Wavefront Control System Research Articles

Balloon flight demonstration of coronagraph focal plane wavefront correction with PICTURE-C

The Planetary Imaging Concept Testbed Using a Recoverable Experiment - Coronagraph (PICTURE-C) mission is designed to directly image debris disks and exozodiacal dust around nearby stars from a high-altitude balloon using a 60 cm diameter off-axis telescope and a vector vortex coronagraph. During its second flight from Fort Sumner, New Mexico, on September 28, 2022, PICTURE-C successfully used its high and low-order wavefront control systems to perform focal plane wavefront correction for the first time on an observatory in a near-space environment. The coronagraph achieved a modest broadband (20%) contrast of 5 × 10 − 6, with performance limited by dynamic pointing transients. The low-order wavefront control system achieved optical pointing stabilization of 1 milliarcseconds (mas) root mean squared (RMS) over 30 second timescales.

Exascale integrated modeling of low-order wavefront sensing and control for the Roman Coronagraph instrument.

Astronomical instruments to detect exoplanets require extreme wavefront stability. For these missions to succeed, comprehensive and precise modeling is required to design and analyze suitable coronagraphs and their wavefront control systems. In this paper, we describe techniques for integrated modeling at scale that is, to the best of our knowledge, 1000 times faster than previously published works. We show how this capability has been used to validate performance and perform uncertainty quantification for the Roman Coronagraph instrument. Finally, we show how this modeling capacity may be necessary to design and build the next generation of space-based coronagraph instruments.

Binary-star Wave Front Control Based on a Common-path Visible Nulling Coronagraph

Exoplanets around multiple-star systems are interesting targets for direct detection. However, it is difficult to suppress unwanted light from all stars in a system. For direct detection of the exoplanets around multiple-star systems, we propose using common-path visible nulling coronagraphs (CP-VNCs). CP-VNCs can suppress diffracted light from stars located on a central null fringe. In addition, a wave front control system is installed to generate a dark hole by rejecting residual stellar speckles of CP-VNCs. For this purpose, spatial light modulators (SLMs) have the ability to generate a dark hole over a wide field on the order of hundreds of λ/D (where λ is the wavelength of light and D is the telescope diameter), owing to their large number of pixels. We perform a laboratory demonstration of high-contrast observation around a binary-star system model using a CP-VNC combined with an SLM. First, we simulate a high-contrast observation over a region close to one of two stars by generating a square dark hole with side lengths of 40 λ 0/D (where λ 0 is the central wavelength of the light sources). The center of the square region was located about 35 λ 0/D and 79 λ 0/D from each star. Measurements of this first scenario find a mean contrast over the dark hole of 3.5 × 10−8. Second, observation of a region with identical separation (100 λ 0/D) from both stars is also demonstrated. In this scenario, a mean contrast of 2.0 × 10−8 is achieved over a circular dark hole with a diameter of 40 λ 0/D.

Halftone Wave Front Control: Numerical Simulation and Laboratory Demonstration

Abstract High-contrast instruments are required for direct imaging of faint exoplanets around bright host stars. In high-contrast instruments, a wave front control system is needed to generate a dark hole by suppressing residual stellar speckles. However, the achievable contrast is limited by the phase quantization error (i.e., finite phase resolution) of wave front control devices, such as deformable mirrors or spatial light modulators. In this paper, we propose a halftone method for wave front control to improve the contrast using a wave front control device with quantized phase modulation. In a numerical simulation, the contrast was improved from 1.4 × 10−9 to 3.8 × 10−10 by halftone wave front control. In addition, we performed a laboratory demonstration in which a spatial light modulator was used for wave front control, and the contrast was improved from 2.2 × 10−7 to 6.0 × 10−8 for a phase resolution of 2π/256.

High-contrast integral field spectrograph (HCIFS): multi-spectral wavefront control and reduced-dimensional system identification.

Any high-contrast imaging instrument in a future large space-based telescope will include an integral field spectrograph (IFS) for measuring broadband starlight residuals and characterizing the exoplanet's atmospheric spectrum. In this paper, we report the development of a high-contrast integral field spectrograph (HCIFS) at Princeton University and demonstrate its application in multi-spectral wavefront control. Moreover, we propose and experimentally validate a new reduced-dimensional system identification algorithm for an IFS imaging system, which improves the system's wavefront control speed, contrast and computational and data storage efficiency.

