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Aligning and commissioning the James Webb Space Telescope's segmented mirrors after launch will last many months and involve the telescope itself, all science instruments, and all parts of the observatory ground system. In an effort to assess and demonstrate readiness of the complete end-to-end system - i.e. the flight optical telescope elements (OTE), the Integrated Science Instruments Module, the on-board operational scripts, and the ground processing infrastructure - we performed two operations tests during the JWST OTIS cryogenic campaign in 2017. They are the Wavefront Sensing and Control Demonstration activities at NASA Johnson Space Center (JSC), where we performed flight-like sensing and control using the flight software to command mirror moves and take measurements, and a "Shadow Mode test" at the Space Telescope Science Institute's Mission Operations Center (MOC), where we demonstrated processing of the JSC data through the entire ground system infrastructure. Overall, these tests demonstrated that the full system that will support OTE commissioning is soundly designed although still not fully mature. This paper focuses on the operations and systems testing aspects and some lessons learned. We also report on a series of Wavefront Rehearsals being held at the MOC that are providing additional opportunities to build team readiness in operating the ground system as a whole using high fidelity observatory simulators

The James Webb Space Telescope’s (Webb’s) deployable primary and secondary mirrors are actively controlled to achieve and maintain precise optical alignment on-orbit. Each of the 18 primary mirror segment assemblies (PMSAs) and the secondary mirror assembly (SMA) are controlled in six degrees of freedom by using six linear actuators in a hexapod arrangement. In addition, each PMSA contains a seventh actuator that adjusts radius of curvature (RoC). The actuators are of a novel stepper motor-based cryogenic two-stage design that is capable of sub-10 nm motion accuracy over a 20 mm range. The nm-level motion of the 132 actuators were carefully tested and characterized before integration into the mirror assemblies. Using these test results as an initial condition, knowledge of each actuator’s length (and therefore mirror position) has relied on software bookkeeping and configuration control to keep an accurate motor step count from which actuator position can be calculated. These operations have been carefully performed through years of Webb test operations using both ground support actuator control software as well as the flight Mirror Control Software (MCS). While the actuator’s coarse stage length is cross-checked using a linear variable differential transformer (LVDT), no on-board cross-check exists for the nm-level length changes of the actuators’ fine stage. To ensure that the software bookkeeping of motor step count is still accurate after years of testing and to test that the actuator position knowledge was properly handed off from the ground software to the flight MCS, a series of optical tests were devised and performed through the Center of Curvature (CoC) ambient optical test campaigns at the Goddard Space Flight Center (GSFC) and during the thermal-vacuum tests of the entire optical payload that were conducted in Chamber A at Johnson Space Center (JSC). In each test, the actuator Fine Step Count (FSC) value is compared to an external measurement provided by an optical metrology tool with the goal of either confirming the MCS database value, or providing a recommendation for an updated calibration if the measured FSC differs significantly from the MCS-based expectation. During ambient testing of the PMSA hexapods, the nm-level actuator length changes were measured with a custom laser deflectometer by measuring tilts of the PMSA. The PMSA RoC fine stage characterization was performed at JSC using multi-wave interferometric measurements with the CoC Optical Assembly (COCOA). Finally, the SMA hexapod fine stage characterization test was performed at JSC using the NIRCam instrument in the “pass-and-a-half” test configuration using a test source from the Aft-Optics System Source Plate Assembly (ASPA). In this paper, each of these three tests, subsequent data analyses, and uncertainty estimations will be presented. Additionally, a summary of the ensemble state of Webb’s actuator fine stages is provided, along with a comparison to a Wavefront Sensing and Control (WFSC)-based requirement for FSC errors as they relate to the optical alignment convergence of the telescope on-orbit.

