The Nancy Grace Roman Space Telescope (“Roman”) was prioritized by the 2010 Decadal Survey in Astronomy and Astrophysics and is NASA’s next astrophysics flagship Observatory. Launching no earlier than 2026, it will conduct several wide field and time domain surveys, as well as conduct an exoplanet census. Roman’s large field of view, agile survey capabilities, and excellent stability enable these objectives, yet present unique engineering and test challenges. The Roman Observatory comprises a Spacecraft and the Integrated Payload Assembly (IPA), the latter of which includes the Optical Telescope Assembly (OTA), the primary science Wide Field Instrument, a technology demonstration Coronagraph Instrument, and the Instrument Carrier, which meters the OTA to each instrument. The Spacecraft supports the IPA and includes the Bus, Solar Array Sun Shield, Outer Barrel Assembly, and Deployable Aperture Cover. It provides all required power, command handling, attitude control, communications, data storage, and stable thermal control functions as well as shading and straylight protection across the entire field of regard. This paper presents the Observatory as it begins integration and test, as well as describes key test and verification activities.
The OTA for the Nancy Grace Roman Space Telescope includes the primary mirror, secondary mirror, and aft optics for guiding light into the Wide Field Instrument and the Coronagraph Instrument. The telescope is taking shape as the tested optical mirror assemblies are integrated. The assemblies have been thermal cycled to the cold temperatures for infrared operation, load tested to launch loads, vibration tested, and optically tested. Testing included launch-level vibration testing of the 2.4-meter light-weighted primary mirror assembly. In addition, the telescope control electronics (TCE) box has been fully assembled and the environmental testing of the TCE is progressing. Pictures and descriptions of the integration and test progress are provided, along with performance results measured at these levels of assemblies. Planning and test equipment preparation for the telescope thermal vacuum testing continues including plans to take advantage of the large dynamic range available with focus diversity phase retrieval and a Shack-Hartmann wavefront sensor for the gravity-sagged primary mirror.
The Nancy Grace Roman Space Telescope (RST) is a Hubble-class telescope with a large field of view for large surveys of the sky, cold temperatures for enabling near infrared imaging, and controlled temperature stability for long exposures and coronagraphy. The OTA includes the primary mirror, secondary mirror, and aft optics for guiding light into the Wide Field Instrument and the Coronagraph Instrument. The testing of the optical assemblies and structures are nearly complete in preparation for telescope integration. Pictures and descriptions of the assemblies are provided, followed by performance results measured at these level of assemblies. The assemblies are nearly complete as they are tested through thermal cycling to cold temperatures for infrared operation, mechanical strength and vibration, and optically testing. Optical surface figure error results are shown for all the optical surfaces.
KEYWORDS: Telescopes, Mirrors, Space telescopes, Signal to noise ratio, Infrared telescopes, Optical fabrication, Observatories, Optical instrument design, Infrared radiation, James Webb Space Telescope
The WFIRST Mission is the next large astrophysical observatory for NASA after the James Webb Space Telescope and is the top priority mission from the 2010 National Academy of Sciences’ decadal survey. The WFIRST OTA includes the inherited primary and secondary mirrors with precision metering structures that are to be integrated to new mirror assemblies to provide optical feeds to the two WFIRST instruments. We present here: (1) the results for the review of the inherited hardware for WFIRST through a thorough technical pedigree process, (2) the status of the effort to establish the capability of the telescope to perform at a cooler operational temperature of 265K, and (3) the status of the work in requirement development for OTA to incorporate the inherited hardware, and (4) the path forward.
NASA’s Wide Field Infrared Survey Telescope (WFIRST) is being designed to deliver unprecedented capability in dark energy and exoplanet science, and to host a technology demonstration coronagraph for exoplanet imaging and spectroscopy. The observatory design has matured since 2013 [“WFIRST 2.4m Mission Study”, D. Content, SPIE Proc Vol 8860, 2013] and we present a comprehensive description of the WFIRST observatory configuration as refined during formulation phase (AKA the phase-A study). The WFIRST observatory is based on an existing, repurposed 2.4m space telescope coupled with a 288 megapixel near-infrared (0.6 to 2 microns) HgCdTe focal plane array with multiple imaging and spectrographic modes. Together they deliver a 0.28 square degree field of view, which is approximately 100 times larger than the Hubble Space Telescope, and a sensitivity that enables rapid science surveys. In addition, the technology demonstration coronagraph will prove the feasibility of new techniques for exoplanet discovery, imaging, and spectral analysis. A composite truss structure meters both instruments to the telescope assembly, and the instruments and the spacecraft are on-orbit serviceable. We present the current design and summarize key Phase-A trade studies and configuration changes that improved interfaces, improved testability, and reduced technical risk. We provide an overview of our Integrated Modeling results, performed at an unprecedented level for a phase-A study, to illustrate performance margins with respect to static wavefront error, jitter, and thermal drift. Finally, we summarize the results of technology development and peer reviews, demonstrating our progress towards a low-risk flight development and a launch in the middle of the next decade.
