An integral field spectrograph (IFS) camera may help fulfill the exoplanet characterization goals of a future Habitable Worlds Observatory. The Roman Space Telescope Coronagraph Instrument Project established the laboratory performance baseline of a combined coronagraph and IFS system with the Prototype Imaging Spectrograph for Coronagraphic Exoplanet Studies (PISCES). New demonstrations are needed to expand on this milestone in terms of contrast, bandwidth, and field of view towards the requirements of the next flagship mission. Here we present the design of a successor to PISCES that can observe a 20 lambda/D field of view with a 30% instantaneous bandwidth at visible wavelengths with a resolving power of 70. The instrument will interface with the coronagraph on the Decadal Survey Testbed 2 located in the High Contrast Imaging Testbed 2 vacuum chamber at NASA's Jet Propulsion Laboratory, to support demonstrations of broadband wavefront sensing and control and data post-processing techniques.
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.
Deformable mirrors (DMs) are a critical technology to enable coronagraphic direct imaging of exoplanets with current and planned ground- and space-based telescopes as well as future mission concepts, such as the Habitable Worlds Observatory (HWO), which aims to image exoplanet types ranging from gas giants to Earth analogs. These missions set several requirements on the DMs such as large actuator count (≥96×96) and resolution smaller than 2.5 pm. This paper presents the first demonstration of single-picometer wavefront control utilizing a new high-resolution, vacuum-compatible DM electronics and a Zernike Wavefront Sensor for measurement. The controller can handle 2,040 actuators with 125 V maximum voltage with 20-bit resolution, resulting in voltage steps of 119 microvolts that allowed us to demonstrate 0.65 pm resolution of the DM surface.
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.
Accurate measurement of exoplanetary masses is a critical step in addressing key aspects of NASA’s science vision. Measuring masses of Earth-analogs around FGK stars out to 10 pc requires sub-microarcsecond astrometric accuracy, which is not within the capabilities of current instrumentation. Thus, new technology will be required to build an astrometric instrument capable of achieving such performance. This will immediately empower the possibility for dedicated astrometric missions, and perhaps most enticingly, it will enable astrometric observing modes to be added to any mission boasting a sufficiently stable direct imaging platform. In this paper, we provide an overview of the scientific goals and technology utilized on NASA’s testbeds dedicated to advancing stellar astrometry for exoplanet detection. The first one, located at the Jet Propulsion Laboratory (JPL), is dedicated to imaging stellar astrometry on sparse fields. The goal of this testbed is to mature the Diffractive Pupil technology to TRL-5, demonstrating high-fidelity performance in a relevant environment. This testbed operates in a vacuum tank at the High Contrast Imaging Testbed (HCIT) at JPL and has demonstrated detection of signals of 1.58e-5 λ/D which is equivalent to 0.75 μas on Hubble. The second testbed is also located at JPL, but it is dedicated to advancing narrow angle relative astrometry to detect exoplanets around nearby binary stars. The key technology in this testbed is a diffractive pupil specially designed to measure the angle between two sources on the sky. This testbed operates in air now, but we are designing a new version of this testbed that will operate in vacuum with the goal of demonstrating sub-microarcsecond accuracy astrometric measurements between binary stars.
Detecting the astrometric signal of exoplanets is a powerful way to determine its mass, but the small size of the signal presents a challenge. For Earth-like planets around Sun-like stars within 10 pc the signal is expected to be a microarcsecond or less. We developed an astrometry testbed to simulate equivalent signals using an illuminated pinhole array and a flexurized central pinhole with picometer resolution. The active wide-field sky simulator is the critical component in the testbed that simulates the small astrometric signals. We report its performance on measuring and correcting optical aberrations caused by thermal distortions and pointing jitter which are later calibrated by a diffractive pupil.
Here we describe the architecture of a compact adaptive optics system that is designed specifically to couple light into a single-mode optical fiber. The dramatic reduction in size of this instrument is afforded by the reduction in pupil size by MEMs deformable mirrors. Or system also enables fast tip/tilt control as well as atmospheric dispersion correction – necessary for extended sky coverage. The heritage and architecture of this instrument will be described. We will also cover performance modeling, and status of the hardware and software build. We will describe our plans to couple AO systems with spectrometers to enable dramatically increased stability with implications for extreme precision radial velocity needed for the detection of exoEarths.
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.
Directly imaging Earth-sized exoplanets with a visible-light coronagraph instrument on a space telescope will require a system that can achieve approximately 10−10 raw contrast and maintain it for the duration of observations (on the order of hours or more). We are designing, manufacturing, and testing Dual Purpose Lyot coronagraph (DPLC) masks that allow for simultaneous wavefront sensing and control using out-of-band light to maintain high contrast in the science focal plane. Our initial design uses a tiered metallic focal plane occultor to suppress starlight in the transmitted coronagraph channel and a dichroic-coated substrate to reflect out-of-band light to a wavefront sensing camera. The occultor design introduces a phase shift such that the reflected channel is a Zernike wavefront sensor. The dichroic coating allows higher-order wavefront errors to be detected which is especially critical for compensating for residual drifts from an actively-controlled segmented primary mirror. A second-generation design concept includes a metasurface to create polarization-dependent phase shifts in the reflected beam, which has several advantages including an extended dynamic range. We will present the focal plane mask designs, characterization, and initial testing at NASA’s High Contrast Imaging Testbed (HCIT) facility
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 .
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.
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.
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.
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.
Measuring the astrometric singal of an exoplanet is an unambiguous method for determining its mass. However, Earth-like planets around Sun-like stars only cause 0.3 uas astrometric signals, which is too small for current instruments to detect. To advance these instruments, an astrometry testbed was created. It can simulate and measure equivalent signals with the use of an illuminated pinhole array and a flexurized pinhole that is translatable to 10 pm resolution. Optical distortions are calibrated with the use of a diffractive pupil. This paper presents the requirements, design, and implementation of the wide-field astrometry testbed’s light source.
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