The James Webb Space Telescope is a large, deployable telescope that will operate at cryogenic temperatures at the Earth-Sun Lagrange 2 point. The Webb Optical Telescope Element (OTE) consists of 18 actively controlled Primary Mirror Segment Assemblies (PMSAs), an actively controlled Secondary Mirror Assembly (SMA), and an Aft-Optics Subsystem (AOS) that contains a fixed Tertiary Mirror and a Fine Steering Mirror. The OTE is combined with the Integrated Science Instrument Module (ISIM) to create the full optical train called OTIS (OTE and ISIM). OTIS has recently undergone cryogenic vacuum testing in Chamber A at Johnson Space Center in Houston, TX. A key outcome of this test was to verify there is adequate range of motion in PMSA and SMA actuators to align them to AOS/ISIM under flight-like conditions. The alignment state of the PMSAs and SMA was measured using photogrammetry and cross-checked optically using a variation of a classical Hartmann test. In the “Pass-and-a-Half” (PAAH) configuration, fiber sources near the Cassegrain focus propagate light through the full optical train and small tilts on the PMSAs create an array of spots on the science instrument detectors, mimicking the effect of a Hartmann mask. Comparison of measured and modeled spot arrays provides the alignment state of the SMA and the global tilt of the primary mirror. This paper will discuss the methodology, testing, and analysis performed to measure the alignment state of OTIS using the Hartmann method and verify the primary and secondary mirrors can be successfully aligned on orbit to meet performance requirements.

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.

The James Webb Space Telescope's (JWST) end-to-end optical system will be tested in a cryogenic vacuum environment before launch at NASA Johnson Space Center’s (JSC) Apollo-era, historic Chamber A thermal vacuum facility. During recent pre-test runs with a prototype “Pathfinder” telescope, the vibration in this environment was found to be challenging for the baseline test approach, which uses phase retrieval of images created by three sub-apertures of the telescope. To address the vibration, an alternate strategy implemented using classic Hartmann test principles combined with precise mirror mechanisms to provide a testing approach that is insensitive to the dynamics environment of the chamber. The measurements and sensitivities of the Hartmann approach are similar to those using phase retrieval over the original sparse aperture test. The Hartmann test concepts have been implemented on the JWST Test Bed Telescope, which provided the rationale and empirical evidence indicating that this Hartmann style approach would be valuable in supplementing the baseline test approach. This paper presents a Hartmann approach implemented during the recent Pathfinder test along with the test approach that is currently being considered for the full optical system test of JWST. Comparisons are made between the baseline phase retrieval approach and the Hartmann approach in addition to demonstrating how the two test methodologies support each other to reduce risk during the JWST full optical system test.

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.

A geometrical phase retrieval (GPR) algorithm is applied to the problem of image stacking in order to extend the capture range of normal phase retrieval (PR) on the James Webb Space Telescope (JWST), and potentially eliminate a lengthy image-stacking process that is based on centroids. Computer simulations are used to establish the capture range of the existing PR algorithm for JWST and demonstrate that it is increased by more than a factor of 10 when combined with GPR, guaranteeing PR capture 95% of the time. An experiment using a scale optical model of JWST was conducted to demonstrate the effectiveness of the GPR algorithm in both coherent and incoherent imaging.

We present several engineering and algorithmic aspects of non-redundant masking (NRM) as they pertain to the James Webb Space Telescope (JWST). NRM's fundamental data structures have multiple uses in wavefront sensing as well in as high resolution imaging. Kernel phases are a full aperture generalization of NRM applicable to moderate and high Strehl ratio images. Eigenphases, the complement to kernel phases, provide wavefront sensing with single in-focus images. Thus this set of phases is relevant to wavefront sensing with routine science images on any Nyquist-sampled science camera on JWST. We attempt to organize these apparently diverse aspects of such Fizeau interferometry into an inter-related picture in order to facilitate their development and potential use on JWST and future space telescopes.