The Wide-Field Infrared Survey Telescope (WFIRST) mission is the next large astrophysics observatory for NASA after the James Webb Space Telescope and is the top priority mission from the 2010 National Academy of Sciences’ decadal survey. The WFIRST Optical Telescope Assembly (OTA) includes inherited composite support structures that were originally designed and tested for room temperature operation; however, the WFIRST mission will require operation at colder temperatures to achieve sufficient sensitivity for the infrared wavelengths. We will present the results and conclusions of testing completed at the coupon and engineering model level to verify that the inherited composite structures will maintain mechanical integrity and performance over the required temperature range. The testing included: (1) characterization testing of constituent material coupons, (2) thermal cycling and static load testing of a representative aft metering structure (AMS) and forward metering structure (FMS), and (3) thermal cycling and dynamic testing of a representative secondary mirror assembly (SMA).
KEYWORDS: Space telescopes, Telescopes, Mirrors, Infrared telescopes, Space operations, Optical telescopes, Coronagraphy, Monte Carlo methods, Infrared astronomy, Infrared radiation
The Wide-Field Infrared Survey Telescope (WFIRST) mission[1] is the top-ranked large space mission in the New Worlds, New Horizon (NWNH) Decadal Survey of Astronomy and Astrophysics. WFIRST will settle essential questions in both exoplanet and dark energy research and will advance topics ranging from galaxy evolution to the study of objects within the galaxy. The WFIRST mission uses a repurposed 2.4-m Forward Optical Telescope assembly (FOA), which, when completed with new aft optics will be an Integrated Optical Assembly (IOA). WFIRST is equipped with a Wide Field Instrument (WFI) and a Coronagraph Instrument (CGI). An Instrument Carrier (IC) meters these payload elements together and to the spacecraft bus (S/C). A distributed ground system receives the data, uploads commands and software updates, and processes the data. After transition from the study phase, Pre-Phase-A (a.k.a., “Cycle 6”) design to NASA Phase A formulation, a significant change to the IOA was initiated; including moving the tertiary mirror from the instrument package to a unified three-mirror anastigmat (TMA) placement, that provides a wide 0.28-sq° instrumented field of view to the Wide Field Instrument (WFI). In addition, separate relays from the primary and secondary mirror feed the Wide Field Instrument (WFI) and Coronagraph Instrument (CGI). During commissioning the telescope is aligned using wavefront sensing with the WFI[2]. A parametric and Monte-Carlo analysis was performed, which determined that alignment compensation with the secondary mirror alone degraded performance in the other instruments. This led to the addition of a second compensator in the WFI optical train to alleviate this concern. This paper discusses the trades and analyses that were performed and resulting changes to the WFIRST telescope architecture.
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.
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.
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.
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.
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.
KEYWORDS: Phase retrieval, Wavefronts, Sensors, James Webb Space Telescope, Wavefront sensors, Cryogenics, Monochromatic aberrations, Monte Carlo methods, Data modeling, Error analysis
Phase retrieval results are presented for the James Webb Space Telescope (JWST) Near InfraRed Spectrograph
(NIRSpec) demonstration model (DM). NIRSpec is one of five science instruments (SIs) comprising the Integrated
Science Instrument Module (ISIM); the NIRSpec is being built for the European Space Agency by a consortium led
by EADS Astrium GmbH. During this initial DM test campaign, focal-sweep images were collected over the
science field of view (FOV) for determining best focus at both ambient and cryogenic (cryo) temperature
environments, and these images were then used as input to the Hybrid Diversity Algorithm (HDA) for phase
retrieval, using Variable Sampling Mapping (VSM). Wavefront estimates from phase retrieval, an error budget, and
diagnostics used to assess phase retrieval stability and convergence are discussed. The ambient phase retrieval
results were compared against wavefront measurements taken with a Shack-Hartmann wavefront sensor.
We present results of a study of a deployable version of the Advanced Technology Large-Aperture Space Telescope
(ATLAST), designed to operate in a Sun-Earth L2 orbit. The primary mirror of the segmented 9.2-meter aperture has 36
hexagonal 1.315 m (flat-to-flat) glass mirrors. The architecture and folding of the telescope is similar to JWST, allowing
it to fit into the 6.5 m fairing of a modest upgrade to the Delta-IV Heavy version of the Evolved Expendable Launch
Vehicle (EELV). We discuss the overall observatory design, optical design, instruments, stray light, wavefront sensing
and control, pointing and thermal control, and in-space servicing options.
