Directly imaging Earth-like exoplanets around Sun-like stars with the future Habitable Worlds Observatory (HWO) will require coronagraphic focal plane masks able to suppress starlight to the 1 × 10−10 contrast levels. Furthermore, to collect enough photons for broadband imaging and detection and to minimize the number of parallel channels for spectroscopic characterization, this level of contrast must be achieved across a 20% bandwidth. Scalar vortex coronagraphs show promise as a polarization-independent alternative to polarizationsensitive vector vortex coronagraphs, but still face chromatic limitations. New scalar vortex mask designs incorporate radial phase dimples to improve the broadband performance. We present initial manufacturing results of prototype masks of these designs including phase metrology and microscope images, in preparation for broadband chromatic characterization and starlight suppression measurements, to be taken on a high contrast imaging testbed. We also present a preliminary narrowband (2%) dark hole result achieving 1.8 × 10−8 average contrast from 3.5-10λ0/D on the High Contrast Spectroscopy Testbed at Caltech. This work aims to advance the technological maturity of scalar vortex coronagraphs as a viable option for consideration for HWO.
With the identified objective of enabling Earth-like exoplanets direct detection, and characterization of their atmospheric content, the Astro2020 report has placed the maturation of exoplanet imaging technology as a key priority for the coming decade. The High Contrast Spectroscopy Testbed (HCST) within the Caltech Exoplanet Technology laboratory serves as an in-air coronagraphic testbed demonstrator, integrating a high order deformable mirror for wavefront control and a vector vortex coronagraph (VVC). HCST has demonstrated excellent in-air contrast performance, achieving 1 × 10−8 raw contrast in broadband light, for both the apodized off-axis segmented pupil configuration and using single mode fiber planet injection. By introducing a low-order wavefront sensor (WFS) that utilizes either the in-band or out-of-band reflected light from the VVC coupled with a tip/tilt mirror, our objective is to address dynamic errors, thereby enhancing the wavefront stability of the experiment. We present in this proceeding the first steps towards a full tip/tilt control loop, starting with the optical design of our low-order camera. We performed a drift test overnight as a diagnostic of the coronagraphic performance stability and to possibly identify causes for the drift. Conclusions show that HCST demonstrate a remarkably stable environment to perform high-contrast imaging experiments, at the level of 1 × 10−8 contrast.
Connecting a coronagraph instrument to a spectrograph via a single-mode optical fiber is a promising technique for characterizing the atmospheres of exoplanets with ground and space-based telescopes. However, due to the small separation and extreme flux ratio between planets and their host stars, instrument sensitivity will be limited by residual starlight leaking into the fiber. To minimize stellar leakage, we must control the electric field at the fiber input. Implicit electric field conjugation (iEFC) is a model-independent wavefront control (WFC) technique in contrast with classical EFC, which requires a detailed optical model of the system. We present here the concept of an iEFC-based WFC algorithm to improve stellar rejection through a single-mode fiber (SMF). As opposed to image-based iEFC, which relies on minimizing intensity in a dark hole region, our approach aims to minimize the amount of residual starlight coupling into an SMF. We present broadband simulation results demonstrating a normalized intensity ≥10−10 for both fiber-based EFC and iEFC. We find that both control algorithms exhibit similar performance for the low wavefront error (WFE) case, however, iEFC outperforms EFC by ≈100x in the high WFE regime. Having no need for an optical model, this fiber-based approach offers a promising alternative to EFC for ground and space-based telescope missions, particularly in the presence of residual WFE.
Current scalar coronagraph focal plane mask designs are performance-limited by chromaticity. We investigate the effects of adding central Roddier and dual zone phase dimples to scalar masks to improve broadband performance by suppressing the chromatic stellar leakage. We present hybrid designs with radial phase dimples integrated with the sawtooth vortex, wrapped vortex, and cosine phase mask. We show that, using these dimples, it is possible to substantially improve the broadband contrast performance of scalar phase masks. We also show that, although adding a phase dimple does not increase the sensitivity to low-order aberrations, suppressing the central leakage of scalar vortex coronagraphs does not restore the aberration sensitivities to their notional state.