The James Webb Space Telescope is a large deployable cryogenic space telescope that is pointed on the sky by control of the attitude of the integrated spacecraft and telescope. The primary mirror has 18 hexagonal Primary Mirror Segment Assemblies (PMSA) that are deployed; 3 on each of two deployable wings, and 12 on a fixed central section of the Primary Mirror Backplane Support Structure. The Secondary Mirror (SM) is deployed from the Secondary Support Structure that folds out from the backplane, and the complete Telescope and Integrated Science Instrument Module are deployed in extension from the spacecraft. The resulting tolerances will result in a "first light" image that has a spread array of 18 individual images for each point source located within the field of view. The initial attitude of the spacecraft will be adjusted to point the telescope to a desired star field for the initial WFSC commissioning process. The deployment tolerances will result in the telescope field of view being offset from the desired location. By use of a sequence of pointings, a mosaic "first light" image that includes the multiplicity of the 18 misaligned segment images may be created that will allow the calibration of the offset between the telescope boresight and the spacecraft attitude control system, allowing subsequent pointing to be done accurately to a fraction of the field of view of the instruments using spacecraft attitude control.

The James Webb Space Telescope (JWST) is a segmented deployable telescope, utilizing 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). When deployed, the PM segments and the SM will be placed in their correct optical positions to within a few mm, with accordingly large wavefront errors. The challenge, therefore, is to position each of these optical elements in order to correct the deployment errors and produce a diffraction-limited telescope, at λ=2μm, across the entire science field. This paper describes a suite of processes, algorithms, and software that has been developed to achieve this precise alignment, using images taken from JWST’s science instruments during commissioning. The results of flight-like end-to-end simulations showing the commissioning process are also presented.

The payload portion of James Web Space Telescope (JWST) consists of a deployable, three mirror anistigmat, telescope and an Integrated Science Instrument Model (ISIM) that contains the scientific instruments. This paper describes the overall process and strategy of aligning the Observatory in an efficient manner that reduces risk and strives to be tolerant of faults in the system. A process has been developed consisting of ground calibration of the instruments and alignment testing of the fixed optics to ensure that the telescope is alignable in space. The overall architecture of the alignment process and the processes to safely and efficiently conduct the optical commissioning is described.

When the secondary mirror (SM) of a Three-Mirror Anastigmat (TMA) telescope is misaligned with respect to the primary mirror (PM), optical wavefront errors are created. In general, the errors take the form of a dominant coma term, common to all field points, in addition to astigmatism and power terms which vary with field position. The magnitude of the field dependent wavefront is usually only a few percent of that of the common coma term, depending on the size of the field of view being considered. The architecture of the James Webb Space Telescope (JWST), however, presents a unique optical problem in that the common term created by misplacement of the SM is compensated by adjustment of the PM segments. As such, the residual field dependent terms become dominant and can be sensed at multiple field points using phase retrieval techniques. In this paper, we present a linear set of equations that describe the multi-field (MF) wavefront errors resulting from a misaligned SM. It is shown that inverting these equations yields corrections for the SM alignment that can independently control the field-dependent astigmatism and the focal-plane tilt. Computer simulations illustrating the correction are presented.

The JWST optical performance verification poses challenges not yet encountered in space based telescopes. The deployable, segmented Primary Mirror and Secondary Mirror require re-alignment after launch rendering ground alignment states moot. The architecture of JWST was designed to accommodate these difficulties by including active positioning of the Secondary Mirror and the Primary Mirror Segments. In fact, the requirements are written such that the active control system shall be used to meet the requirements. Therefore many of the optical requirements are necessarily based on modeling and simulations of the post-launch, re-alignment of the telescope. This paper provide an overview of a process of computer simulation using an end-to-end integrated model that is used to statistically evaluate the on-orbit, re-alignment performance based on the uncertainties in the integration and test program and deployments.

