NASA’s Habitable Worlds Observatory will consist of a segmented telescope and high contrast coronagraph to characterize exoplanets for habitability. Achieving this objective requires an ultra-stable telescope with wavefront stability of picometers in certain critical modes. The NASA funded Ultra-Stable Large Telescope Research and Analysis – Technology Maturation program has matured key component-level technologies in 10 areas spanning an “ultra-stable” architecture, including active components like segment edge sensors, actuators and thermal hardware, passive components like low distortion mirrors and stable structures, and supporting capabilities like precision metrology. This paper will summarize the final results from the four-year ULTRA-TM program, including advancements in performance and/or path-to-flight readiness, TRL/MRL maturation, and recommendations for future work.
The James Webb Space Telescope (JWST) launched on December 25, 2021, and its optical performance in orbit has been even better than predicted pre-flight. The static wavefront error (WFE) is less than half the value specified for the requirement of having diffraction-limited image quality at 2 microns in the NIRCam shortwave channel, enabling the observatory to deliver both sharper images and higher sensitivity than anticipated. In addition to the excellent image quality, the optical stability has also exceeded expectations, both in terms of high-frequency dynamic contributions (which would be perceived as part of “static WFE”) and in terms of drifts over minutes, hours, and days. Stability over long timescales is critical for several important science cases, including exoplanet transit spectroscopy and coronagraphy. JWST’s stability success was achieved through detailed design and testing, with several important lessons learned for future observatories, especially the Habitable Worlds Observatory that is expected to need even higher levels of stability. We review the stability architecture, how it was technologically demonstrated, the ground test results and improvements, the on-orbit results, and the lessons learned.
NASA’s Habitable Worlds Observatory will consist of a segmented telescope and high contrast coronagraph to characterize exoplanets for habitability. Achieving this objective requires an ultra-stable telescope with wavefront stability of picometers in certain critical modes. The NASA funded Ultra-Stable Large Telescope Research and Analysis – Technology Maturation program continues to mature key component-level technologies for this new regime of “ultra-stable optical systems,” including active components like segment edge sensors, actuators and thermal hardware, passive components like low distortion mirrors and stable structures, and supporting capabilities like precision metrology. This paper will present an update to the latest results from hardware testbeds and simulations in the areas listed above. It will also contain a correction to previously published results of Ball’s Integrated Demo, which consists of a capacitive sensor and three actuators operating in closed loop.
Segmented-mirror telescopes such as JWST and Keck provide a particular challenge when first pointed to the sky: to access a suitably isolated star with which to align the mirror segments, one must first determine the sky location to which the telescope is pointed. Prior to stacking, the primary mirror segments each produce a separate image of the sky; the expected result is a confusing image in which the star field is convolved with the randomly pointed segments so that each star appears multiple times. To establish the initial sky pointing of JWST, we have developed a pair of novel and complementary approaches for identifying the field. The first approach uses image pairs in which a single primary mirror segment is tilted from its initial pointing by a small amount. This motion of the segment produces a corresponding motion of the stellar images from that segment, allowing us to resolve the ambiguity between the array of stellar images and the array of segment images. The second performs a pattern match within a single image to identify the repeating pattern of the segment array (i.e., the star pattern in the field of view) which can then be matched against an astrometric catalog. Both algorithms produce a resulting array of star positions from which the astrometry.net engine can identify the sky location. We describe the application of these algorithms to both simulated JWST NIRCam images and actual images acquired with MOSFIRE on Keck I, explain how we employed these approaches during the initial stage of JWST primary mirror commissioning, and speculate on future applications for mirrors with more segments.
The 2020 decadal survey presents a clear message of the grand astronomy goals of the next decade and beyond, and of the urgent need for technology maturation that will enable the next flagship observatory to observe potentially habitable exoplanets. For a segmented implementation of a large ultra-stable telescope, low TRL areas such as segment sensing and control at the picometer scale have been identified as critical areas for significant technological improvements to accomplish the survey’s grand goals. We present exciting results on picometer scale sensing and actuation in certain temporal and spatial bandwidths as key advances towards addressing this technology gap. We have designed and demonstrated a capacitive testbed for informing different edge sensing architectures, and qualified our novel ultra-fine stage actuator using an optical distance measuring interferometer. We have also integrated the capacitive sensor and our ultra-fine stage actuator for an integrated demo with few picometer noise floor, sensing and actuator resolution. These key results will roll into the design of subscale demonstrations of these components in a future flight-like layout.
