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) 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.
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.
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 design considerations for astronomical space telescopes cover many disciplines but can be simplified into two overarching constraints: the desire to maximize science while adhering to budgetary constraints. More than ever, understanding the cost implications up front will be critical to success. Science performance can be translated into a set of simple performance metrics that set the requirements for design options. Cost is typically estimated by considering mass, complexity, technology maturity, and heritage. With this in mind, we survey the many diverse design considerations for a space telescope and, where appropriate, relate them to these basic performance metrics. In so doing, we hope to provide a roadmap for future space telescope designers on how best to optimize the design to maximize science and minimize total cost.
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.
KEYWORDS: Wavefronts, Coronagraphy, Space telescopes, James Webb Space Telescope, Telescopes, Wavefront sensors, Mirrors, Phase modulation, Nulling interferometry, Point spread functions
The Advanced Technology for Large Aperture Space Telescope (ATLAST) concept was assessed as one of the NASA
Astrophysics Strategic Mission Concepts (ASMC) studies. Herein we discuss the 9.2-meter diameter segmented aperture
version and its wavefront sensing and control (WFSC) with regards to coronagraphic detection and spectroscopic
characterization of exoplanets. The WFSC would consist of at least two levels of sensing and control: (i) an outer coarser
level of sensing and control to phase and control the segments and secondary mirror in a manner similar to the James
Webb Space Telescope but operating at higher temporal bandwidth, and (ii) an inner, coronagraphic instrument based,
fine level of sensing and control for both amplitude and wavefront errors operating at higher temporal bandwidths. The
outer loop would control rigid-body actuators on the primary and secondary mirrors while the inner loop would control
one or more segmented deformable mirror to suppress the starlight within the coronagraphic field-of-view. Herein we
discuss the visible nulling coronagraph (VNC) and the requirements it levies on wavefront sensing and control and show
the results of closed-loop simulations to assess performance and evaluate the trade space of system level stability versus
control bandwidth.
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.
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).
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.
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.
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: 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.
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.
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, Imaging systems, Systems modeling, Diffraction, Data modeling, Process control, Mirrors, Monochromatic aberrations, Radio propagation, Fourier transforms
Pupil imaging performance is analyzed from the perspective of physical optics. A multi-plane diffraction model is constructed by propagating the scalar electromagnetic field, surface by surface, along the optical path comprising the pupil imaging optical system. Modeling results are compared with pupil images collected in the laboratory. The experimental setup, although generic for pupil imaging systems in general, has application to the James Webb Space Telescope (JWST) optical system characterization where the pupil images are used as a constraint to the wavefront sensing and control process. Practical design considerations follow from the diffraction modeling which are discussed in the context of the JWST Observatory.
KEYWORDS: James Webb Space Telescope, Point spread functions, Space telescopes, Telescopes, Sensors, Code v, Linear filtering, Mirrors, Cameras, Wavefronts
The James Webb Space Telescope (JWST) is a segmented deployable telescope that will require on-orbit alignment
using the Near Infrared Camera as a wavefront sensor. The telescope will be aligned by adjusting seven degrees of
freedom on each of 18 primary mirror segments and five degrees of freedom on the secondary mirror to optimize the
performance of the telescope and camera at a wavelength of 2 microns. With the completion of these adjustments, the
telescope focus is set and the optical performance of each of the other science instruments should then be optimal
without making further telescope focus adjustments for each individual instrument. This alignment approach requires
confocality of the instruments after integration and alignment to the composite metering structure, which will be verified
during instrument level testing at Goddard Space Flight Center with a telescope optical simulator. In this paper, we
present the results from a study of several analytical approaches to determine the focus for each instrument. The goal of
the study is to compare the accuracies obtained for each method, and to select the most feasible for use during optical
testing.
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: James Webb Space Telescope, Point spread functions, Mirrors, Error analysis, Image segmentation, Stars, Stochastic processes, Space telescopes, Monte Carlo methods, Sensors
To achieve and maintain the fine alignment of its segmented primary mirror the James Webb Space Telescope (JWST) plans to use focus-diverse wavefront sensing (WFS) techniques with science camera imagery. The optical requirements for JWST are such that the error contribution from the WFS itself must be limited tp 10nm rms over the controllable degrees of freedom of the telescope. In this paper, we will explore the requirements on the target selection and imaging requirements necessary to achieve the desired level of WFS accuracy. Using Monte Carlo simulations we explore the WFS error as a function of wavefront aberrations level, defocus-diversity level, optical bandwidth and imaging signal-to-noise ratio to establish the key imaging requirements. By taking into account practical integration time limits along with the distribution of the defocused point-spread functions, we establish the bright and faint star magnitude limits suitable for WFS target selection.