Regime-switching model detection map for direct exoplanet detection in ADI sequences

Context. Beyond the choice of wavefront control systems or coronographs, advanced data processing methods play a crucial role in disentangling potential planetary signals from bright quasi-static speckles. Among these methods, angular differential imaging (ADI) for data sets obtained in pupil tracking mode (ADI sequences) is one of the foremost research avenues, considering the many observing programs performed with ADI-based techniques and the associated discoveries. Aims. Inspired by the field of econometrics, here we propose a new detection algorithm for ADI sequences, deriving from the regime-switching model first proposed in the 1980s. Methods. The proposed model is very versatile as it allows the use of PSF-subtracted data sets (residual cubes) provided by various ADI-based techniques, separately or together, to provide a single detection map. The temporal structure of the residual cubes is used for the detection as the model is fed with a concatenated series of pixel-wise time sequences. The algorithm provides a detection probability map by considering two possible regimes for concentric annuli, the first one accounting for the residual noise and the second one for the planetary signal in addition to the residual noise. Results. The algorithm performance is tested on data sets from two instruments, VLT/NACO and VLT/SPHERE. The results show an overall better performance in the receiver operating characteristic space when compared with standard signal-to-noise-ratio maps for several state-of-the-art ADI-based post-processing algorithms.

Design and Modeling of the Off-Axis Parabolic Deformable (OPD) Mirror Laboratory

Coronagraph-equipped direct imaging missions need an active wavefront control system to cancel out the optical aberrations that degrade the performance of the coronagraphs. A fast steering mirror is used to control Line-of-Sight (LoS) pointing error caused by the telescope jitter. In addition to controlling other low-order aberrations such as astigmatism and coma, high stroke, high actuator density deformable mirrors (DMs) are also used to control the electric field at the required high spatial frequencies. We are designing a testbed to verify a different deformable architecture, where the powered optic in the optical train are controllable and have lower actuator count compared to the existing DMs with flat nominal surfaces. This simplifies the packaging issue for space missions and reduces both cost and risk of having the entire coronagraph instrument's performance depending on one or two high-actuator count DMs. The testbed would also be capable of testing different low-order wavefront sensing algorithms, which focuses in the near-term on a new adaptive Kalman filtering and gradient decent method to estimate the harmonic LoS errors that affect space telescopes. In long run, we would test different machine learning techniques to estimate low-order aberrations and non-linear algorithms for digging the region of high contrast called the dark holes (DH).

Low-rank plus sparse decomposition for exoplanet detection in direct-imaging ADI sequences

Data processing constitutes a critical component of high-contrast exoplanet imaging. Its role is almost as important as the choice of a coronagraph or a wavefront control system, and it is intertwined with the chosen observing strategy. Among the data processing techniques for angular differential imaging (ADI), the most recent is the family of principal component analysis (PCA) based algorithms. PCA serves, in this case, as a subspace projection technique for constructing a reference point spread function (PSF) that can be subtracted from the science data for boosting the detectability of potential companions present in the data. Unfortunately, when building this reference PSF from the science data itself, PCA comes with certain limitations such as the sensitivity of the lower dimensional orthogonal subspace to non-Gaussian noise. Inspired by recent advances in machine learning algorithms such as robust PCA, we aim to propose a localized subspace projection technique that surpasses current PCA-based post-processing algorithms in terms of the detectability of companions at near real-time speed, a quality that will be useful for future direct imaging surveys. We used randomized low-rank approximation methods recently proposed in the machine learning literature, coupled with entry-wise thresholding to decompose an ADI image sequence locally into low-rank, sparse, and Gaussian noise components (LLSG). This local three-term decomposition separates the starlight and the associated speckle noise from the planetary signal, which mostly remains in the sparse term. We tested the performance of our new algorithm on a long ADI sequence obtained on beta Pictoris with VLT/NACO. Compared to a standard PCA approach, LLSG decomposition reaches a higher signal-to-noise ratio and has an overall better performance in the receiver operating characteristic space. (abridged).

Methods and limitations of focal plane sensing, estimation, and control in high-contrast imaging

Coronagraphy is a very promising method for directly imaging exoplanets, but the performance of a coronagraph is highly sensitive to quasi-static aberrations within the telescope. The resultant speckles are suppressed in the final focal plane using a wavefront control system that estimates the field at the final focal plane to avoid any noncommon path error. This requires a set of probe images that modulate the field so that it may be estimated. With an estimate of the focal plane electric field, a control law is defined to suppress the speckle field so that the planet can be imaged. Characterizing the planet requires that the speckle field be suppressed simultaneously over the bandpass of interest. The choice of control law, bandpass, estimator, and probing methodology has implications in the control solutions and contrast performance. Here, we compare wavefront probing, estimation, and control algorithms, and describe their practical implementation.