The recently released Astro2020 Decadal Survey recommends a large IR/O/UV space telescope that can observe potentially habitable exoplanets. Achieving this goal requires a telescope with wavefront stability on the order of picometers in some modes. The Ultra-Stable Large Telescope Research and Analysis – Technology Maturation (ULTRATM) program has matured key component-level technologies for this new regime of “ultra-stable optical systems,” including active components like segment edge sensors, actuators and thermal sensing and control hardware, as well as passive components like low distortion mirror mounts and stable composites for structures. Hardware testbeds have demonstrated component performance in the desired regime and with path-to-flight properties and simulations have applied those results to the flight system. These component level demonstrations are a critical step to enable subsequent subsystem and system level demonstrations of an ultra-stable telescope.
The James Webb Space Telescope (JWST) is a segmented deployable telescope, currently operating at L2. The telescope utilizes 6 degrees of freedom for adjustment of the Secondary Mirror (SM) and 7 degrees of freedom for adjustment of each of its 18 segments in the Primary Mirror (PM). After deployment, the PM segments and the SM arrived in their correct optical positions to within a ~1 mm, with accordingly large wavefront errors. A Wavefront Sensing and Controls (WFSC) process was executed to adjust each of these optical elements in order to correct the deployment errors and produce diffraction-limited images across the entire science field. This paper summarizes the application of the WFSC process.
Commissioning the Webb telescope to realize the observatory’s full capability necessitated the development of robust wavefront sensing and control processes. These processes rely on techniques that were adapted or newly innovated for the mission, and further adaptation of these techniques may be expected for future segmented telescopes. Over the course of mission development, these techniques were refined to form a baseline wavefront commissioning plan that assumes several conditions and performance requirements are met. Herein we present efforts carried out to define and develop contingency concepts of operation for Webb telescope commissioning, and the mission-level approach to managing the response to deviations from the baseline plan in the event of significant off-nominal or anomaly scenarios encountered by the wavefront team. An overview of selected contingencies is presented along with more detailed example model cases and instances of interest encountered in flight.
To achieve the ambitious science goal of performing direct imaging of earth-like exoplanets with a high contrast coronagraph, future space-based astronomical telescopes will require wavefront stability several orders of magnitude beyond state-of-the-art. The Ultra-Stable Large Telescope Research and Analysis – Technology Maturation (ULTRA-TM) program is maturing key component-level technologies for this new regime of “ultra-stable optical systems” through hardware testbeds that demonstrate component performance in the desired picometer regime and with path-to-flight properties. This paper describes the initial results from these testbeds – which address key capabilities across the ultrastable architecture and include active components like segment edge sensors, actuators and thermal sensing and control hardware, as well as passive components like low distortion mirror mounts and stable composites for structures. These promising experimental results are the first steps in our team’s technical maturation plan to credibly enable a large, ultrastable telescope in space. The resulting component, sub-system and system roadmaps are meant to support planning for technology development efforts for future NASA missions.
To achieve the ambitious goal of directly imaging exo-Earths with a coronagraph, future space-based astronomical telescopes will require wavefront stability several orders of magnitude beyond state-of-the-art. The Ultra-Stable Large Telescope Research and Analysis – Technology Maturation (ULTRA-TM) program will mature critical technologies for this new regime of “ultra-stable optical systems” through component-level hardware demonstrations.
This paper describes the progress towards demonstrating performance of these technologies in the picometer regime and with flight-like properties – including active systems like segment sensing and actuation and thermal sensing and control, as well as passive systems like low distortion mirror mounts and composite structures. Raising the TRL of these technologies will address the most difficult parts of the stability problem with the longest lead times and provide significant risk reduction for their inclusion in future mission concepts.
