There are some systems that have been traditionally regarded as too complex for simulation, this mindset results in expensive protypes to conduct build and break scenarios. As the need to understand increasingly complex systems evolves, so must the tools. This work seeks to demonstrate that not only is simulation possible with a complex multi-physics problem, but it is accurate, while providing incredible time and cost savings when compared to alternative methods. Simulation of complex systems early in the design and development phases can reduce the number of prototypes created, the number of test flights required and provide design insights earlier in the product life cycle. Design modification while still in the development phase facilitates potential for greater flexibility. Failure to include simulation early, can result in more costly prototyping, greater number of test flights required and the further into a product life cycle issues are discovered, the more limited the options are for modification. Simulation can provide early insights and cost savings.
KEYWORDS: Mirrors, Finite element methods, Data modeling, Process modeling, Prisms, Thermography, Systems modeling, Beam steering, Gradient-index optics, Optical testing
For many optical applications, we need more efficient ways to create complete models of the system performance, including optical, thermal, and structural effects. Current models are difficult to create and prone to error. More efficient methods would lower costs and enable new kinds of studies. We examine the ideal STOP workflow for two systems. First, we model an optical test for a light-weighted mirror with the goal of determining its on-orbit shape. Second, we inspect the workflow for a compact steering prism system with some absorption of the incident beam. We identify challenges to implementation and discuss possible solutions.
Our new Contrast Optimization technique allows for robust and efficient optimization on the system MTF at a given spatial frequency. The method minimizes the wavefront differences between pairs of rays separated by a pupil shift corresponding to the targeted spatial frequency, which maximizes the MTF. Further computational efficiency is achieved by using Gaussian Quadrature to determine the pattern of rays sampled. Examples are given to demonstrate the advantages of the technique.
We present a new high-contrast imaging testbed designed to provide complete solutions in wavefront sensing, control and starlight suppression with complex aperture telescopes. The testbed was designed to enable a wide range of studies of the effects of such telescope geometries, with primary mirror segmentation, central obstruction, and spiders. The associated diffraction features in the point spread function make high-contrast imaging more challenging. In particular the testbed will be compatible with both AFTA-like and ATLAST-like aperture shapes, respectively on-axis monolithic, and on-axis segmented telescopes. The testbed optical design was developed using a novel approach to define the layout and surface error requirements to minimize amplitude induced errors at the target contrast level performance. In this communication we compare the as-built surface errors for each optic to their specifications based on end-to-end Fresnel modelling of the testbed. We also report on the testbed optical and optomechanical alignment performance, coronagraph design and manufacturing, and preliminary first light results.
Accurate models of optical performance are an essential tool for astronomers, both for planning scientific observations ahead of time, and for a wide range of data analysis tasks such as point-spread-function (PSF)-fitting photometry and astrometry, deconvolution, and PSF subtraction. For the James Webb Space Telescope, the WebbPSF program provides a PSF simulation tool in a flexible and easy-to-use software package available to the community and implemented in Python. The latest version of WebbPSF adds new support for spectroscopic modes of JWST NIRISS, MIRI, and NIRSpec, including modeling of slit losses and diffractive line spread functions. It also provides additional options for modeling instrument defocus and/or pupil misalignments. The software infrastructure of WebbPSF has received enhancements including improved parallelization, an updated graphical interface, a better configuration system, and improved documentation. We also present several comparisons of WebbPSF simulated PSFs to observed PSFs obtained using JWST's flight science instruments during recent cryovac tests. Excellent agreement to first order is achieved for all imaging modes cross-checked thus far, including tests for NIRCam, FGS, NIRISS, and MIRI. These tests demonstrate that WebbPSF model PSFs have good fidelity to the key properties of JWST's as-built science instruments.
The James Webb Space Telescope (JWST) Optical Simulation Testbed (JOST) is a tabletop workbench to study aspects of wavefront sensing and control for a segmented space telescope, including both commissioning and maintenance activities. JOST is complementary to existing optomechanical testbeds for JWST (e.g. the Ball Aerospace Testbed Telescope, TBT) given its compact scale and flexibility, ease of use, and colocation at the JWST Science & Operations Center. We have developed an optical design that reproduces the physics of JWST's three-mirror anastigmat using three aspheric lenses; it provides similar image quality as JWST (80% Strehl ratio) over a field equivalent to a NIRCam module, but at HeNe wavelength. A segmented deformable mirror stands in for the segmented primary mirror and allows control of the 18 segments in piston, tip, and tilt, while the secondary can be controlled in tip, tilt and x, y, z position. This will be sufficient to model many commissioning activities, to investigate field dependence and multiple field point sensing & control, to evaluate alternate sensing algorithms, and develop contingency plans. Testbed data will also be usable for cross-checking of the WFS&C Software Subsystem, and for staff training and development during JWST's five- to ten-year mission.