KEYWORDS: Systems modeling, James Webb Space Telescope, Observatories, Thermal modeling, Data modeling, Mathematical modeling, Solid modeling, Space telescopes, Performance modeling, Distortion
The James Web Space Telescope (JWST) is a large, infrared-optimized space telescope scheduled for launch in 2014.
The imaging performance of the telescope will be diffraction limited at 2μm, defined as having a Strehl ratio >0.8.
System-level verification of critical performance requirements will rely on integrated observatory models that predict the
wavefront error accurately enough to verify that allocated top-level wavefront error of 150 nm root-mean-squared (rms)
through to the wave-front sensor focal plane is met. Furthermore, responses in several key disciplines are strongly crosscoupled.
The size of the lightweight observatory structure, coupled with the need to test at cryogenic temperatures,
effectively precludes validation of the models and verification of optical performance with a single test in 1-g. Rather, a
complex series of incremental tests and measurements are used to anchor components of the end-to-end models at
various levels of subassembly, with the ultimate verification of optical performance is by analysis using the assembled
models. The assembled models themselves are complex and require the insight of technical experts to assess their ability
to meet their objectives. This paper describes the modeling approach used on the JWST through the detailed design
phase.
Future large UV-optical space telescopes offer new and exciting windows of scientific parameter space. These
telescopes can be placed at L2 and borrow heavily from the James Webb Space Telescope (JWST) heritage. For
example, they can have similar deployment schemes, hexagonal mirrors, and use Wavefront Sensing and Control
(WFSC) technologies developed for JWST. However, a UV-optical telescope requires a 4x improvement in
wavefront quality over JWST to be diffraction-limited at 500 nm. Achieving this tolerance would be difficult using
a passive thermal architecture such as the one employed on JWST. To solve this problem, our team has developed a
novel Hybrid Sensor Active Control (HSAC) architecture that provides a cost effective approach to building a
segmented UV-optical space telescope. In this paper, we show the application of this architecture to the ST-2020
mission concept and summarize the technology development requirements.
KEYWORDS: James Webb Space Telescope, Phase retrieval, Wavefronts, Monochromatic aberrations, Telescopes, Space telescopes, Data modeling, Wavefront sensors, Sensors, Aerospace engineering
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).
KEYWORDS: Wavefronts, James Webb Space Telescope, Point spread functions, Mirrors, Space telescopes, Wavefront sensors, Motion models, Observatories, Telescopes, Spatial frequencies
This paper is part five of a series on the ongoing optical modeling activities for the James Webb Space Telescope
(JWST). The first two papers discussed modeling JWST on-orbit performance using wavefront sensitivities to
predict line of sight motion induced blur, and stability during thermal transients. The third paper investigates the
aberrations resulting from alignment and figure compensation of the controllable degrees of freedom (primary and
secondary mirrors), which may be encountered during ground alignment and on-orbit commissioning of the
observatory, and the fourth introduced the software toolkits used to perform much of the optical analysis for JWST.
The work here models observatory operations by simulating line-of-sight image motion and alignment drifts over a
two-week period. Alignment updates are then simulated using wavefront sensing and control processes to calculate
and perform the corrections. A single model environment in Matlab is used for evaluating the predicted
performance of the observatory during these operations.
KEYWORDS: Mirrors, James Webb Space Telescope, Wavefronts, Image segmentation, Control systems, Detection and tracking algorithms, Telescopes, Space telescopes, Zemax, Interfaces
A MATLAB toolbox has been developed for wavefront control of segmented optical systems. The toolbox
is applied to the optical models of the James Webb Space Telescope (JWST) in general and to the JWST
Testbed Telescope (TBT) in particular, implementing both unconstrained and constrained wavefront
optimization to correct for possible misalignments of the segmented primary mirror or the monolithic
secondary mirror. The optical models are implemented in the ZEMAX optical design program and
information is exchanged between MATLAB and ZEMAX via the Dynamic Data Exchange (DDE)
interface. The model configuration is managed using the Extensible Markup Language (XML) protocol.
The optimization algorithm uses influence functions for each adjustable degree of freedom of the optical
model. Both iterative and non-iterative algorithms have been developed that converge to a local minimum
of the root-mean-square (rms) wavefront error using singular value decomposition (SVD) of the control
matrix of influence functions. The toolkit is highly modular and allows the user to choose control
strategies for the degrees-of-freedom (DOF) on a given iteration and also allows the wavefront
convergence criterion to be checked on each iteration. As the influence functions are nonlinear over the full
control parameter space, the toolkit also allows for trade-offs between frequency of updating the local
influence functions and execution speed. The functionality of the toolbox and the validity of the underlying
algorithms have been verified through extensive simulations.
