Kong Q. Ha

Integrated modeling activities for the James Webb Space Telescope: optical jitter analysis

This is a continuation of a series of papers on the integrated modeling activities for the James Webb Space Telescope (JWST). Starting with the linear optical model discussed in part one, and using the optical sensitivities developed in part two, we now assess the optical image motion and wavefront errors from the structural dynamics. This is often referred to as “jitter” analysis. The optical model is combined with the structural model and the control models to create a linear structural/optical/control model. The largest jitter is due to spacecraft reaction wheel assembly disturbances which are harmonic in nature and will excite spacecraft and telescope structural. The structural/optic response causes image quality degradation due to image motion (centroid error) as well as dynamic wavefront error. Jitter analysis results are used to predict imaging performance, improve the structural design, and evaluate the operational impact of the disturbance sources.

The Role of Integrated Modeling in the Design and Verification of the James Webb Space Telescope

The James Web Space Telescope (JWST) is a large, infrared-optimized space telescope scheduled for launch in 2011. System-level verification of critical optical performance requirements will rely on integrated modeling to a considerable degree. In turn, requirements for accuracy of the models are significant. 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 steps are planned by which the components of the end-to-end models are validated at various levels of subassembly, and the ultimate verification of optical performance is by analysis using the assembled models. This paper describes the critical optical performance requirements driving the integrated modeling activity, shows how the error budget is used to allocate and track contributions to total performance, and presents examples of integrated modeling methods and results that support the preliminary observatory design. Finally, the concepts for model validation and the role of integrated modeling in the ultimate verification of observatory are described.

End-to-end performance modeling of the James Webb Space Telescope (JWST) Observatory

The JWST Observatory currently under development for NASA and its international partners contains a 6.5 meter diameter cryogenic telescope and a suite of 4 highly sensitive instruments which will collect imagery and spectroscopic data over the spectral range of 0.6 to 30 micrometers. The Observatory architecture contains a number of innovative and aggressive technologies including a light-weight primary mirror made up of 18 individually controllable segments, a large sunshield to permit stable low temperature operation, and a nested multi-loop pointing and tracking subsystem to establish milli-arc second line of sight stability. Detailed analytical models are being developed for each of the individual elements of the Observatory. The work described in this paper draws on these models so as to create a high level end-to-end model for the total Observatory. The principal thrust of this end-to-end model (OPTOOL) is to verify that the Observatory meets its overall image quality requirements. These requirements are codified in terms of Strehl ratio, encircled energy, and image anisotropy, and are applied at wavelengths of 1,2, and 5.6 micrometers. OPTOOL is implemented through Matlab (Version 7.0.1) with a Fourier optics based approach for PSF calculations, and uses a direct integration calculation to permit high spatial sampling of the PSF. Polychromatic PSFs are calculated using the Observatory band pass characteristic and an assumed constant Jansky level target. The focal plane arrays of each of the 4 instruments are also included in the model so that realistic signal with noise imagery can be simulated. Exit pupil optical path difference (OPD) maps can be generated using combinations of Zernike polynomials or shaped power spectral densities. Aberrations can be applied to the entire pupil or to the individual segments which make up the pupil. Global exit pupil OPD maps can also be imported and used to generate predicted Point Spread Functions (PSFs). Sample results are presented.

Optical modeling activities for NASA's James Webb Space Telescope (JWST): Part V. Operational alignment updates

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.

Optical modeling activities for the James Webb Space Telescope (JWST) project: II. Determining image motion and wavefront error over an extended field of view with a segmented optical system

This is the second of a series on the optical modeling activities for the JWST government systems engineering integrated modeling team. Starting with the linear optical model discussed in the first installment, we develop centroid and wavefront error sensitivities for the special case of a segmented optical system such as JWST, where the primary mirror consists of 18 individual segments. Our approach extends standard sensitivity matrix methods used for systems consisting of monolithic optics, where the image motion is approximated by averaging ray coordinates at the image and residual wavefront error is determined with global tip/tilt removed. We develop an exact formulation using the linear optical model, and extend it to cover multiple field points for performance prediction at each instrument aboard JWST. This optical model is then driven by thermal and dynamic structural perturbations in an integrated modeling environment. Results are presented.

Preliminary jitter stability results for the large UV/optical/infrared (LUVOIR) surveyor concept using a non-contact vibration isolation and precision pointing system

The need for high payload dynamic stability and ultra-stable mechanical systems is an overarching technology need for large space telescopes such as the Large Ultraviolet / Optical / Infrared (LUVOIR) Surveyor concept. The LUVOIR concept includes a 15-meter-diameter segmented-aperture telescope with a suite of serviceable instruments operating over a range of wavelengths between 100 nm to 2.5 μm. Wavefront error (WFE) stability of less than 10 picometers RMS of uncorrected system WFE per wavefront control step represents a drastic performance improvement over current space-based telescopes being fielded. Through the utilization of an isolation architecture that involves no mechanical contact between the telescope and the host spacecraft structure, a system design is realized that maximizes the telescope dynamic stability performance without driving stringent technology requirements on spacecraft structure, sensors or actuators. Through analysis of the LUVOIR finite element model and linear optical model, the wavefront error and Line- Of-Sight (LOS) jitter performance is discussed in this paper when using the Vibration Isolation and Precision Pointing System (VIPPS) being developed cooperatively with Lockheed Martin in addition to a multi-loop control architecture. The multi-loop control architecture consists of the spacecraft Attitude Control System (ACS), VIPPS, and a Fast Steering Mirror on the instrument. While the baseline attitude control device for LUVOIR is a set of Control Moment Gyroscopes (CMGs), Reaction Wheel Assembly (RWA) disturbance contribution to wavefront error stability and LOS stability are presented to give preliminary results in this paper. CMG disturbance will be explored in further work to be completed.

An update on the role of systems modeling in the design and verification of the James Webb Space Telescope

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.