The Habitable Worlds Observatory will have uniquely stringent wavefront stability requirements, in the single-digit picometers for observations lasting days, to preserve coronagraph contrast for imaging earth-like exoplanets. This need will be addressed using high-precision Wavefront Sensing and Control methods, including continuous picometerprecision metrology and control of the Optical Telescope Assembly (OTA). This paper reviews methods for initializing and maintaining the OTA wavefront, evolved from those used for the James Webb Space Telescope, but extended to much higher precision. It concludes by identifying performance targets for WFSC technology development, to help guide NASA technology investments.
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.
We propose an approach for coarse alignment of a segmented space telescope using science instrument images. The recommended steps go from large post launch rigid body misalignments to within the capture range of coarse phasing where segment piston error is the predominant residual wavefront error. These steps include five data collection and analysis methods comprising of metrology capture, segment capture and identification, segment translation, segment stacking, and fine alignment. Using a proposed architecture for the NASA Habitable Worlds Observatory (HWO) we describe the details of our recommended approach for each telescope alignment step. We then compare this recommended sequence to alternative alignment progressions used in existing segmented testbeds and telescopes in terms of number of data collections required. This model-based demonstration establishes that the recommended coarse and fine alignment sequence performs more efficiently in time and resource cost, handing off to coarse and fine phasing activities further along the telescope commissioning process.
Current approaches for phasing of segmented space telescopes have required complex dedicated optics and mechanisms, such as Dispersed Hartmann sensors or grisms. These methods do not scale well as the number of segments increases. The broadband phasing approach used at the Keck Observatory does scale well and can work on space telescopes without the need for any additional hardware. We show that this method implemented as white light interferometry (WLI), using a standard imaging detector and filters, has a capture range limited only by the range of the segment actuators and can easily phase the mirrors to within the capture range of single wavelength phasing methods. An analysis of the Keck broadband phasing performance is presented and used to develop a formula for implementation of WLI on other segmented telescopes. As an example, a WLI implementation for the NASA Habitable Worlds Observatory telescope is developed and demonstrated via detailed wave-optics simulations. The implementation, performance and limitations of the proposed WLI method are discussed in detail in the paper.
KEYWORDS: Coronagraphy, Electric fields, Scalable video coding, Model based design, Wavefront sensors, Cameras, Spiral phase plates, Wavefronts, Design, Space telescopes
Future space telescope coronagraph instruments hinge on the integration of high-performance masks and precise wavefront sensing and control techniques to create dark holes essential for exoplanet detection. Recent advancements in wavefront control algorithms might exhibit differing performances depending on the coronagraph used. This research investigates three model-free and model-based algorithms in conjunction with either a vector vortex coronagraph or a scalar vortex coronagraph under identical 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 in narrowband and broadband light from the In-Air Coronagraph Testbed at the Jet Propulsion Laboratory. We find that model-free dark hole digging methods achieve broadband contrasts comparable to model-based methods, and we highlight the calibration costs of model-free methods compared with model-based approaches. This study also reports the first time that electric field conjugation with the self-coherent camera has been applied for simultaneous multi-subband correction with a field stop. This study compares the advantages and disadvantages of each of these wavefront sensing and control algorithms with respect to their potential for future space telescopes.
Due to the reporting from the Astro2020: Decadal Survey for Astronomy and Astrophysics, the direct imaging of Earth-like exoplanets has become a leading priority for research and development in astrophysics. In response to this survey, NASA has selected the Habitable Worlds Observatory (HWO) as its next astrophysics flagship mission. However, the current state-of-the-art coronagraph technology is not sufficient to achieve the required contrast (approximately 10−10) with HWO’s proposed configuration. Improving the technology readiness level of coronagraphs for HWO will require extensive research and development. To increase its capacity to support this objective and to provide the community with a new resource to perform versatile coronagraph technology development, NASA’s High Contrast Imaging Testbed (HCIT) facility at the Jet Propulsion Laboratory (JPL) has commissioned the Decadal Survey Testbed 2 (DST2), a state-of-the-art vacuum coronagraph testbed. It’s commissioned architecture incorporates a 2K-actuator Boston Micromachines deformable mirror and a traditional Lyot coronagraph and features a flexible design capable of accommodating different coronagraph technologies including tip/tilt mirrors, segmented apertures, reflective pupils, disturbance injection, wavefront sensing and control, and the integration of additional sources or telescope simulators with minimal reconfiguration. DST2 has repeatably achieved 1 × 10−9 monochromatic raw contrast, and 3.3 × 10−9 raw contrast in a 10% bandwidth centered at 550nm in a 180 degree dark hole from 3λ/D to 9λ/D. Additionally, DST2 uses new 20-bit DM electronics capable of a 650 femtometer DM actuator resolution which meets future requirements for HWO. As of August 2023, DST2 has begun operations to meet scientific milestones set by the principal investigators of NASA Strategic Astrophysics Technologies awards. Parallel to these priorities, work to determine contrast limitations will be performed.