The Solar TErrestrial RElations Observatory (STEREO), the third mission in NASA's Solar Terrestrial Probes program,
was launched in 2006 on a two year mission to study solar phenomena. STEREO consists of two nearly identical
satellites, each carrying an Extreme Ultraviolet Imager (EUVI) telescope as part of the Sun Earth Connection Coronal
and Heliospheric Investigation instrument suite. EUVI is a normal incidence, 98mm diameter, Ritchey-Chrétien
telescope designed to obtain wide field of view images of the Sun at short wavelengths (17.1-30.4nm) using a CCD
detector. The telescope entrance aperture is divided into four quadrants by a mask near the secondary mirror spider
veins. A mechanism that rotates another mask allows only one of these sub-apertures to accept light over an exposure.
The EUVI contains no focus mechanism. Mechanical models predict a difference in telescope focus between ambient
integration conditions and on-orbit operation. We describe an independent check of the ambient, ultraviolet, absolute
focus setting of the EUVI telescopes after they were integrated with their respective spacecraft. A scanning Hartmann-like
test design resulted from constraints imposed by the EUVI aperture select mechanism. This inexpensive test was
simultaneously coordinated with other integration and test activities in a high-vibration, clean room environment. The
total focus test error was required to be better than ±0.05mm. We cover the alignment and test procedure, sources of
statistical and systematic error, data reduction and analysis, and results using various algorithms for determining focus.
The results are consistent with other tests of instrument focus alignment and indicate that the EUVI telescopes meet the
ambient focus offset requirements. STEREO and the EUVI telescopes are functioning well on-orbit.
KEYWORDS: Wavefronts, Point spread functions, Image segmentation, Wavefront sensors, James Webb Space Telescope, Phase retrieval, Sensors, Telescopes, Space telescopes, Signal to noise ratio
Image-based wavefront sensing algorithms are being used to characterize the 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 co-added 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 each PSF frame individually and average the resulting wavefronts. The choice of co-add methodology
is particularly noteworthy for segmented-mirror telescopes that are subject to noise that causes uncorrelated motions
between groups of segments. Using models and data from the James Webb Space Telescope (JWST) Testbed Telescope
(TBT), 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 co-add method. Of
particular interest, segment piston is more accurately recovered in "image-plane space" co-adding, while segment tip/tilt
is recovered in "pupil-plane space" co-adding.
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.
KEYWORDS: Digital signal processing, Field programmable gate arrays, Computer architecture, Wavefront sensors, Magnesium, Algorithm development, Distributed computing, James Webb Space Telescope, Signal processing, Image processing
Image-based wavefront sensing provides significant advantages over interferometric-based wavefront sensors such as
optical design simplicity and stability. However, the image-based approach is computationally intensive, and therefore,
applications utilizing the image-based approach gain substantial benefits using specialized high-performance computing
architectures. The development and testing of these computing architectures are essential to missions such as James
Webb Space Telescope (JWST), Terrestrial Planet Finder-Coronagraph (TPF-C and CorSpec), and the Spherical
Primary Optical Telescope (SPOT). The algorithms implemented on these specialized computing architectures make
use of numerous two-dimensional Fast Fourier Transforms (FFTs) which necessitate an all-to-all communication when
applied on a distributed computational architecture. Several solutions for distributed computing are presented with an
emphasis on a 64 Node cluster of digital signal processors (DSPs) and multiple DSP field programmable gate arrays
(FPGAs), offering a novel application of low-diameter graph theory. Timing results and performance analysis are
presented. The solutions offered could be applied to other computationally complex all-to-all communication problems.
Testbed results are presented demonstrating high-speed image-based wavefront sensing and control for a spherical primary optical telescope (SPOT). The testbed incorporates a phase retrieval camera coupled to a 3-Mirror Vertex testbed (3MV) at the NASA Goddard Space Flight Center. Actuator calibration based on the Hough transform is discussed as well as several supercomputing architectures for image-based wavefront sensing. Timing results are also presented based on various algorithm implementations using a cluster of 64 TigerSharc TS101 DSP's (digital-signal processors).
KEYWORDS: James Webb Space Telescope, Phase retrieval, Wavefronts, Mirrors, Digital signal processing, Algorithm development, Image segmentation, Data modeling, Space telescopes, Telescopes
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.
KEYWORDS: Wavefront sensors, Data modeling, Diffraction, James Webb Space Telescope, Phase retrieval, Spectrographs, Mirrors, Cameras, Point spread functions, Astronomical imaging
An analysis is presented that illustrates how the James Webb Space Telescope (JWST) fine-phasing process can be carried out using the Near-Infrared Spectrograph (NIRSpec) data collected at the science focal plane. The analysis considers a multi-plane diffraction model which properly accounts for the microshutter diffractive element placed at the first relay position of the spectrograph. Wavefront sensing results are presented based on data collected from the NASA Goddard Microshutter Optical Testbed.
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