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 (ULTRA- TM) 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.

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.

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 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.

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 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.

Two of the large mission concepts developed for the 2020 Astronomy and Astrophysics Decadal Survey - the Large UV/Optical/Infrared (LUVOIR) Surveyor and the Habitable Exoplanet (HabEx) Observatory - aim to perform direct imaging and spectroscopy of exo-Earth candidates with high-contrast coronagraphs. Meeting this ambitious science goal will require architectures with large-area primary mirrors to achieve the high resolution and greater photon flux needed to detect faint objects, as well as instrumentation to suppress light from a host star ~10 billion times brighter than the exoplanet of interest. Achieving contrast of 10−10 at visible wavelengths using a coronagraph ushers in a new regime where the corresponding wavefront stability is expressed in units of picometers rather than nanometers. The NASA-funded “Ultra-Stable Large Telescope Research and Analysis - Technology Maturation” (ULTRA-TM) program is working to mature critical technologies for ultra-stable optical systems to support these mission concepts and enable ground-breaking science. This paper describes the progress towards demonstrating performance of enabling technologies through component-level hardware testbeds - specifically for picometer-capable edge sensors and actuators with flight-like properties, which are needed for active sensing and control of large, segmented primary mirrors like LUVOIR. Key performance requirements include resolution, range, and operational bandwidth, which are derived from wavefront stability budgets traceable to the top-level coronagraph performance and science goals driving the ultra-stability need. Raising the TRL of these technologies will address some of the most difficult parts of the stability problem with the tightest requirements and longest lead times, as well as provide significant risk reduction for their inclusion in future mission concepts.

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.

Our ability to answer scientific questions about exoplanets hinges on satisfying an age-old astronomical requirement: a sufficient sample size. Thus, the yield of exoplanets is critical to understanding the scientific impact of future missions. We discuss how the yield of directly-imaged exoplanets depends on mission scale and survey design.

This whitepaper discusses the diversity of exoplanets that could be detected by future observations, so that comparative exoplanetology can be performed in the upcoming era of large space-based flagship missions. The primary focus will be on characterizing Earth-like worlds around Sun-like stars. However, we will also be able to characterize companion planets in the system simultaneously. This will not only provide a contextual picture with regards to our Solar system, but also presents a unique opportunity to observe size dependent planetary atmospheres at different orbital distances. We propose a preliminary scheme based on chemical behavior of gases and condensates in a planet's atmosphere that classifies them with respect to planetary radius and incident stellar flux.

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.

Recent advances in the physical vapor deposition (PVD) of protective fluoride films have raised the far-ultraviolet (FUV: 912-1600 angstrom) reflectivity of aluminum-based mirrors closer to the theoretical limit. The greatest gains, at more than 20%, have come for lithium fluoride-protected aluminum, which has the shortest wavelength cutoff of any conventional overcoat. Despite the success of the NASA FUSE mission, the use of lithium fluoride (LiF)-based optics is rare, as LiF is hygroscopic and requires handling procedures that can drive risk. With NASA now studying two large mission concepts for astronomy, Large UV-Optical-IR Surveyor (LUVOIR) and the Habitable Exoplanet Imaging Mission (HabEx), which mandate throughput down to 1000 A, the development of LiF-based coatings becomes crucial. This paper discusses steps that are being taken to qualify these new enhanced LiF-protected aluminum (eLiF) mirror coatings for flight. In addition to quantifying the hygroscopic degradation, we have developed a new method of protecting eLiF with an ultrathin (10-20 angstrom) capping layer of a non-hygroscopic material to increase durability. We report on the performance of eLiF-based optics and assess the steps that need to be taken to qualify such coatings for LUVOIR, HabEx, and other FUV-sensitive space missions. (C) 2017 Optical Society of America

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.