization on the high contrast spectroscopy testbed. We also performed wavefront correction on the in-air coronagraph testbed at NASA’s Jet Propulsion Laboratory and achieved an average raw contrasts of 3.2 × 10 − 8 in monochromatic light and 2.2 × 10 − 7 across a 10% bandwidth.
Directly imaging Earth-like exoplanets (“exoEarths”) with a coronagraph instrument on a space telescope requires a stable wavefront with optical path differences limited to tens of picometers RMS during exposure times of a few hours. Although the structural dynamics of a segmented mirror can be directly stabilized with telescope metrology, another possibility is to use a closed-loop wavefront sensing and control system in the coronagraph instrument that operates during the science exposures to actively correct the wavefront and relax the constraints on the stability of the telescope. We present simulations of the temporal filtering provided using the example of LUVOIR-A, a 15-m segmented telescope concept. Assuming steady-state aberrations based on a finite-element model of the telescope structure, we (1) optimize the system to minimize the wavefront residuals, (2) use an end-to-end numerical propagation model to estimate the residual starlight intensity at the science detector, and (3) predict the number of exoEarth candidates detected during the mission. We show that telescope dynamic errors of 100 pm RMS can be reduced down to 30 pm RMS with a magnitude 0 star, improving the contrast performance by a factor of 15. In scenarios where vibration frequencies are too fast for a system that uses natural guide stars, laser sources can increase the flux at the wavefront sensor to increase the servo-loop frequency and mitigate the high temporal frequency wavefront errors. For example, an external laser with an effective magnitude of −4 allows the wavefront from a telescope with 100 pm RMS dynamic errors and strong vibrations as fast as 16 Hz to be stabilized with residual errors of 10 pm RMS thereby increasing the number of detected planets by at least a factor of 4.
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 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.
Future space missions such as LUVOIR and HabEx require large apertures and coronagraphs with active wavefront control to image and characterize faint exoplanets.
ECLIPS is the coronagraph instrument on LUVOIR, an 8-15m segmented telescope. The Apodized Pupil Lyot Coronagraph is the baselined mask technology to enable the required 10e−10 contrast for observations in the habitable zones of nearby stars.
Its performance depends on wavefront sensing and control to compensate for dynamic and static aberrations induced by beam shear, segment, and global wavefront errors. Here we present how these aberrations are simulated and evaluate the ExoEarth yield in different scenarios.
The Astro2020 decadal survey recommended a ~6m IR/O/UV telescope equipped with a coronagraph instrument to directly image exoEarths in the habitable zone of their host star. A telescope of such size may need to be segmented to be folded and then carried in current launch vehicles. However, a segmented primary mirror introduces the potential for mid spatial frequency optical wavefront instabilities during the science operations that would degrade the coronagraph performance. A coronagraph instrument with a wavefront sensing and control (WS&C) system can stabilize the wavefront with a picometer precision at high temporal frequencies (<1Hz). In this work, we study a realistic set of aberrations based on a finite element model of a slightly bigger (8m circumscribed, 6.7m inscribed diameter) segmented telescope with its payload. We model an adaptive optics (AO) system numerically to compute the post-AO residuals. The residuals then feed an end-to-end model of a vector vortex coronagraph instrument. The long exposure contrast thus obtained is finally used in an ExoEarth yield method calculation to understand the overall benefits of the adaptive optics system in the flagship mission success.
One of the primary science goals of the Large UV/Optical/Infrared Surveyor (LUVOIR) mission concept is to detect and characterize Earth-like exoplanets around nearby stars with direct imaging. The success of its integrated instrument ECLIPS (Extreme Coronagraph for Living Planetary Systems) depends on the ability to stabilize the wavefront from a large segmented mirror at a level of a few picometers during an exposure time of a few hours. To relax the constraints on the mechanical stability, ECLIPS will be equipped with a wavefront sensing and control (WS&C) architecture to correct wavefront errors at high temporal frequencies (<~1 Hz). These errors are expected to be dominated by spacecraft structural dynamics exciting vibrations at the segmented primary mirror. In this work, we present detailed simulations of the WS&C system within the ECLIPS instrument and the resulting contrast performance. This study assumes realistic wavefront aberrations based on a Finite Element Model of the telescope and the spacecraft structural dynamics. Wavefront residuals are then computed according to a model of the adaptive optics system that includes numerical propagation to simulate realistic images on the wavefront sensor and an analytical model of the temporal performance. An end-to-end numerical propagation model of ECLIPS is then used to estimate the residual starlight intensity distribution on the science detector. We show that the contrast performance depends strongly on the target star magnitude and advocate for the use of laser metrology to mitigate high temporal frequency wavefront errors and increase the mission yield.
Coronagraph instruments on future space telescopes will rely on deformable mirrors to create high contrast images for the direct detection of exoplanets. As part of the NASA Exoplanet Explorations Program’s coronagraph technology development efforts, two sets of Micro-Electro-Mechanical Systems (MEMS) deformable mirrors manufactured by Boston Micromachines Corporation were exposed to vibration and thermal cycles representative of launch conditions. The first set of mirrors were 952-actuator Kilo-DMs that successfully demonstrated 100% actuator survival and achieved ~1e- 8 contrast after the environmental test. The second round used an engineering-grade 2048-actuator deformable mirror on which few changes were identified after the environmental test. However, each actuator that changed behavior was flagged as anomalous beforehand or was directly adjacent to a defective actuator. From this result, we hypothesized that typical actuators on a science grade deformable mirror are robust to environmental testing. A third set of 2048-actuator deformable mirrors have been procured for a planned test to characterize the deformable mirrors using interferometric measurements, contrast results on a new in-air coronagraph testbed, and infrared microscopy on the internal structure of the MEMS devices.
After completing its duty in one the vacuum chambers in the High Contrast Imaging Testbed (HCIT) facility at NASA’s Jet Propulsion Laboratory, the General Purpose Coronagraph Testbed (GPCT) has been retrofitted as the In-Air Coronagraph Testbed (IACT), with the purpose of verification of Boston Micromachines (BMC) 50x50 MEMS deformable mirrors (MEMS DM) performance by contrast comparison before and after random vibration testing. The testbed is configured as a vortex coronagraph, with one MEMS DM in the pupil plane to create a half dark hole in monochromatic light. High wavefront stability is achieved using an environmentally isolating enclosure, based on the enclosure design previously used on Caltech’s High Contrast Spectroscopy Testbed (HCST).
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.
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
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.
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