This work presents a detailed current performance analysis for the telescope, pointing, and coronagraph com- ponent subsystems of the Segmented Aperture Interferometric Nulling Testbed (SAINT). The project pairs an active segmented mirror with the Visible Nulling Coronagraph (VNC) towards demonstrating capabilities for the future space observatories needed to directly detect and characterize Earth-sized worlds around nearby stars. We describe approaches to optimize subsystem wavefront sensing and control parameters, summarizing relevant scal- ing relations between these parameters, residual errors, and observed contrast measurements. Preliminary results from diagnostic testing under various control states are presented along with intermediate contrast measurements towards demonstrating the full system.
This work presents updates to the coronagraph and telescope components of the Segmented Aperture Interferometric Nulling Testbed (SAINT). The project pairs an actively-controlled macro-scale segmented mirror with the Visible Nulling Coronagraph (VNC) towards demonstrating capabilities for the future space observatories needed to directly detect and characterize a significant sample of Earth-sized worlds around nearby stars in the quest for identifying those which may be habitable and possibly harbor life. Efforts to improve the VNC wavefront control optics and mechanisms towards repeating narrowband results are described. A narrative is provided for the design of new optical components aimed at enabling broadband performance. Initial work with the hardware and software interface for controlling the segmented telescope mirror is also presented.
We present a progress report on the development of new broadband mirror coatings that demonstrate ⪆ 80% reflectivities from 1020−5000Å. Four different coating recipes are presented as candidates for future far-ultraviolet (FUV) sensitive broadband observatories. Three samples were first coated with aluminum (Al) and lithium fluoride (LiF) at the NASA Goddard Space Flight Center (GSFC) using a new high-temperature physical vapor deposition (PVD) process. Two of these samples then had an ultrathin (10−20 Å) protective coat of either magnesium fluoride (MgF2) or aluminum fluoride (AlF3) applied using atomic later deposition (ALD) at the NASA Jet Propulsion Laboratory (JPL). A fourth sample was coated with Al and a similar high temperature PVD coating of AlF3. Polarized reflectivities into the FUV for each sample were obtained through collaboration with the Synchrotron Ultraviolet Radiation Facility at the National Institute of Standards and Technology. We present a procedure for using these reflectivities as a baseline for calculating the optical constants of each coating recipe. Given these results, we describe plans for improving our measurement methodology and techniques to develop and characterize these coating recipes for future FUV missions.
A wide array of general astrophysics studies including detecting and characterizing habitable exoplanets could be enabled by a future large segmented telescope with sensitivity in the UV, optical, and infrared bands. When paired with a starshade or coronagraph, such an observatory could enable direct imaging and detailed spectroscopic observations of nearby Earth-like habitable zone planets. Over the past several years, a laboratory-based Visible Nulling Coronagraph (VNC) has evolved to reach requisite contrasts over a ~ 1 nm bandwidth at narrow source angle separation using a segmented deformable mirror in one arm of a Mach-Zehnder layout. More recent efforts targeted broadband performance following the addition of two sets of half-wave Fresnel rhomb achromatic phase shifters (APS) with the goal of reaching 10-9 contrast, at a separation of 2λ/D, using a 40 nm (6%) bandwidth single mode fiber source. Here we present updates on the VNC broadband nulling effort, including approaches to addressing system contrast limitations.
KEYWORDS: Space telescopes, Interferometry, Telescopes, Mirrors, Coronagraphy, Image segmentation, Point spread functions, Exoplanets, Control systems, James Webb Space Telescope
This work presents an overview of the Segmented Aperture Interferometric Nulling Testbed (SAINT), a project
that will pair an actively-controlled macro-scale segmented mirror with the Visible Nulling Coronagraph (VNC).
SAINT will incorporate the VNC’s demonstrated wavefront sensing and control system to refine and quantify
end-to-end high-contrast starlight suppression performance. This pathfinder testbed will be used as a tool
to study and refine approaches to mitigating instabilities and complex diffraction expected from future large
segmented aperture telescopes.