The High Contrast Spectroscopy Testbed (HCST) in the Exoplanet Technology laboratory (ETlab) at Caltech is an in-air testbed hosting all key components necessary to demonstrate coronagraphic performance in broadband light with the vortex coronagraph. The testbed was recently upgraded with the aim of closing the remaining technological gaps in terms of pupil segmentation and to demonstrate our main objective: achieving 10−8 contrast in broadband light. As part of the HCST redesign, a new apodizer has been manufactured using carbon nanotubes acting as microdots to encode a grayscale pattern. This technology presents a very low reflectance conducive to highly efficient amplitude apodization. We demonstrate raw contrasts of 1 × 10−8 with the apodized vortex coronagraph, both in narrowband and broadband light (10% bandwidth), and with the use of wavefront control. The use of an apodizer enables to deal with the diffraction caused by the segment gaps and successfully eliminates this source of starlight leakage for the vortex coronagraph to the 10−8 level.
The Habitable Worlds Observatory mission will require coronagraphs capable of achieving contrasts of 1e-10 to detect exo-Earths. The choice of coronagraph depends on finding a solution that is achromatic within a 20% bandwidth, insensitive to low order aberrations and polarization independent. We present two scalar vortex phase mask designs which employ a Roddier phase dimple and a dual zone phase dimple to improve the achromatic performance by addressing the chromatic stellar leakage not handled by the vortex. We show that using these dimples, it is possible to substantially improve the broadband contrast performance of existing scalar vortex phase masks.
A pathway to maximizing the scientific yield of future exoplanet imaging instruments is to develop technologies that give access to high levels of contrast over large bandwidths. Combining the use of Deformable Mirrors (DMs) for wavefront control and a Single Mode Fiber (SMF) in the image plane has been proven to enhance the contrast achievable for broadband characterization of exoplanet atmospheres. Indeed, the modal selectivity of the SMF aids with the rejection of speckles in the vicinity of the companion. Furthermore, the stroke requirements on the DMs needed to achieve high levels of raw contrast are relaxed given the reduced spatial frequencies needed. Here we report on the status of our SMF wavefront control experiments at the High-Contrast Spectroscopy Testbed (HCST) in the Exoplanet Technology Laboratory at Caltech: we have reached 1×10−8 raw contrast over 20% bandwidth with an SMF.
Connecting a coronagraph instrument to a spectrograph via a single-mode optical fiber is a promising technique for characterizing the atmospheres of exoplanets with ground and space-based telescopes. However, due to the small separation and extreme flux ratio between planets and their host stars, instrument sensitivity will be limited by residual starlight leaking into the fiber. To minimize stellar leakage, we must control the electric field at the fiber input. Implicit Electric fFeld Conjugation (iEFC) is a model-independent wavefront control technique in contrast with classical Electric Field Conjugation (EFC) which requires a detailed optical model of the system. We present here the concept of an iEFC-based wavefront control algorithm to improve stellar rejection through a single-mode fiber. As opposed to image-based iEFC which relies on minimizing intensity in a dark hole region, our approach aims to minimize the amount of residual starlight coupling into a single-mode fiber. We present broadband simulation results demonstrating a normalized intensity ≥ 10−10 for both fiber-based EFC and iEFC. We find that both control algorithms exhibit similar performance for the low wavefront error case, however, iEFC outperforms EFC by ≈ 100x in the high wavefront error regime. Having no need for an optical model, this fiber-based approach is theoretically easier to implement than conventional EFC on future ground and space-based telescope missions.
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.
MICADO is the ELT first light instrument, an imager working at the diffraction limit of the telescope thanks to two adaptive optics (AO) modes: a single conjugate one (SCAO), available at the instrument first light and developed by the MICADO consortium, and a multi conjugate one (MCAO), developed by the MORFEO consortium.
