Carl Blaurock

Reaction Wheel Disturbance Modeling, Jitter Analysis, and Validation Tests for Solar Dynamics Observatory

The Solar Dynamics Observatory (SDO) aims to study the Suns influence on the Earth by understanding the source, storage, and release of the solar energy, and the interior structure of the Sun. During science observations, the jitter stability at the instrument focal plane must be maintained to less than a fraction of an arcsecond for two of the SDO instruments. To meet these stringent requirements, significant amount of analysis and test effort have been devoted to predicting the jitter induced from various disturbance sources. One of the largest disturbances onboard is the reaction wheel. This paper presents the SDO approach on reaction wheel disturbance modeling and jitter analysis. It describes the verification and calibration of the disturbance model, and ground tests performed for validating the reaction wheel jitter analysis. To mitigate the reaction wheel disturbance effects, the wheels will be limited to operate at low wheel speeds based on the current analysis. An on-orbit jitter test algorithm is also presented in the paper which will identify the true wheel speed limits in order to ensure that the wheel jitter requirements are met.

Precision telescope pointing and spacecraft vibration isolation for the Terrestrial Planet Finder Coronagraph

The Terrestrial Planet Finder Coronagraph is a visible-light coronagraph to detect planets that are orbiting within the Habitable Zone of stars. The coronagraph instrument must achieve a contrast ratio stability of 2e-11 in order to achieve planet detection. This places stringent requirements on several spacecraft subsystems, such as pointing stability and structural vibration of the instrument in the presence of mechanical disturbance: for example, telescope pointing must be accurate to within 4 milli-arcseconds, and the jitter of optics must be less than 5 nm. This paper communicates the architecture and predicted performance of a precision pointing and vibration isolation approach for TPF-C called Disturbance Free Payload (DFP)* . In this architecture, the spacecraft and payload fly in close-proximity, and interact with forces and torques through a set of non-contact interface sensors and actuators. In contrast to other active vibration isolation approaches, this architecture allows for isolation down to zero frequency, and the performance of the isolation system is not limited by sensor characteristics. This paper describes the DFP architecture, interface hardware and technical maturity of the technology. In addition, an integrated model of TPF-C Flight Baseline 1 (FB1) is described that allows for explicit computation of performance metrics from system disturbance sources. Using this model, it is shown that the DFP pointing and isolation architecture meets all pointing and jitter stability requirements with substantial margin. This performance relative to requirements is presented, and several fruitful avenues for utilizing performance margin for system design simplification are identified.

Integrated modeling of optical performance for the Terrestrial Planet Finder structurally connected interferometer

The Terrestrial Planet Finder (TPF) mission, to be launched in 2014 as a part of NASAs Origins Program, will search for Earth-like planets orbiting other stars. One main concept under study is a structurally connected interferometer. Integrated modeling of all aspects of the flight system is necessary to ensure that the stringent dynamic stability requirements imposed by the mission are met. The MIT Space Systems Laboratory has developed a suite of analysis tools known as DOCS (Disturbances Optics Controls Structures) that provides a MATLAB environment for managing integrated models and performing analysis and design optimization. DOCS provides a framework for identifying critical subsystem design parameters and efficiently computing system performance as a function of subsystem design. Additionally, the gradients of the performance outputs with respect to design variables can be analytically computed and used for automated exploration and optimization of the design space. The TPF integrated model consists of a structural finite element model, optical performance model, reaction wheel isolation stage, and attitude/optical control systems. The integrated model is expandable and upgradeable due to the modularity of the state-space subsystem models. Optical performance under reaction wheel disturbances is computed, and the effects of changing design parameters are explored. The results identify redesign options that meet performance requirements with improved margins, reduced cost and minimized risk.