The contrast and angular resolution required to directly image and characterize mature exoplanetary systems place stringent requirements on the space-based telescopes and starlight suppression systems needed to study spatial distributions of debris disks, exozodiacal dust, and individual planets at multiple epochs in their orbits. A nulling interferometer (nuller) is a coronagraphic suppression system that can be used with all telescope types, including those with obscured and segmented apertures envisioned for upcoming and future observatories. One of the challenges for detection and characterization of exoplanetary signals is achieving high contrast with broad spectral coverage. This work presents design concepts for broadband nulling over four parallel ∼20% bandpasses spanning the visible spectrum. Contrast-limiting effects of stellar angular extent, residual chromaticity of broadband phase shifters, and aperture diffraction are considered to reach simultaneous ≲2×10−8 contrast over separations spanning 0.2 to 0.9 arc sec for a 2.4-m telescope observing a Sun-like star at 10 pc. With added dark hole wavefront control and postprocessing point spread function subtraction techniques to further reduce scattered starlight, such a system could be capable of detecting the very the nearest Earth-like exoplanets and spectral characterization of several nearby extrasolar gas giants.
This paper discusses charge blooming and starlight saturation – two potential technical problems – when using an Electron Multiplying Charge Coupled Device (EMCCD) type detector in a high-contrast instrument for imaging exoplanets. These problems especially affect an interferometric type coronagraph – coronagraphs that do not use a mask to physically block starlight in the science channel of the instrument. These problems are presented using images taken with a commercial Princeton Instrument EMCCD camera in the Goddard Space Flight Center’s (GSFC), Interferometric Coronagraph facility. In addition, this paper discusses techniques to overcome such problems. This paper also discusses the development and architecture of a Field Programmable Gate Array and Digital-to-Analog Converter based shaped clock controller for a photon-counting EMCCD camera. The discussion contained here will inform high-contrast imaging groups in their work with EMCCD detectors.
The key to broadband operation of the Visible Nulling Coronagraph (VNC) is achieving a condition of quasi-achromatic destructive interference between combined beams. Here we present efforts towards meeting this goal using Fresnel rhombs in each interferometric arm as orthogonally aligned half wave phase retarders. The milestone goal of the demonstration is to achieve 1 × 10−9 contrast at 2λ/D over a 40 nm bandpass centered at 633 nm. Rhombs have been designed and fabricated, and a multi-step approach to alignment using coarse positioners for each rhomb and pair has been developed to get within range of piezo stages used for fine positioning. The previously demonstrated narrowband VNC sensing and control approach that uses a segmented deformable mirror is being adapted to broadband to include fine positioning of the piezo-mounted rhombs, all demonstrated in a low-pressure environment.
We explore the use of hexagonal segment tip-tilt-piston deformable mirrors alone and paired with pinhole spatial filter arrays for high-order wavefront correction of nulling interferometers used for visible light study of exoplanetary systems at 107 to 1010 contrast within regions extending ∼0.1 to 6 arc s from a parent star. A similar system has been proposed using a single-mode fiber array as an alternative to using multiple deformable mirrors to correct both phase aberrations and balance electric field amplitude, the benefit being drastically reduced component and control complexity. Performance is compared using measured deformable mirror data for hexagonal arrays consisting of a number of rings NR=2 to 18, emphasizing the trade between throughput and the additional contrast gained from suppressing wavefront errors introduced by the deformable mirror at spatial frequencies Λ≥NR that are otherwise present in the image at corresponding field locations. Taking into account effects of loss of throughput and vignetting, the nulled signal-to-noise ratio is shown to improve for filtered systems in the outer portion of the field of view. Modeled performance shows no significant change in signal-to-noise in the inner field of view.
Exoplanet coronagraphy will be driven by the telescope architectures available and thus the system designer must have available one or more suitable coronagraphic instrument choices that spans the set of telescope apertures, including filled (off-axis), obscured (e.g. with secondary mirror spiders and struts), segmented apertures, such as JWST, and interferometric apertures. In this work we present one such choice of coronagraph, known as the visible nulling coronagraph (VNC), that spans all four types of aperture and also employs differential sensing and control.