Searching for nearby habitable worlds with direct imaging and spectroscopy will require a telescope large enough to provide angular resolution and sensitivity to planets around a significant sample of stars. Segmented telescopes are a compelling option to obtain such large apertures. However, these telescope designs have a complex geometry (central obstruction, support structures, segmentation) that makes high-contrast imaging more challenging. We are developing a new high-contrast imaging testbed at STScI to provide an integrated solution for wavefront control and starlight suppression on complex aperture geometries. We present our approach for the testbed optical design, which defines the surface requirements for each mirror to minimize the amplitude-induced errors from the propagation of out-of-pupil surfaces. Our approach guarantees that the testbed will not be limited by these Fresnel propagation effects, but only by the aperture geometry. This approach involves iterations between classical ray-tracing optical design optimization, and end-to-end Fresnel propagation with wavefront control (e.g. Electric Field Conjugation / Stroke Minimization). The construction of the testbed is planned to start in late Fall 2013.
KEYWORDS: Point spread functions, James Webb Space Telescope, Coronagraphy, Wavefronts, Telescopes, Device simulation, Space telescopes, Fourier transforms, Data modeling, Interfaces
Experience with the Hubble Space Telescope has shown that accurate models of optical performance are extremely
desirable to astronomers, both for assessing feasibility and planning scientific observations, and for data analyses
such as point-spread-function (PSF)-fitting photometry and astrometry, deconvolution, and PSF subtraction.
Compared to previous space observatories, the temporal variability and active control of the James Webb Space
Telescope (JWST) pose a significantly greater challenge for accurate modeling. We describe here some initial
steps toward meeting the community's need for such PSF simulations. A software package called WebbPSF
now provides the capability for simulating PSFs for JWST's instruments in all imaging modes, including direct
imaging, coronagraphy, and non-redundant aperture masking. WebbPSF is intended to provide model PSFs
suitable for planning observations and creating mock science data, via a straightforward interface accessible
to any astronomer; as such it is complementary to the sophisticated but complex-to-use modeling tools used
primarily by optical designers. WebbPSF is implemented using a new exible and extensible optical propagation
library in the Python programming language. While the initial version uses static precomputed wavefront
simulations, over time this system is evolving to include both spatial and temporal variation in PSFs, building
on existing modeling efforts within the JWST program. Our long-term goal is to provide a general-purpose PSF
modeling capability akin to Hubble's Tiny Tim software, and of sufficient accuracy to be useful to the community.
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: Actuators, Mirrors, Wavefronts, Space telescopes, Error analysis, James Webb Space Telescope, Optimization (mathematics), Optical engineering, Kinematics, Monte Carlo methods
The performance of a segmented space telescope depends in part upon the ability to maintain the alignment and phasing of its primary mirror segments. Failures of segment control actuators pose a threat to mission success, but their effects can be mitigated by using the remaining segment actuators to optimize the pose of each affected segment. This paper considers the effect of actuator failures on the final wavefront error of a segmented space telescope whose primary mirror consists of 18 hexagonal segments, each controlled by a 3-6 hexapod. Optimization algorithms that minimize the wavefront error for single- and multiple-failure cases are developed, and simulation results are presented. When one actuator fails, the affected segment can still attain a pose with zero wavefront error by exploiting the rotational symmetry of the primary. When two actuators fail, the resulting wavefront error depends upon which hexapod legs fail and at what lengths; cases where both legs of a bipod fail are an order of magnitude worse than other cases. Finally, Monte Carlo simulations of many failures randomly distributed across an initially well-phased segmented primary show that more than 10% of the actuators must fail before the root-mean-square wavefront error degrades significantly.
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.
We developed a distortion measurement technique that works in snapshot mode. Distortion information across the full field of view can be captured in a single short exposure. To do this, a Ronchi ruling is placed in the object and image planes of the system under test. The undistorted ruling in the image plane interferes with the distorted image of the ruling, producing a Moire fringe pattern that can be analyzed in several ways. Phase shifting can be carried out by shifting the Ronchi ruling in object space. The technique is insensitive to vibration and turbulence. Measurements were routinely made with P-V noise levels of 1 μm on measured chief ray locations in 20 mm image planes (0.01%). Repeated measurements showed disagreements on the 6 μm level across a 20 mm image plane (0.03% repeatability).
