The Habitable Worlds Observatory will revolutionize our understanding of the universe by directly detecting biosignatures on extrasolar planets and allow us to answer the question if we are alone in the universe. To accomplish the tight science goals associated with this mission, the development of an ultrastable observatory with a coronagraphic instrument is necessary. The observatory itself may need to stay stable on the order of 10 picometers over a wavefront control cycle, orders of magnitude more stable than what is required on current space missions. The metrology to verify stability requirements must be roughly a factor of ten more stable. The ultrastable laboratory at NASA’s Goddard Space Flight Center has further stabilized its testbed to allow for dynamic measurements on diffuse and specular objects on the order of single picometers, and we are currently measuring drifts on the orders of tens of picometers over different temporal bands. This paper will discuss the mechanical updates to the testbed setup, the analysis performed on several test articles, and the path forward on the road to measuring achieving the required stability for Habitable Worlds Observatory.
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.
The James Webb Space Telescope (JWST) launched on December 25th, 2021. The observatory was deployed and commissioned during its first six months. The Optical Telescope Element includes both large deployments of the Deployable Tower Assembly, Secondary Mirror Support Structure and the Primary Mirror Wings and it includes the deployment and alignment of the 18 primary mirror segments and the secondary mirror. The initial phase included cooldown and ice mitigation efforts followed by telescope deployments, mirror deployment and wavefront sensing and control. This paper will discuss the entire OTE commissioning from deployment through alignment including a discussion of results and lessons learned.
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.
NASA’s James Webb Space Telescope (JWST) is a 6.6m diameter, segmented, deployable telescope for cryogenic IR space astronomy. The JWST Observatory architecture includes the Optical Telescope Element (OTE) and the Integrated Science Instrument Module (ISIM) element which contains four science instruments (SI), including a guider. The SIs and guider are mounted to a composite metering structure with outer envelope approximate measurements of 2.2x2.2x1.7m. These SI units are integrated to the ISIM structure and optically tested at NASA Goddard Space Flight Center as an instrument suite using an Optical telescope element SIMulator (OSIM). OSIM is a high-fidelity, cryogenic JWST simulator that features a ~1.5m diameter powered mirror. The SIs are aligned to the flight structure’s coordinate system under ambient, clean room conditions using opto-mechanical metrology and customized interfaces. OSIM is aligned to the ISIM mechanical coordinate system at the cryogenic operating temperature via internal mechanisms and feedback from alignment sensors and metrology in six degrees of freedom. SI performance, including focus, pupil shear, pupil roll, boresight, wavefront error, and image quality, is evaluated at the operating temperature using OSIM. This work reports on the as-run ambient assembly and ambient alignment steps for the flight ISIM, including SI interface fixtures and customization and kinematic mount adjustment. The ISIM alignment plan consists of multiple steps to meet the “absolute” alignment requirements of the SIs and OSIM to the flight coordinate system. In this paper, we focus on key aspects of absolute, optical-mechanical alignment. We discuss various metrology and alignment techniques. In addition, we summarize our approach for dealing with and the results of ground-test factors, such as gravity.
KEYWORDS: James Webb Space Telescope, Optical components, Space telescopes, Optical testing, Sensors, Calibration, Data modeling, Human-machine interfaces, Error analysis, Analytical research
NASA’s James Webb Space Telescope (JWST) is a 6.5m diameter, segmented, deployable telescope for cryogenic IR space astronomy. The JWST Observatory includes the Optical Telescope Element (OTE) and the Integrated Science Instrument Module (ISIM), that contains four science instruments (SI) and the Fine Guidance Sensor (FGS). The SIs are mounted to a composite metering structure. The SIs and FGS were integrated to the ISIM structure and optically tested at NASA's Goddard Space Flight Center using the Optical Telescope Element SIMulator (OSIM). OSIM is a full-field, cryogenic JWST telescope simulator. SI performance, including alignment and wavefront error, was evaluated using OSIM. We describe test and analysis methods for optical performance verification of the ISIM Element, with an emphasis on the processes used to plan and execute the test. The complexity of ISIM and OSIM drove us to develop a software tool for test planning that allows for configuration control of observations, implementation of associated scripts, and management of hardware and software limits and constraints, as well as tools for rapid data evaluation, and flexible re-planning in response to the unexpected. As examples of our test and analysis approach, we discuss how factors such as the ground test thermal environment are compensated in alignment. We describe how these innovative methods for test planning and execution and post-test analysis were instrumental in the verification program for the ISIM element, with enough information to allow the reader to consider these innovations and lessons learned in this successful effort in their future testing for other programs.