We present a monolithic multispectral camera (MMC) for high contrast direct imaging of inner exoplanetary environments. The primary scientific goal of the camera is to enable eight color characterization of jovian exoplanets and interplanetary dust and debris distributions around nearby stars. Technological highlights of the design include: 1. Diffraction limited resolution at 350 nm through active optical aberration correction; 2. Greater than million-to-one contrast at narrow star separation using interferometry and post-processing techniques; 3. Demonstration of deep broadband interferometric nulling and interband image stability through the use of monolithic optical assemblies; 4. Optimization of multispectral throughput while minimizing components.
The Planetary Imaging Concept Testbed Using a Rocket Experiment (PICTURE 36.225 UG) was designed
to directly image the exozodiacal dust disk of ǫ Eridani (K2V, 3.22 pc) down to an inner radius of 1.5 AU.
PICTURE carried four key enabling technologies on board a NASA sounding rocket at 4:25 MDT on October
8th, 2011: a 0.5 m light-weight primary mirror (4.5 kg), a visible nulling coronagraph (VNC) (600-750 nm), a
32x32 element MEMS deformable mirror and a milliarcsecond-class fine pointing system.
Unfortunately, due to a telemetry failure, the PICTURE mission did not achieve scientific success. Nonetheless,
this flight validated the flight-worthiness of the lightweight primary and the VNC. The fine pointing system,
a key requirement for future planet-imaging missions, demonstrated 5.1 mas RMS in-flight pointing stability.
We describe the experiment, its subsystems and flight results. We outline the challenges we faced in developing
this complex payload and our technical approaches.
We present progress in the development of the monolithic achromatic nulling interference coronagraph (MANIC),
an optic designed for enabling direct detection and characterization of exoplanetary systems around nearby stars.
MANIC is a fully symmetric implementation of a rotational shearing interferometer consisting of fused quartz
prisms and a symmetric beamsplitter optically contacted in an arrangement that geometrically flips the fields
in the TR and RT arms about orthogonal axes such that upon recombination, a centro-symmetric, theoretically
achromatic null is produced. In addition to a small inner working angle (⪅ 1λ/D), built-in alignment and
stability are inherent benefits of the compact monolithic design, which make MANIC a competitive alternative
to conventional discrete element nullers proposed for imaging exoplanetary environments. Following MANIC's
initial fabrication, the path error between its TR and RT arms was measured. This measurement was used to
fabricate compensator plates of varying thicknesses that were bonded to the optic to reduce dispersion imbalance,
thereby improving broadband nulling performance. In performing this correction, initial OPD was reduced from
949 ± 44 nm to 63 ± 10 nm, which in the absence of any other asymmetries, corresponds to an increase in a
107 R-band (λc = 648 nm) nulling bandpass from monochromatic to 25%, or at the 106 level, from 5% to 50%.
Current benchtop laser and polychromatic nulling strategies are described. The potential science return from
using MANIC on a sub-orbital platform is discussed.
We present progress in the development of the monolithic achromatic nulling interference coronagraph (MANIC),
a nulling optic designed to enable direct imaging of nearby Jupiter-like exoplanets. The experimental testbed
for measuring the optical path difference (OPD) between the two arms of the nuller and characterizing the
nuller's performance is described. The OPD measurement will be used to determine the relative thicknesses of
compensator plates needed to complete MANIC's fabrication. Demonstrating the performance of the monolith
will include sub-aperture nulling of laser and white-light sources using a single PZT-controlled delay line on one
half of a bisected input beam.
We report progress on a nulling coronagraph intended for direct imaging of extrasolar planets. White light is suppressed
in an interferometer, and phase errors are measured by a second interferometer. A 1020-pixel MEMS deformable mirror
in the first interferometer adjusts the path length across the pupil. A feedback control system reduces deflections of the
deformable mirror to order of 1 nm rms.
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