The James Webb Space Telescope (JWST) consists of an optical telescope element (OTE) that sends light to five science instruments. The initial steps for commissioning the telescope are performed with the Near-Infrared Camera (NIRCam) instrument, but low-order optical aberrations in the remaining science instruments must be determined (using phase retrieval) in order to ensure good performance across the entire field of view. These remaining instruments were designed to collect science data, and not to serve as wavefront sensors. Thus, the science cameras are not ideal phase-retrieval imagers for several reasons: they record under-sampled data and have a limited range of diversity defocus, and only one instrument has an internal, narrowband filter. To address these issues, we developed the capability of sensing these aberrations using an extension of image-based iterative-transform phase retrieval called Variable Sampling Mapping (VSM). The results show that VSM-based phase retrieval is capable of sensing low-order aberrations to a few nm RMS from images that are consistent with the non-ideal conditions expected during JWST multi-field commissioning. The algorithm is validated using data collected from the JWST Testbed Telescope (TBT).

From its orbit around the Earth-Sun second Lagrange point some million miles from Earth, the James Webb Space Telescope (JWST) will be uniquely suited to study early galaxy and star formation with its suite of infrared instruments.[1] To maintain exceptional image quality using its 6.6 meter segmented primary mirror, wavefront sensing and control (WFS&C) is vital to ensure the optical alignment of the telescope throughout the mission. After deployment of the observatory structure and mirrors from the "folded" launch configuration, WFS&C is used to align the telescope[2], as well as maintain that alignment. WFS&C verification includes the verification of the software and its incorporated algorithms, along with the supporting aspects of the integrated ground segment, instrumentation, and telescope through increasing levels of assembly. The software and process are verified with the Integrated Telescope Model (ITM), which is a Matlab/Simulink integrated observatory model which interfaces to CodeV/OSLO/IDL. In addition to lower level testing, the Near-Infrared Camera[3] (NIRCam) with its wavefront sensing optical components is verified with the other instruments with a cryogenic optical telescope simulator (OSIM) before moving on to the final WFS&C testing in Chamber A at the Johnson Space Center (JSC) where additional observatory verification occurs.

The Wavefront Sensing and Controls commission process for the James Webb Space Telescope will be described, and data from supporting experiments will be presented.

Image-based wavefront sensing algorithms are being used to characterize optical performance for a variety of current and planned astronomical telescopes. Phase retrieval recovers the optical wavefront that correlates to a series of diversity-defocused point-spread functions (PSFs), where multiple frames can be acquired at each defocus setting. Multiple frames of data can be coadded in different ways; two extremes are in "image-plane space," to average the frames for each defocused PSF and use phase retrieval once on the averaged images, or in "pupil-plane space," to use phase retrieval on every set of PSFs individually and average the resulting wavefronts. The choice of coadd methodology is particularly noteworthy for segmented-mirror telescopes that are subject to noise that causes uncorrelated motions between groups of segments. Using data collected on and simulations of the James Webb Space Telescope Testbed Telescope (TBT) commissioned at Ball Aerospace, we show how different sources of noise (uncorrelated segment jitter, turbulence, and common-mode noise) and different parts of the optical wavefront, segment and global aberrations, contribute to choosing the coadd method. Of particular interest, segment piston is more accurately recovered in "image-plane space" coadding, while segment tip/tilt is recovered in "pupil-plane space" coadding.

NASA's Technology Readiness Level (TRL)-6 is documented for the James Webb Space Telescope (JWST) Wavefront Sensing and Control (WFSC) subsystem. The WFSC subsystem is needed to align the Optical Telescope Element (OTE) after all deployments have occurred, and achieves that requirement through a robust commissioning sequence consisting of unique commissioning algorithms, all of which are part of the WFSC algorithm suite. This paper identifies the technology need, algorithm heritage, describes the finished TRL-6 design platform, and summarizes the TRL-6 test results and compliance. Additionally, the performance requirements needed to satisfy JWST science goals as well as the criterion that relate to the TRL-6 Testbed Telescope (TBT) performance requirements are discussed

The one-meter Testbed Telescope (TBT) has been developed at Ball Aerospace to facilitate the design and implementation of the wavefront sensing and control (WFSC) capabilities of the James Webb Space Telescope (JWST). We have recently conducted an "end-to-end" demonstration of the flight commissioning process on the TBT. This demonstration started with the Primary Mirror (PM) segments and the Secondary Mirror (SM) in random positions, traceable to the worst-case flight deployment conditions. The commissioning process detected and corrected the deployment errors, resulting in diffraction-limited performance across the entire science FOV. This paper will describe the commissioning demonstration and the WFSC algorithms used at each step in the process.

