Large aperture telescope commonly features segment mirrors and a coarse phasing step is needed
to bring these individual segments into the fine phasing capture range. Dispersed Fringe Sensing
(DFS) is a powerful coarse phasing technique and its alteration is currently being used for JWST.
An Advanced Dispersed Fringe Sensing (ADFS) algorithm is recently developed to improve the
performance and robustness of previous DFS algorithms with better accuracy and unique
solution. The first part of the paper introduces the basic ideas and the essential features of the
ADFS algorithm and presents the some algorithm sensitivity study results. The second part of the
paper describes the full details of algorithm validation process through the advanced wavefront
sensing and correction testbed (AWCT): first, the optimization of the DFS hardware of AWCT
to ensure the data accuracy and reliability is illustrated. Then, a few carefully designed algorithm
validation experiments are implemented, and the corresponding data analysis results are shown.
Finally the fiducial calibration using Range-Gate-Metrology technique is carried out and a
<10nm or <1% algorithm accuracy is demonstrated.
KEYWORDS: Calibration, Sensors, Monte Carlo methods, James Webb Space Telescope, Signal detection, Error analysis, Detection and tracking algorithms, Space telescopes, Wavefronts, Telescopes
Dispersed Fringe Sensing (DFS) is an elegant method of coarse phasing segmented mirrors. DFS performance
accuracy is dependent upon careful calibration of the system as well as other factors such as internal optical
alignment, system wavefront errors, and detector quality. Novel improvements to the algorithm have led to
substantial enhancements in DFS performance. In this paper, we present Advanced DFS, an advancement of
the DFS algorithm, which allows the overall method to be less sensitive to calibration errors. This is achieved
by correcting for calibration errors, which appear in the fitting equations as a signal phase term. This paper will
outline a brief analytical explanation of the improvements, results of advanced DFS processed simulations and
experimental advanced DFS results.
We evaluate how well the performance of the Thirty Meter Telescope (TMT) can be maintained against thermally
induced errors during a night of observation. We first demonstrate that using look-up-table style correction for
TMT thermal errors is unlikely to meet the required optical performance specifications. Therefore, we primarily
investigate the use of a Shack-Hartmann Wavefront Sensor (SH WFS) to sense and correct the low spatial
frequency errors induced by the dynamic thermal environment. Given a basic SH WFS design, we position
single or multiple sensors within the telescope field of view and assess telescope performance using the JPL
optical ray tracing tool MACOS for wavefront simulation. Performance for each error source, wavefront sensing
configuration, and control scheme is evaluated using wavefront error, plate scale, pupil motion, pointing error,
and the Point Source Sensitivity (PSSN) as metrics. This study provides insight into optimizing the active optics
control methodology for TMT in conjunction with the Alignment and Phasing System (APS) and primary mirror
control system (M1CS).
The primary mirror segment aberrations after shape corrections with warping harness have been identified as
the single largest error term in the Thirty Meter Telescope (TMT) image quality error budget. In order to better
understand the likely errors and how they will impact the telescope performance we have performed detailed
simulations. We first generated unwarped primary mirror segment surface shapes that met TMT specifications.
Then we used the predicted warping harness influence functions and a Shack-Hartmann wavefront sensor model
to determine estimates for the 492 corrected segment surfaces that make up the TMT primary mirror. Surface
and control parameters, as well as the number of subapertures were varied to explore the parameter space. The
corrected segment shapes were then passed to an optical TMT model built using the Jet Propulsion Laboratory
(JPL) developed Modeling and Analysis for Controlled Optical Systems (MACOS) ray-trace simulator. The
generated exit pupil wavefront error maps provided RMS wavefront error and image-plane characteristics like
the Normalized Point Source Sensitivity (PSSN). The results have been used to optimize the segment shape
correction and wavefront sensor designs as well as provide input to the TMT systems engineering error budgets.
The baseline wavefront sensing and control for James Webb Space Telescope (JWST) includes the Dispersed Hartmann
Sensors (DHS) for segment mirror coarse phasing. The two DHS devices, residing on the pupil wheel of the JWST's
Near Infrared Camera (NIRCam) Short Wavelength Channel (SWC), can sense the JWST segment mirror pistons by
measuring the heights of 20 inter-segment edges from the dispersed fringes. JWST also incorporates two identical
grisms in the NIRCam's Long Wavelength Channel (LWC). The two grisms, designed as the Dispersed Fringe Sensor
(DFS), are used as the backup sensor for JWST segment mirror coarse phasing. The versatility of DFS enables a very
flexible JWST segment coarse phasing process and the DFS is designed to have larger piston capture range than that of
DHS, making the coarse phasing more robust. The DFS can also be a useful tool during JWST ground integration and
test (I&T). In this paper we describe the DFS design details and use the JWST optical model to demonstrate the DFS
coarse phasing process during flight and ground I&T.