This paper describes the critical instrument alignment terms associated with the six-degree of freedom alignment of each the Science Instrument (SI) in the James Webb Space Telescope (JWST), including focus, pupil shear, pupil clocking, and boresight. We present the test methods used during cryogenic-vacuum tests to directly measure the performance of each parameter, the requirements levied on each, and the impact of any violations of these requirements at the instrument and Observatory level.
KEYWORDS: James Webb Space Telescope, Wavefronts, Monte Carlo methods, Space telescopes, Wavefront sensors, Aerospace engineering, Silicon, Sensors, Detection and tracking algorithms, Mirrors
The science instruments (SIs) comprising the James Webb Space Telescope (JWST) Integrated Science Instrument
Module (ISIM) were tested in three cryogenic-vacuum test campaigns in the NASA Goddard Space Flight Center
(GSFC)'s Space Environment Simulator (SES) test chamber.
In this paper, we describe the results of optical wavefront-error performance characterization of the SIs. The wavefront
error is determined using image-based wavefront sensing, and the primary data used by this process are focus sweeps, a
series of images recorded by the instrument under test in its as-used configuration, in which the focal plane is
systematically changed from one image to the next. High-precision determination of the wavefront error also requires
several sources of secondary data, including 1) spectrum, apodization, and wavefront-error characterization of the optical
ground-support equipment (OGSE) illumination module, called the OTE Simulator (OSIM), 2) f/# and pupil-distortion
measurements made using a pseudo-nonredundant mask (PNRM), and 3) pupil-geometry predictions for each SI field
point tested, which are complicated because of a tricontagon-shaped outer perimeter and small holes that appear in the
exit pupil due to the way that different light sources are injected into the optical path by the OGSE. One set of
wavefront-error tests, for the coronagraphic channel of the Near-Infrared Camera (NIRCam) Longwave instruments, was
performed using data from transverse-translation diversity (TTD) sweeps instead of focus sweeps, in which a subaperture
is translated and/or rotated across the exit pupil of the system from one image to the next.
Several optical-performance requirements that were verified during this ISIM Element-level testing are levied on the
uncertainties of various wavefront-error-related quantities rather than on the wavefront errors themselves. This paper
also gives an overview of the methodology, based on Monte Carlo simulations of the wavefront-sensing analysis of
focus-sweep data, used to establish the uncertainties of the wavefront-error maps.
The James Webb Space Telescope (JWST) project is an international collaboration led by NASA’s Goddard Space
Flight Center (GSFC) in Greenbelt, MD. JWST is NASA’s flagship observatory that will operate nearly a million miles
away from Earth at the L2 Lagrange point. JWST’s optical design is a three-mirror anastigmat with four main optical
components; 1) the eighteen Primary Mirror Segment Assemblies (PMSA), 2) a single Secondary Mirror Assembly
(SMA), 3) an Aft-Optics Subsystem (AOS) consisting of a Tertiary Mirror and Fine Steering Mirror, and 4) an
Integrated Science Instrument Module consisting of the various instruments for JWST. JWST’s optical system has been
designed to accommodate a significant amount of alignment capability and risk with the PMSAs and SMA having rigid
body motion available on-orbit just for alignment purposes. However, the Aft-Optics Subsystem (AOS) and Integrated
Science Instrument Module (ISIM) are essentially fixed optical subsystems within JWST, and therefore the cryogenic
alignment of the AOS to the ISIM is critical to the optical performance and mission success of JWST.
In support of this cryogenic alignment of the AOS to ISIM, an array of fiber optic sources, known as the AOS Source
Plate Assembly (ASPA), are placed near the intermediate image location of JWST (between the secondary and tertiary
mirrors) during thermal vacuum ground-test operations. The AOS produces images of the ASPA fiber optic sources at
the JWST focal surface location, where they are captured by the various science instruments. In this manner, the AOS
provides an optical yardstick by which the instruments within ISIM can evaluate their relative positions to and the
alignment of the AOS to ISIM can be quantified. However, since the ASPA is located at the intermediate image location
of the JWST three-mirror anastigmat design, the images of these fiber optic sources produced by the AOS are highly
aberrated with approximately 2-3μm RMS wavefront error consisting mostly of 3rd-order astigmatism and coma. This is
because the elliptical tertiary mirror of the AOS is used off of its ideal foci locations without the compensating
wavefront effects of the JWST primary and secondary mirrors. Therefore, the PSFs created are highly asymmetric with
relatively complex structure and the centroid and encircled energy analyses traditionally used to locate images are not
sufficient for ensuring the AOS to ISIM alignment.