The planned Habitable Worlds Observatory (HWO) will be designed to find and characterize Earth-like planets in the habitable zones of sun-like stars. This mission will require an instrument capable of making measurements of planet spectra at high contrast and small inner working angles. Previous demonstrations of existing coronagraph designs have not achieved these requirements at the 20% bandwidths or better required for full spectroscopic characterization, even in a laboratory environment. One technique that shows promise for improving bandwidth is the allocation of Deformable Mirror (DM) degrees of freedom to improve bandwidth by relaxing requirements on the size of the coronagraph’s focal plane correction region, or Dark Zone (DZ). In this work, we provide details and results of a laboratory demonstration of a wide-band coronagraph in NASA’s High-Contrast Imaging Testbed (HCIT) at the Jet Propulsion Laboratory (JPL). We show that a D-shaped region on one half of the focal plane can be corrected to a contrast of 4×10−10 in a 20% bandwidth using a simple Lyot coronagraph with two DMs. We also provide details of future plans to improve contrast and bandwidth by further reducing the dark region to just the light coupled into a single-mode fiber in the coronagraph’s focal plane.
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 .
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.
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.
The unparalleled theoretical performance of an ideal vector vortex coronagraph makes it one of the most promising technologies for directly imaging exoplanets with a future, off-axis space telescope. However, the image contrast required for observing the light reflected from Earth-sized planets (∼ 10−10) has yet to be demonstrated in a laboratory setting. With recent advances in the manufacturing of liquid crystal vector vortex waveplates as well as system-level performance improvements on our testbeds, we have achieved raw contrast of 1.57×10−9 and 5.86×10−9 in 10% and 20% optical bandwidths, respectively, averaged over 3-10 λ/D separations on one side of the pseudo-star. The former represents a factor of 10 improvement over the previously reported performance. We will show experimental comparisons of the contrast achieved and a function of spectral bandwidth. We provide estimates of the limiting error terms and discuss the improvements needed to close the gap in contrast performance required for future exoplanet imaging space telescopes.
One of the most profound scientific questions is whether there are other Earth-like worlds. To image such exoplanets directly, the Astro2020 Decadal Survey highly recommends technology development for a future 6- meter, space-based observatory equipped with a coronagraph. The vector vortex coronagraph (VVC) is a top candidate for providing the necessary 10−10 planet-to-star contrast ratio needed at small angular separations to image and characterize Earth-like exoplanets. The VVC is best suited for an unobscured aperture, but for a 6-meter, space-based observatory the primary mirror may need to be segmented. In this paper, we describe our laboratory experiments testing a charge four VVC with a segmented, off-axis entrance pupil. The extra diffraction from the pupil segmentation was suppressed with the single Boston Micromachines deformable mirror in the optical system. We achieved a mean normalized intensity of 4.7 × 10−9 from 3 − 10 λ/D in a 10% spectral bandwidth in the Decadal Survey Testbed at the Jet Propulsion Laboratory. To determine our current limitations and to enable 10−10 contrast in future tests, we compare to other testbed results with a monolithic aperture.