This contribution presents an overview of the SCAO module while MICADO and its SCAO are in the last phase of their final design review. We focus on the SCAO architecture choices and present the final design of the SCAO subsystems: the Green Doughnut structure, the SCAO wavefront sensor, the SCAO calibration unit, the SCAO ICS (i.e. AOCS) and the SCAO RTC. We also present the SCAO global performance in terms of AO correction, obtained from an error budget that includes contributors estimated from AO end-to-end simulations as well as instrumental contributors. Finally, we present the current SCAO subsystems prototyping and the main milestones of the SCAO AIT plan.
MICADO is the ELT near-infrared first light imager. It will provide diffraction limited images thanks to single-conjugate adaptive optics (SCAO) mode provided inside the MAORY module. Numerical simulations were performed using COMPASS to assess the overall SCAO performance, exploring WFS design parameters and associated calibration procedures.
We present the optimizations developed to deal with pyramid wavefront sensor specific calibrations expected at the ELT (optimal modal basis, petalling, optical gains & NCPA management,). We then evaluate the impact of the AO loop frequency and RTC latency and others specific SCAO optimization parameters (modulation amplitude, number of controlled modes, etc) in various flux and turbulence conditions. We finally evaluate the impact of some of the ELT errors contributors such as M1 reflectivity errors, M1 phase aberrations, M1 missing segments, M4 mis-registration, telescope windshake & vibrations.
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 performance: their primary mirror segmentation implies phasing errors or even missing segments, and the size of the telescope imposes large spiders, generating low-wind effect as already observed on the Very Large Telescope (VLT)/SPHERE instrument or at the Subaru telescope, or adaptive-optics-due petaling, studied in simulations in the ELT case. The ongoing development of coronagraphs has then to take into account their sensitivity to such errors. We propose an innovative method to generate coronagraphs robust to primary mirror phasing errors and low-wind and adaptive-optics-due petaling effect. This method is based on the apodization of the segment or petal instead of the entire pupil, this apodization being then repeated to mimic the pupil redundancy. We validate this so-called Redundant Apodized Pupil (RAP) method on a James Webb Space Telescope-like pupil composed of 18 hexagonal segments segments to align, and on the VLT architecture in the case of residual low-wind effect.
The detection and characterization of Earth-like exoplanets around Sun-like stars for future flagship missions requires coronagraphs to achieve contrasts on the order of 10−10 at close angular separations and over large spectral bandwidths (≥20%). We present our progress thus far on exploring the potential for scalar vortex coronagraphs (SVCs) in direct exoplanet imaging. SVCs are an attractive alternative to vector vortex coronagraphs (VVCs), which have recently demonstrated 6 ×10−9 raw contrast in 20% broadband light but are polarization dependent. SVCs imprint the same phase ramp on the incoming light and do not require polarization splitting, but are inherently limited by their chromatic behavior. Several SVC designs have been proposed in recent years to solve this issue by modulating or wrapping the azimuthal phase function according to specific patterns. For one such design, the staircase SVC, we present our best experimental SVC results demonstrating raw contrast of 2 ×10−7 in 10% broadband light. Since SVC broadband performance and aberration sensitivities are highly dependent on topology, we conducted a comparative study of several SVC designs to optimize for high contrast across a range of bandwidths. Furthermore, we present a new coronagraph optimization tool to predict performance in order to find an achromatic solution.
A key challenge of high contrast imaging (HCI) is to differentiate a speckle from an exoplanet signal. The sources of speckles are a combination of atmospheric residuals and aberrations in the non-common path. Those non-common path aberrations (NCPA) are particularly challenging to compensate for as they are not directly measured, and because they include static, quasi-static and dynamic components. The proposed method directly addresses the challenge of compensating the NCPA. The algorithm DrWHO - Direct Reinforcement Wavefront Heuristic Optimisation - is a quasi-real-time compensation of static and dynamic NCPA for boosting image contrast. It is an image-based lucky imaging approach, aimed at finding and continuously updating the ideal reference of the wavefront sensor (WFS) that includes the NCPA, and updating this new reference to the WFS. Doing so changes the point of convergence of the AO loop. We show here the first results of a post-coronagraphic application of DrWHO. DrWHO does not rely on any model nor requires accurate wavefront sensor calibration, and is applicable to non-linear wavefront sensing situations. We present on-sky performances using a pyramid WFS sensor with the Subaru coronagraph extreme AO (SCExAO) instrument.