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.
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 primary mirror of the James Webb Space Telescope (JWST) consists of 18 segments and is 6.6 meters in diameter.
A sequence of commissioning steps is carried out at a single field point to align the segments. At that single field point,
though, the segmented primary mirror can compensate for aberrations caused by misalignments of the remaining
mirrors. The misalignments can be detected in the wavefronts of off-axis field points. The Multifield (MF) step in the
commissioning process surveys five field points and uses a simple matrix multiplication to calculate corrected positions
for the secondary and primary mirrors. A demonstration of the Multifield process was carried out on the JWST Testbed
Telescope (TBT). The results show that the Multifield algorithm is capable of reducing the field dependency of the TBT
to about 20 nm RMS, relative to the TBT design nominal field dependency.
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.
We have designed and built a testbed demonstrating an angle sensor that measures the relative angular position between
two free-flying spacecraft when used in conjunction with a distance-metrology system. In flight, one spacecraft would
carry an LED beacon while the other would carry the sensor system. Our fixed, staring testbed sensor demonstrated a 10
degree capture range with 0.1 arcsec resolution over the inner 1 arcmin of field, and an update rate of over 100 Hz. The
testbed showed this performance for simulated spacecraft separations of 100 to 1000 meters.
We discuss the basic concepts that have been useful in our work designing multiple aperture telescopes with wide fields of view. We examine combining errors at zero field and errors that are linear as a function of field. An easy optimization for satisfying the sine condition to eliminate linear piston errors is given. Methods for estimating the RMS wavefront errors for the lower-order combining errors are given.
Gossamer mirrors have the potential to reach 100 meter baselines in space because of their very light weight. We explore a type of system that uses an array of flat gossamer mirrors as a primary mirror. Using wavefront reconstruction, we can easily estimate the fields of view for these systems. We report the fields of view as a function of the free parameters for these systems.
We establish the groundwork for a phase theory applicable to multiple-aperture systems. To do this, we define ideal behavior as the phase behavior of an off-axis system that has inherent rotational symmetry. Then we examine the phase behavior of a more general system that has only a single plane of symmetry. This system represents a branch of an actual synthetic aperture system. The comparison of the two systems leads to conditions for which the plane symmetric system has ideal behavior. As a result of this comparison, design rules that are commonly applied to multiple aperture systems appear naturally, including the well-known requirement that the exit pupil is a scaled copy of the entrance pupil. The theory also shows that in reflective synthetic telescopes, fewer mirrors are required to achieve ideal behavior if the mirrors are off- axis sections of an axially-symmetric parent system, rather than on-axis mirrors. The phase theory that we present is cohesive, provides useful design guidelines, and can be considered an addition to wave aberration theory.
Increased performance for optical telescopes has historically come from larger apertures, from technological advances for the telescope components, such as detectors, and from access to better sites, such as space. Little has changed in the basic telescope design for a century. These conventional designs have served us well and will continue to do so with the Next Generation Space Telescope. There is an upper limit to the size of thsi type of telescope, set by the capacity to launch the required mass. For future space telescopes of 50, 100, 500 meter apertures, we have developed a new type of optical design. We use a primary reflector made from segments of flat and near-flat membranes. The secondary reflector and subsequent optics are supported in separate spacecraft, flying in formation with the primary reflector. In addition, each spacecraft maintains sunshields to keep the optics shaded from the sun. This paper explores the optical design issues for this type of giant space telescope.
We establish the groundwork for a phase theory applicable to multiple-aperture systems. We examine the phase behavior of a reference system with rotational symmetry, and then examine the phase behavior of a system with only a single plane of symmetry assumed. A comparison of the two systems leads to the mathematical conditions for which one branch of a multiple-aperture system has ideal behavior.
The 22.8 m Large Binocular Telescope Interferometer will be a uniquely powerful tool for imaging and nulling interferometry at thermal infrared wavelengths (2 - 20 micrometers ) because of the LBT's unusual combination of low emissivity, high spatial resolution, broad (u,v)-plane coverage, and high photometric sensitivity. The gregorian adaptive secondary mirrors permit beam combination after only three warm reflections. They also control the relative pathlength, wavefront tip/tilt, and focus of the two telescope beams, thus greatly simplifying the complexity of the beam-combiner. The resulting four-mirror beam-combiner reimages the original focal plane and also images the telescope pupil onto a cold stop to limit thermal background. At first-light in 2004, an all-reflective, cooled beam-combiner can provide a 2 arcmin diameter field for Fizeau-style imaging as well as the low thermal background and achromaticity required for nulling interferometry.
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