A novel approach combining phase retrieval and spatial metrology was developed to both locate the images with respect
to the AOS and provide calibration information for eventual AOS to ISIM alignment verification. During final JWST
OTE and ISIM (OTIS) testing, only a single thru-focus image will be collected by the instruments. Therefore, tools and
processes were developed to perform single-image phase retrieval on these highly aberrated images such that any single
image of the ASPA source can provide calibrated knowledge of the instruments’ position relative to the AOS. This paper
discusses the results of the methodology, hardware, and calibration performed to ensure that the AOS and ISIM are
aligned within their respective tolerances at JWST OTIS testing.
KEYWORDS: James Webb Space Telescope, Wavefront sensors, Space telescopes, Observatories, Image segmentation, Stars, Point spread functions, Wavefronts, Space operations, Mirrors
The James Webb Space Telescopes segmented primary and deployable secondary mirrors will be actively con- trolled to achieve optical alignment through a complex series of steps that will extend across several months during the observatory's commissioning. This process will require an intricate interplay between individual wavefront sensing and control tasks, instrument-level checkout and commissioning, and observatory-level calibrations, which involves many subsystems across both the observatory and the ground system. Furthermore, commissioning will often exercise observatory capabilities under atypical circumstances, such as fine guiding with unstacked or defocused images, or planning targeted observations in the presence of substantial time-variable offsets to the telescope line of sight. Coordination for this process across the JWST partnership has been conducted through the Wavefront Sensing and Control Operations Working Group. We describe at a high level the activities of this group and the resulting detailed commissioning operations plans, supporting software tools development, and ongoing preparations activities at the Science and Operations Center. For each major step in JWST's wavefront sensing and control, we also explain the changes and additions that were needed to turn an initial operations concept into a flight-ready plan with proven tools. These efforts are leading to a robust and well-tested process and preparing the team for an efficient and successful commissioning of JWSTs active telescope.
In measuring the figure error of an aspheric optic using a null lens, the wavefront contribution from the null lens must be
independently and accurately characterized in order to isolate the optical performance of the aspheric optic alone.
Various techniques can be used to characterize such a null lens, including interferometry, profilometry and image-based
methods. Only image-based methods, such as phase retrieval, can measure the null-lens wavefront in situ – in single-pass,
and at the same conjugates and in the same alignment state in which the null lens will ultimately be used – with no
additional optical components. Due to the intended purpose of a null lens (e.g., to null a large aspheric wavefront with a
near-equal-but-opposite spherical wavefront), characterizing a null-lens wavefront presents several challenges to image-based
phase retrieval: Large wavefront slopes and high-dynamic-range data decrease the capture range of phase-retrieval
algorithms, increase the requirements on the fidelity of the forward model of the optical system, and make it difficult to
extract diagnostic information (e.g., the system F/#) from the image data. In this paper, we present a study of these
effects on phase-retrieval algorithms in the context of a null lens used in component development for the Climate
Absolute Radiance and Refractivity Observatory (CLARREO) mission. Approaches for mitigation are also discussed.
KEYWORDS: James Webb Space Telescope, Phase retrieval, Digital signal processing, Algorithm development, Integrated modeling, Wavefront sensors, Space telescopes, Computer architecture, Silicon, Commercial off the shelf technology
The James Webb Space Telescope (JWST) is the successor to the Hubble Space Telescope and will be NASA's
premier observatory of the next decade. Image-based wavefront sensing (phase retrieval) is the primary method for
ground testing and on-orbit commissioning. For ground tests at NASA's Goddard Space Flight Center (GSFC) and
Johnson Space Center (JSC), near-real-time analysis is critical for ensuring that pass/fail criteria are met before
completion of a specific test. To address this need we have developed a computational architecture for image
processing and phase retrieval. Using commercially available off-the-shelf hardware and software, we have
designed, implemented, and tested a solution for high-speed parallel computing. The architecture is a hybrid
solution utilizing both CPUs and GPUs and exploiting the unique advantages of each. Discussions are presented of
the architecture, performance, and current limitations.
Many modern optical designs employ diamond-turned optical components and utilize phase retrieval for metrology
during testing, assembly, and commissioning. The accuracy of the wavefronts obtained by phase retrieval depends on
the fidelity of the system model used during the retrieval, including knowledge of the pupil amplitude, and the
relationship between the digital sample spacing in the pupil and each point spread function (PSF), i.e., the plate scale.
However, recent simulations have shown that errors in the estimation of both the plate scale and unknown pupil
vignetting can both lead to mid-spatial-frequency groove-like errors in the wavefront maps obtained by phase retrieval.
In particular, these errors manifest themselves as concentric rings resembling diamond-turning tooling marks, and can
therefore easily confound metrology results involving diamond-turned components. Furthermore it was found that only
moderate amounts of pupil vignetting, and errors in sampling ratio as low as 2% produced groove errors consistent in
magnitude with typical diamond-turning specifications. This paper presents the results of this study on the magnitude
and nature of these artifacts and their impact on telescope metrology.
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