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.
The NGST Wavefront Control Testbed (WCT) is a joint technology program managed by the Goddard Space Flight Center (GSFC) and the Jet Propulsion Laboratory (JPL) for the purpose of developing technologies relevant to the NGST optical system. The WCT provides a flexible testing environment that supports the development of wavefront sensing and control algorithms that may be used to align and control a segmented optical system. WCT is a modular system consisting of a Source Module (SM), Telescope Simulator Module (TSM) and an Aft-Optics (AO) bench. The SM incorporates multiple sources, neutral density filters and bandpass filters to provide a customized point source for the TSM. The telescope simulator module contains a flip-in mirror that selects between a small deformable mirror and three actively controlled spherical mirror segments. The TSM is capable of delivering a wide range of aberrated, unaberrated, continuous and segmented wavefronts to the AO optical bench for analysis. The AO bench consists of a series of reflective and transmissive optics that images the exit pupil of the TSM onto a 349 actuator deformable mirror that is used for wavefront correction. A Fast Steering Mirror (FSM) may be inserted into the system (AO bench) to investigate image stability and to compensate for systematic jitter when operated in a closed loop mode. We will describe the optical design and performance of the WCT hardware and discuss the impact of environmental factors on system performance.
A wide field (6x6 arcmin2) Rapid Infrared-Visible Multi-Object Spectrometer (RIVMOS) has been designed and is being fabricated at NASA's GSFC as part of the Next Generation Space Telescope (NGST) development and new technology demonstration. The primary goal is to demonstrate that the microshutter arrays, currently being designed for the NGST Near Infrared Spectrometer (NIRSpec) as programmable 2D selection masks, can achieve the optical performance required for faint object imaging and spectroscopy.
We developed an original optical design that includes both reflective and refractive optics. The primary goal of the design was to achieve high imaging quality in both imaging and spectroscopy modes over a very wide spectral range with all spherical surfaces. The required optical performance is achieved for both multi-object spectroscopy and camera imaging over the entire field-of-view. The optical design consists of six optical subsystems including (1) an image relay consisting of a three-mirror anastigmat (TMA), (2) the microshutter assembly, (3) a triplet collimating optic, (4) a grism/filter assembly, (5) a pupil imaging optic, and (6) a five element telecentric camera design.
The all-spherical optical design reduces construction costs and facilitates fabrication of the optical assembly while maintaining an encircled energy of 2 pixels within the FOV for wavelengths between 0.6 and 5.0 microns. Three spectral resolution modes (R = 50, 2000, 4000) will be available for multi-object spectroscopy as well as cross-dispersed echelle spectroscopy at the highest spectral resolution. The low resolution mode will be provided by the direct view prism, whereas silicon grisms will be used for higher resolving power. This design provides an extremely wide spectral range, wide field, very compact, high resolution imager-spectrometer with multi-object capability.
By segmenting and folding the primary mirror, quite large telescopes can be packed into the nose cone of a rocket. Deployed after launch, initial optical performance can be quite poor, due to deployment errors, thermal deformation, fabrication errors and other causes. We describe an automatic control system for capturing, aligning, phasing, and deforming the optics of such a telescope, going from initial cm-level wavefront errors to diffraction-limited observatory operations. This system was developed for the Next Generation Space Telescope and is being tested on the NGST Wavefront Control Testbed.
This paper describes the results of a few of the initial series of tests being conducted with the first configuration of the Next Generation Space Telescope Wavefront sensing and Control Testbed (WCT1). WCT1 is a 1:1, f/16.6 re-imaging system, incorporating two deformable mirrors located at pupil conjugate positions with 6 actuators/diameter (SM/DM) and 20 actuators/diameter (AO/DM). A CCD on a precision stage is used for obtaining defocused images providing phase diversity for wavefront determination using phase retrieval. In a typical experiment, wavefront error is injected into the optical path with the SM/DM and then corrected using the more densely actuated AO/DM. Wavefront analysis is provided via a phase retrieval algorithm, and control software is used to reshape the AO/DM and correct the wavefront. A summary of the results of some initial tests are presented.
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