Following the success of the Decadal Survey Testbed (DST), the HCIT team at JPL has developed a companion testbed, the Decadal Survey Testbed 2 (DST2), that further implements lessons learned from DST and from recent modeling work at JPL in support of the HabEx concept. Commissioning for DST2 is currently targeted for Fall 2021. Here we report on the detailed design of DST2 and status of integration and testing highlighting comparisons/changes from the original DST. Expected performance is summarized here, with details of the modeling effort provided in Noyes et al. also in these proceedings.
Coronagraph instruments rely on predictable and stable deformable mirror (DM) surface displacement to achieve the contrast required to detect Earth-sized exoplanets in the habitable zone of their host star. Anomalous DM behavior, such as unstable or pinned actuators, can limit contrast in coronagraphs. Simulating how these undesired behaviors affect the performance of a high contrast imaging architecture is important for developing requirements on their associated hardware. Simulating a vortex coronagraph (VC) with two deformable mirrors, this study quantifies how the number of pinned actuators affects the performance of Focal Plane Wavefront Sensing and Control algorithms using both Grid Search Electric Field Conjugation (EFC) and Planned EFC, which uses Beta-Bumping. The simulations also quantify how various types of voltage noise such as zero-mean Gaussian noise, zero-mean periodic noise, and drift can affect the contrast of a VC during an observation run. A tolerance of a change in the Mean Normalized Intensity of 1 × 10−11 is allocated to both types of error. If Planned EFC is used, only 1 pinned actuator on both DMs can be tolerated. If only pure Grid Search EFC is used the DMs cannot have any pinned actuators. For the case of zero-mean Gaussian noise and zero-mean periodic noise, one can tolerate a noise standard deviation of no more than σ = 0.45 mV. For drift, one can only tolerate σ = 0.30 mV or less. These results show that the DM electronics and the DMs themselves need to be nearly defect free to avoid having more than 1 pinned actuator. It is important that the electronics designer attempts to minimize the noise by not only selecting high quality components but also control the output voltage to minimize drift.
Future space telescopes with coronagraph instruments will use a wavefront sensor (WFS) to measure and correct for phase errors and stabilize the stellar intensity in high-contrast images. The HabEx and LUVOIR mission concepts baseline a Zernike wavefront sensor (ZWFS), which uses Zernike’s phase contrast method to convert phase in the pupil into intensity at the WFS detector. In preparation for these potential future missions, we experimentally demonstrate a ZWFS in a coronagraph instrument on the Decadal Survey Testbed in the High Contrast Imaging Testbed facility at NASA’s Jet Propulsion Laboratory. We validate that the ZWFS can measure low- and mid-spatial frequency aberrations up to the control limit of the deformable mirror (DM), with surface height sensitivity as small as 1 pm, using a configuration similar to the HabEx and LUVOIR concepts. Furthermore, we demonstrate closed-loop control, resolving an individual DM actuator, with residuals consistent with theoretical models. In addition, we predict the expected performance of a ZWFS on future space telescopes using natural starlight from a variety of spectral types. The most challenging scenarios require ∼1 h of integration time to achieve picometer sensitivity. This timescale may be drastically reduced by using internal or external laser sources for sensing purposes. The experimental results and theoretical predictions presented here advance the WFS technology in the context of the next generation of space telescopes with coronagraph instruments.
We have previously introduced a high throughput multiplexing computational spectral imaging device. The device measures scalar projections of pseudo-arbitrary spectral filters at each spatial pixel. This paper discusses simulation and initial experimental progress in performing computational spectral unmixing by taking advantage of the natural sparsity commonly found in the fractional abundances. The simulation results show a lower unmixing error compared to traditional spectral imaging devices. Initial experimental results demonstrate the ability to directly perform spectral unmixing with less error than multiplexing alone.
Traditional approaches to persistent surveillance generate prodigious amounts of data, stressing storage, communication,
and analysis systems. As such, they are well suited for compressed sensing (CS) concepts. Existing
demonstrations of compressive target tracking have utilized time-sequences of random patterns, an approach
that is sub-optimal for real world dynamic scenes. We have been investigating an alternative architecture that
we term SCOUT-the Static Computational Optical Undersampled Tracker-which uses a pair of static masks
and a defocused detector to acquire a small number of measurements in parallel. We will report on our working
prototypes that have demonstrated successful target tracking at 16x compression.
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