An effective multi-field wavefront control (WFC) approach is demonstrated for an actuated, segmented space
telescope using wavefront measurements at the exit pupil, and the optical and computational implications of this
approach are discussed. The integration of a Kalman Filter as an optical state estimator into the wavefront control
process to further improve the robustness of the optical alignment of the telescope will also be discussed. Through a
comparison of WFC performances between on-orbit and ground-test optical system configurations, the connection (and a
possible disconnection) between WFC and optical system alignment under these circumstances are analyzed. Our
MACOS-based [2] computer simulation results will be presented and discussed.
KEYWORDS: Telescopes, Optical transfer functions, Error analysis, Space telescopes, Point spread functions, Wavefronts, Systems modeling, Spatial frequencies, Computer simulations, Thirty Meter Telescope
We investigate a new metric, Normalized Point Source Sensitivity (PSSN), for characterizing the seeing limited
performance of the Thirty Meter Telescope. As the PSSN metric is directly related to the photometric error of
background limited observations, it truly represents the efficiency loss in telescope observing time. The PSSN
metric properly accounts for the optical consequences of wavefront spatial frequency distributions due to different
error sources, which makes it superior to traditional metrics such as the 80% encircled energy diameter. We
analytically show that multiplication of individual PSSN values due to individual errors is a good approximation
for the total PSSN when various errors are considered simultaneously. We also numerically confirm this feature
for Zernike aberrations, as well as for the numerous error sources considered in the TMT error budget using a
ray optics simulator, Modeling and Analysis for Controlled Optical Systems. We also discuss other pertinent
features of the PSSN including its relations to Zernike aberration and RMS wavefront error.
We consider high-resolution optical modeling of the Thirty Meter Telescope for the purpose of error budget and instrumentation trades utilizing the Modeling and Analysis for Controlled Optical Systems tool. Using this ray-trace and diffraction model we have simulated the TMT optical errors related to multiple effects including segment alignment and phasing, segment surface figures, temperature, and gravity. We have then modeled the effects of each TMT optical error in terms of the Point Source Sensitivity (a multiplicative image plane metric) for a seeing limited case and an adaptive optics corrected case (for the NFIRAOS). This modeling provides the information necessary to rapidly conduct design trades with respect to the planned telescope instrumentation and to optimize the telescope error budget.
Optical State Estimation provides a framework for both separating errors in test optics from the target system and deducing the state of multiple optics in a telescope beam train using wavefront as well as pre-test component measurements including the knowledge of their level of error. Using this framework, we investigate the feasibility of simplifying the interferometric alignment configuration of NASA's James Webb Space Telescope, a large segmented-aperture cryogenic telescope, using a single, static auto-collimating flat instead of six such flats, resulting in a reduced sub-aperture sampling.
KEYWORDS: James Webb Space Telescope, Wavefronts, Error analysis, Phase modulation, Optical alignment, Telescopes, Space telescopes, Filtering (signal processing), Monte Carlo methods, Control systems
An effective multi-field wavefront control (WFC) approach is demonstrated for the James Webb Space Telescope (JWST) on-orbit optical telescope element (OTE) fine-phasing using wavefront measurements at the NIRCam pupil, and the optical and computational implications of this approach are discussed. The integration of a Kalman Filter as an optical state estimator into the JWST wavefront control process to further improve the robustness of the fine-phasing JWST OTE alignment will also be discussed. Through a comparison of WFC performances between the JWST on-orbit and ground-test optical system configurations, the connection (and a possible disconnection) between WFC and optical system alignment under these circumstances are analyzed. Our MACOS-based [2] computer simulation results will be presented and discussed.
KEYWORDS: Error analysis, James Webb Space Telescope, Wavefronts, Telescopes, Phase modulation, Space telescopes, Optical testing, Interferometers, Actuators, Monte Carlo methods
The use of wavefront measurements to deduce the state of multiple optics in a telescope beam train - their misalignments and figure errors - can be confused by the fact that there are multiple potential sources for the same measured error. This talk applies Kalman filtering techniques as a tool for separating true telescope errors from artifactual testing errors in the alignment and testing of NASA's James Webb Space Telescope, a large segmented-aperture cryogenic telescope to be launched after 2010.
To accomplish micro-arcsecond astrometric measurement, stellar interferometers such as SIM require the measurement of internal optical path length delay with an accuracy of ~10 picometers level. A novel common-path laser heterodyne interferometer suitable for this application was proposed and demonstrated at JPL. In this paper, we present some of the experimental results from a laboratory demonstration unit and design considerations for SIM's internal metrology beam launcher.
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