Active compensation of aperture discontinuities for WFIRST-AFTA: analytical and numerical comparison of propagation methods and preliminary results with a WFIRST-AFTA-like pupil

The new frontier in the quest for the highest contrast levels in the focal plane of a coronagraph is now the correction of the large diffractive artifacts effects introduced at the science camera by apertures of increasing complexity. The coronagraph for the WFIRST/AFTA mission will be the first of such instruments in space with a two Deformable Mirrors wavefront control system. Regardless of the control algorithm for these multi Deformable Mirrors, they will have to rely on quick and accurate simulation of the propagation effects introduced by the out-of-pupil surface. In the first part of this paper, we present the analytical description of the different approximations to simulate these propagation effects. In Annex A, we prove analytically that, in the special case of surfaces inducing a converging beam, the Fresnel method yields high fidelity for simulations of these effects. We provide numerical simulations showing this effect. In the second part, we use these tools in the framework of the Active Compensation of Aperture Discontinuities technique (ACAD) applied to pupil geometries similar to WFIRST-AFTA. We present these simulations in the context of the optical layout of the High-contrast imager for Complex Aperture Telescopes, which will test ACAD on a optical bench. The results of this analysis show that using the ACAD method, an apodized pupil lyot coronagraph and the performance of our current deformable mirrors, we are able to obtain, in numerically simulations, a dark hole with an AFTA-like pupil. Our numerical simulation shows that we can obtain contrast better than $2.10^{-9}$ in monochromatic light and better than 3.e-8 with 10% bandwidth between 5 and 14 lambda/D.

Wavefront control in space with MEMS deformable mirrors for exoplanet direct imaging

To meet the high contrast requirement of 1×10 −10 to image an Earth-like planet around a sun-like star, space telescopes equipped with coronagraphs require wavefront control systems. Deformable mirrors (DMs) are a key element of a wavefront control system, as they correct for imperfections, thermal distortions, and diffraction that would otherwise corrupt the wavefront and ruin the contrast. The goal of the CubeSat DM technology demonstration mission is to test the ability of a microelectromechanical system (MEMS) DM to perform wavefront control on-orbit on a nanosatellite platform. We consider two approaches for an MEMS DM technology demonstration payload that will fit within the mass, power, and volume constraints of a CubeSat: (1) a Michelson interferometer and (2) a Shack-Hartmann wavefront sensor. We clarify the constraints on the payload based on the resources required for supporting CubeSat subsystems drawn from subsystems that we have developed for a different CubeSat flight project. We discuss results from payload laboratory prototypes and their utility in defining mission requirements.

Mirror actively deformed and regulated for applications in space: design and performance

The need for both high quality images and lightweight structures is one of the main drivers in space telescope design. An efficient wavefront control system will become mandatory in future large observatories, retaining performance while relaxing specifications in the global system's stability. We present the mirror actively deformed and regulated for applications in space project, which aims to demonstrate the applicability of active optics for future space instrumentation. It has led to the development of a 24-actuator, 90-mm-diameter active mirror, able to compensate for large lightweight primary mirror deformations in the telescope's exit pupil. The correcting system has been designed for expected wavefront errors from 3-m-class lightweight primary mirrors, while also taking into account constraints for space use. Finite element analysis allowed an optimization of the system in order to achieve a precision of correction better than 10 nm rms. A dedicated testbed has been designed to fully characterize the integrated system performance in representative operating conditions. It is composed of: a telescope simulator, an active correction loop, a point spread function imager, and a Fizeau interferometer. All conducted tests demonstrated the correcting mirror performance and has improved this technology maturity to a TRL4.

嵌入式波前处理与控制系统设计

嵌入式波前处理与控制系统设计

Weighted Fried reconstructor and spatial-frequency response optimization of Shack–Hartmann wavefront sensing

Two modified Fried wavefront reconstructors are proposed, both based on an enhanced geometry that combines and balances those of Fried and Hudgin with an additional weight. The optimal weights for both of them are derived with the analytical frequency response functions, which can provide near-unity spatial frequency response over broad bandwidth to the Shack-Hartmann wavefront sensing system. Comparisons between the proposed reconstructors and the classical ones are presented in the frequency domain, and simulations have further confirmed the frequency characteristic and the reconstruction performance of the new reconstructors. It is expected that the optimized reconstructors can help improve the performance of adaptive optics systems for high-power laser wavefront control and other relevant optical systems that require high-accuracy wavefront sensing.