The one-meter Testbed Telescope (TBT) has been developed at Ball Aerospace to facilitate the design and implementation of the wavefront sensing and control (WFS&C) capabilities of the James Webb Space Telescope (JWST). The TBT is used to develop and verify the WFS&C algorithms, check the communication interfaces, validate the WFS&C optical components and actuators, and provide risk reduction opportunities for test approaches for later full-scale cryogenic vacuum testing of the observatory. In addition, the TBT provides a vital opportunity to demonstrate the entire WFS&C commissioning process. This paper describes recent WFS&C commissioning experiments that have been performed on the TBT.

The opto-mechanical design of the 6.6 meter James Webb Space Telescope (JWST), with its actively-controlled secondary and 18-segment primary mirror, presents unique challenges from a system engineering perspective. To maintain the optical alignment of the telescope on-orbit, a process called wavefront sensing and control (WFS&C) is employed to determine the current state of the mirrors and calculate the optimal mirror move updates. The needed imagery is downloaded to the ground, where the WFS&C algorithms to process the images reside, and the appropriate commands are uploaded to the observatory. Rather than use a dedicated wavefront sensor for the imagery as is done in most other applications, a science camera is used instead. For the success of the mission, WFSC (2) when an existing requirement is insufficient to meet the need, a requirement change is initiated; and finally (3) when a needed requirement is missing, a new requirement is established in the corresponding document. This approach, deemed a "best practice" at the customer's independent audit, allows for program confidence that the necessary requirements are complete, while still maintaining the responsibility for the requirement with the most appropriate entity. This paper describes the details and execution of the approach; the associated WFSC and the implementation of the primary database tool for the project, DOORS (Dynamic Object-Oriented Requirements System).

From its orbit around the Earth-Sun second Lagrange point some million miles from Earth, the James Webb Space Telescope (JWST) will be uniquely suited to study early galaxy and star formation with its suite of infrared instruments. To maintain exceptional image quality using its 6.6 meter segmented primary mirror, wavefront sensing and control (WFS&C) is vital to ensure the optical alignment of the telescope throughout the mission. WFS&C design architecture includes using the Near-Infrared Camera (NIRCam) to provide imagery for ground-resident image processing algorithms which determine the optimal alignment of the telescope. There are two distinct mission phases for WFS&C, both of which use algorithms and NIRCam imagery to determine the required segment updates. For the first phase, WFS&C commissioning, the telescope is taken from its initial deployed state with each of the 18 primary mirror segments acting like independent telescopes, to its final phased state with each segment acting in concert as a part of a single mirror. The second phase, Wavefront Monitoring and Maintenance, continues for the rest of the mission. Here the wavefront quality is evaluated, and when needed, the mirror positions are updated to bring it back to an optimal configuration. This paper discusses the concept of operations for the commissioning and on-going maintenance of the telescope alignment using WFS&C.

An image-based wavefront sensing and control algorithm for the James Webb Space Telescope (JWST) is presented. The algorithm heritage is discussed in addition to implications for algorithm performance dictated by NASA's Technology Readiness Level (TRL) 6. The algorithm uses feedback through an adaptive diversity function to avoid the need for phase-unwrapping post-processing steps. Algorithm results are demonstrated using JWST Testbed Telescope (TBT) commissioning data and the accuracy is assessed by comparison with interferometer results on a multi-wave phase aberration. Strategies for minimizing aliasing artifacts in the recovered phase are presented and orthogonal basis functions are implemented for representing wavefronts in irregular hexagonal apertures. Algorithm implementation on a parallel cluster of high-speed digital signal processors (DSPs) is also discussed.