Gary E. Mosier

Wavefront sensing and control for a Next-Generation Space Telescope

The Next Generation Space Telescope will depart from the traditional means of providing high optical quality and stability, namely use of massive structures. Instead, a benign orbital environment will provide stability for a large, flexible, lightweight deployed structure, and active wavefront controls will compensate misalignments and figure errors induced during launch and cool-down on orbit. This paper presents a baseline architecture for NGST wavefront controls, including initial capture and alignment, segment phasing, wavefront sensing and deformable mirror control. Simulations and analyses illustrate expected scientific performance with respect to figure error, misalignments, and thermal deformation.

Integrated modeling and dynamics simulation for the Next Generation Space Telescope (NGST)

NGST represents a challenging problem from the point of view of maintaining a milli-arcsecond level pointing accuracy and diffraction limited wavefront performance in the presence of dynamic onboard disturbances during science observations in a cryogenic environment. A Dynamics-Optics-Controls- Structures framework is being developed in support of the NGST dynamics and controls modeling program.

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.

Fine pointing control for a Next-Generation Space Telescope

The Next Generation Space Telescope will provide at least ten times the collecting area of the Hubble Space Telescope in a package that fits into the shroud of an expendable launch vehicle. The resulting large, flexible structure provides a challenge to the design of a pointing control system for which the requirements are at the milli-arcsecond level. This paper describes a design concept in which pointing stability is achieved by means of a nested-loop design involving an inertial attitude control system (ACS) and a fast steering mirror (FSM). A key to the integrated control design is that the ACS controllers has a bandwidth well below known structural modes and the FSM uses a rotationally balanced mechanism which should not interact with the flexible modes that are within its control bandwidth. The ACS controller provides stable pointing of the spacecraft bus with star trackers and gyros. This low bandwidth loop uses nearly co-located sensors and actuators to slew and acquire faint guide stars in the NIR camera. This controller provides a payload reference stable to the arcsecond level. Low-frequency pointing errors due to sensor noise and dynamic disturbances are suppressed by a 2-axis gimbaled FSM locate din the instrument module. The FSM servo bandwidth of 6 Hz is intended to keep the guide star position stable in the NIR focal plane to the required milli-arcsecond level. The mirror is kept centered in its range of travel by a low-bandwidth loop closed around the ACS. This paper presents the result of parametric trade studies designed to assess the performance of this control design in the presence of modeled reaction wheel disturbances, assumed to be the principle source of vibration for the NGST, and variations in structural dynamics. Additionally, requirements for reaction wheel disturbance levels and potential vibration isolation subsystems were developed.

Wavefront Control for a Segmented Deployable Space Telescope

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.

Attitude control system conceptual design for the X-ray timing explorer

The X-ray Timing Explorer (XTE) satellite is the next in a long series of Explorer-class missions developed by NASA. It will study the structure and dynamics of compact X-ray sources, neutron stars, white dwarfs, and other stellar objects with X-ray energy emissions. The demanding pointing requirement of XTE are driving the attitude control system design. This design is further complicated by large moving instruments which impart significant momentum on the spacecraft. The attitude control system concept to meet the XTE science objectives is discussed.

Design for an 8-meter monolithic UV/OIR space telescope

ATLAST-8 is an 8-meter monolithic UV/optical/NIR space observatory to be placed in orbit at Sun-Earth L2 by NASAs planned Ares V cargo launch vehicle. ATLAST-8 will yield fundamental astronomical breakthroughs. A one year mission concept study has developed a detailed point design for the optical telescope assembly and spacecraft. The mission concept assumes two enabling technologies: NASAs planned Ares-V launch vehicle (scheduled for 2019) and autonomous rendezvous and docking (AR&D). The unprecedented Ares-V payload and mass capacity enables the use of a massive, monolithic, thin-meniscus primary mirror - similar to a VLT or Subaru. Furthermore, it enables simple robust design rules to mitigate cost, schedule and performance risk. AR&D enables on-orbit servicing, extending mission life and enhancing science return.

Framework for multidisciplinary integrated modeling and analysis of space telescopes

This paper presents a comprehensive framework for integrated modeling, simulation and analysis of optical telescopes. This framework is called DOCS (Dynamics-Optics-Controls-Structures) and supports model development, model integration, analysis and multidisciplinary design optimization of this class of precision opto-mechanical systems. First the research background and literature in this young filed is discussed. Next the structure and nominal process of an integrated modeling, simulation and analysis study for a generic optical telescope using the DOCS framework is discussed in detail. The major steps include subsystems modeling, model assembly, model reduction and conditioning, initial performance assessment, sensitivity analysis, uncertainty analysis, redesign, design optimization and isoperformance analysis. Such a comprehensive analysis is demonstrated for the NEXUS Space Telescope precursor mission. This mission was designed as a technology testbed for the Next Generation Space Telescope. The challenge is to achieve a very tight pointing accuracy with a sub-pixel line-of-sight (LOS) jitter budget and a root-mean-square (RMS) wavefront error smaller than λ/50 despite the presence of electronic and mechanical disturbance sources. The framework suggested in this paper has the potential for becoming a general prescription for analyzing future, innovative telescope projects. Significant challenges remain in enabling fast simulations for large models, analytical sensitivity analysis for all sub-models, incorporation of slow-varying thermal or impulsive transient effects and the effective use of experimental results.

Joint-space adaptive control of a redundant telerobot manipulator

The design of a joint-space adaptive control-scheme for controlling the slave arm motion of a dual-arm telerobot system developed to study the telerobotic operations in space is presented. Each slave arm of the dual-arm system is a kinematically redundant manipulator with 7 degrees of freedom (DOF). Using the concept of model reference adaptive control (MRAC) and the Lyapunov direct method, the authors derive an adaptation algorithm which adjusts the PD (proportional and derivative) controller gains of the control scheme. The development of the adaptive control scheme assumes that the slave arm motion is noncompliant and slowly varying. The implementation of the derived control scheme does not require the computation of manipulator dynamics which makes the control scheme sufficiently fast for real-time applications. A computer simulation study performed for the 7-DOF slave arm shows that the developed control scheme can efficiently adapt to sudden change in payloads while tracking various test trajectories such as ramps or sinusoids with negligible position errors.<<ETX>>

ATLAST-8 Mission Concept Study for 8-Meter Monolithic UV/Optical Space Telescope

ATLAST-8m is an 8-meter monolithic UV/optical/NIR space observatory which could be placed in orbit at Sun-Earth L2 by a heavily lift launch vehicle. Two development study cycles have resulted in a detailed concept including a dual foci optical design; several primary mirror launch support and secondary mirror support structural designs; spacecraft propulsion, power and pointing control design; and thermal design. ATLAST-8m is designed to yield never before achieved performance to obtain fundamental astronomical breakthroughs.