Wavefront Control System for Phase Compensation in Hard X-ray Optics

A highly precise adaptive optical system that can be used in the hard X-ray region was developed. To achieve highly precise control of the wavefront shape, we discussed an optical system with a bendable mirror of deformation accuracy better than 0.4 nm RMS. Using the system, we demonstrated the controllability of the wavefront of a 15 nm hard X-ray nanobeam. The intensity profile of the wavefront-modified beam was in good agreement with the wave-optically calculated profile.

Modeling and performance limits of a large aperture high-resolution wavefront control system based on a liquid crystal spatial light modulator

The aberration introduced by the primary optical element of a lightweight large aperture telescope can be corrected with a diffractive optical element called the liquid crystal spatial light modulator. Such aberration is usually very large, which makes the design and modeling of such a system difficult. A method to analyze the system is introduced, and the performance limitation of the system is studied through extensive modeling. An experimental system is demonstrated to validate the analysis. The connection between the modeling data and the experimental data is given.

Closed loop, DM diversity-based, wavefront correction algorithm for high contrast imaging systems

High contrast imaging from space relies on coronagraphs to limit diffraction and a wavefront control systems to compensate for imperfections in both the telescope optics and the coronagraph. The extreme contrast required (up to 10(-10) for terrestrial planets) puts severe requirements on the wavefront control system, as the achievable contrast is limited by the quality of the wavefront. This paper presents a general closed loop correction algorithm for high contrast imaging coronagraphs by minimizing the energy in a predefined region in the image where terrestrial planets could be found. The estimation part of the algorithm reconstructs the complex field in the image plane using phase diversity caused by the deformable mirror. This method has been shown to achieve faster and better correction than classical speckle nulling.

Reflectivity and optical surface height requirements in a broadband coronagraph 1Contrast floor due to controllable spatial frequencies

We derive the broadband contrast floor in a coronagraphic telescope having nonideal optical surfaces. We consider only fundamental spatial frequencies within the control bandwidth of the coronagraph's deformable mirror. Cross terms arising from the beating of spatial frequencies beyond the deformable mirror control bandwidth will be considered in a second paper. Two wavefront control systems are analyzed:a zero-path difference Michelson interferometer with two deformable mirrors at a pupil image, and a sequential pair of deformable mirrors with one placed at a pupil image. We derive requirements on optical surface figure and reflectivity uniformity for both cases.

Closed-loop Wavefront Correction for High Contrast Imaging: The “peak-a-boo” Algorithm

High contrast imaging from space must overcome photon noise of the diffracted star light and scattered light from optical components defects. The very high contrast required (up to $10^{-10}$ for terrestrial planets) puts severe requirements on the wavefront control system, as the achievable contrast is limited by the quality of the wavefront. In this paper we present a closed loop correction method for the shaped pupil coronagraph to minimize the energy in a pre-defined region in the image where terrestrial planets would be found. The reconstruction part uses three intensity measurements in the image plane with a pinhole added to the shaped pupil for diversity. This method has been shown in simulations to be effective in the presence of photon noise, deformable mirror modeling errors and overcoming defective actuators in the deformable mirror.

Fusion of adaptive beam steering and optimization-based wavefront control for laser communications in atmosphere

The performance of mobile laser communication systems operating within Earth's atmosphere is generally limited by the pointing errors due to movement of the platforms and mechanical vibrations. In addition, atmospheric turbulence causes changes of the refractive index along the propagation path, creating random redistribution of the optical energy in the spatial domain. Under adverse conditions these effects lead to increased bit error rate. While traditional approaches provide separate treatment of these problems, suggesting high-bandwidth beam steering systems for tracking and wavefront control for the mitigation of atmospheric effects, the two tasks can be integrated. This paper presents a hybrid laser beam-steering–wavefront-control system comprising an electrically addressed spatial light modulator (SLM) installed on the Omni-Wrist sensor mount. The function of the Omni-Wrist is to provide coarse steering over a wide range of pointing angles, while that of the SLM is twofold: wavefront correction and fine steering. The control law for the Omni-Wrist is synthesized using a decentralized approach that provides independent access to the azimuth and declination channels; calculation of the required phase profile for the SLM is optimization-based. This paper presents the control algorithms, the approach to coordinating the operation of the two systems, and the results.