The presence of a six legged 50cm-wide spider supporting the secondary mirror of the Extremely Large Telescope (ELT) breaks the spatial continuity of the incoming wave-front. Atmospheric turbulence, low wind effect and thermo-mechanical drift of the deformable mirror are all potential contributors to discontinuities between the six segments of the ELT pupil. It is therefore necessary to measure these differential pistons in order to reconstruct the full wave-front. The pyramid wave-front sensor is currently the preferred design for adaptive optics systems. However, it was shown to be a poor differential piston sensor in the visible, under partial turbulence correction, leading to a severe degradation of the image quality. Using the COMPASS adaptive optics (AO) simulator, we first investigate strategies to ensure the spatial continuity of the correction applied on the deformable mirror. These methods present some limitations in strong seeing conditions, when the corrugated phase varies a lot below the spider legs, and lead to a significant degradation of the Strehl Ratio. To tackle this critical issue, we propose as a second step to couple the continuity hypothesis with a petalometer: a sensor specifically designed for sensing the differential piston. As candidates, we compare an unmodulated pyramid, a Zernike wavefront-sensor and a Zernike coupled with a field stop. We present results regarding their sensitivity and their reliability when working in operation, in presence of realistic AO residuals.
With the upcoming giant class of telescopes, Adaptive Optics (AO) has become more essential than ever before to get access to the full potential offered by those telescopes. The complexity of such AO systems is reaching extreme heights, and disruptive developments will have to be made in order to build them. One of the critical component of a AO system is the Real Time Controller (RTC) which will have to compute the slopes and the Deformable Mirror (DM) commands at high frequency, in a range of 0.5 to several kHz. Since the complexity of the computations involved in the RTC is increasing with the size of the telescope, fulfilling RTC requirements for Extremely Large Telescope (ELT) class is a challenge. As an example, the MICADO SCAO (Single Conjugate Adaptive Optics) system requires around 1 TMAC/s for the RTC to get sufficient performance. This complexity brings the need for High Performance Computing (HPC) techniques and standards, such as the use of hardware accelerator like GPU. On top of that, building a RTC is often project-dependent as the components and the interfaces change from one instrument to an other. The COSMIC platforms aims at developing a common AO RTC platform which is meant to be powerful, modular and available to the AO community. This development is a joint effort between Observatoire de Paris and the Australian National University (ANU) in collaboration with the Subaru Telescope. We focus here on the current status of the core hard real-time component of this platform. The H-RTC pipeline is composed of Business Units (BU): each BU is an independent process in charge of one particular operation, such as Matrix Vector Multiply (MVM) or centroid computation, that can be made on CPU or on GPU. BUs read and write data on Shared Memory (SHM) handled by the CACAO framework. Synchronization between each BU can then be made either by using semaphore or by busy waiting on the GPU to ensure very low jitter. The RTC pipeline can then be controlled through a Python interface. One of the key point of this architecture is that the interfaces of a BU with the various SHM is abstracted, so adding a new BU in the collection of available ones is straight forward. This approach allows a high performance, scalable, modular and configurable RTC pipeline that could fit the needs of any AO system configuration. Performance has been measured on a MICADO SCAO scale RTC pipeline with around 25,000 slopes by 5,000 actuators on a DGX-1 system equipped with 8 Tesla V100 GPUs. The considered pipeline is composed of two BUs : the first one takes an input the raw pyramid WFS image (produced by simulator), applies on it dark and flat references, and then extract the useful pixel from the image. The second BU performs the MVM and the integration of the commands following a classical integrator command law. Synchronization between the BU is made through GPU busy waiting on the BU inputs. Performance obtained shows a mean latency up to 235 μs using 4 GPUs, with a jitter of 4.4 μs rms and a maximum jitter of 30 μs
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