Dynamic analysis of TMT

Dynamic disturbance sources affecting the optical performance of the Thirty Meter Telescope (TMT) include unsteady wind forces inside the observatory enclosure acting directly on the telescope structure, unsteady wind forces acting on the enclosure itself and transmitted through the soil and pier to the telescope, equipment vibration either on the telescope itself (e.g. cooling of instruments) or transmitted through the soil and pier, and potentially acoustic forces. We estimate the characteristics of these disturbance sources using modeling anchored through data from existing observatories. Propagation of forces on the enclosure or in support buildings through the soil and pier to the telescope base are modeled separately, resulting in force estimates at the telescope pier. We analyze the resulting optical consequences using integrated modeling that includes the telescope structural dynamics, control systems, and a linear optical model. The dynamic performance is given as a probability distribution that includes the variation of the external wind speed and observing orientation with respect to the wind, which can then be combined with dome seeing and other time- or orientation-dependent components of the overall error budget. The modeling predicts acceptable dynamic performance of TMT.

Passive isolator design for jitter reduction in the Terrestrial Planet Finder Coronagraph

Terrestrial Planet Finder (TPF) is a mission to locate and study extrasolar Earthlike planets. The TPF Coronagraph (TPF-C), planned for launch in the latter half of the next decade, will use a coronagraphic mask and other optics to suppress the light of the nearby star in order to collect visible light from such planets. The required contrast ratio of 5e-11 can only be achieved by maintaining pointing accuracy to 4 milli-arcseconds, and limiting optics jitter to below 5 nm. Numerous mechanical disturbances act to induce jitter. This paper concentrates on passive isolation techniques to minimize the optical degradation introduced by disturbance sources. A passive isolation system, using compliant mounts placed at an energy bottleneck to reduce energy transmission above a certain frequency, is a low risk, flight proven design approach. However, the attenuation is limited, compared to an active system, so the feasibility of the design must be demonstrated by analysis. The paper presents the jitter analysis for the baseline TPF design, using a passive isolation system. The analysis model representing the dynamics of the spacecraft and telescope is described, with emphasis on passive isolator modeling. Pointing and deformation metrics, consistent with the TPF-C error budget, are derived. Jitter prediction methodology and results are presented. Then an analysis of the critical design parameters that drive the TPFC jitter response is performed.

Solar Dynamics Observatory On-Orbit Jitter Testing, Analysis, and Mitigation Plans

The recently launched Solar Dynamics Observatory (SDO) has two science instruments onboard that required sub-arcsecond pointing stability. Significant effort has been spent pre-launch to characterize the disturbances sources and validating jitter level at the component, sub-assembly, and spacecraft levels. However, an end-to-end jitter test emulating the flight condition was not performed on the ground due to cost and risk concerns. As a result, the true jitter level experienced on orbit remained uncertain prior to launch. Based on the pre-launch analysis, several operational constraints were placed on the observatory aimed to minimize the instrument jitter levels. If the actual jitter is below the analysis predictions, these operational constraints can be relaxed to reduce the burden of the flight operations team. The SDO team designed a three-day jitter test, utilizing the instrument sensors to measure pointing jitter up to 256 Hz. The test results were compared to pre-launch analysis predictions, used to determine which operational constraints can be relaxed, and analyzed for setting the jitter mitigation strategies for future SDO operations.

Design and performance of the Terrestrial Planet Finder coronagraph

Terrestrial Planet Finder Coronagraph, one of two potential architectures, is described. The telescope is designed to make a visible wavelength survey of the habitable zones of at least thirty stars in search of earth-like planets. The preliminary system requirements, optical parameters, mechanical and thermal design, operations scenario and predicted performance is presented. The 6-meter aperture telescope has a monolithic primary mirror, which along with the secondary tower, are being designed to meet the stringent optical tolerances of the planet-finding mission. Performance predictions include dynamic and thermal finite element analysis of the telescope optics and structure, which are used to make predictions of the optical performance of the system

Modeling wind-buffeting of the Thirty Meter Telescope

The Thirty Meter Telescope project is designing a 30m diameter ground-based optical telescope. Unsteady wind loads on the telescope structure due to turbulence inside the telescope enclosure impact the delivered image quality. A parametric model is described that predicts the optical performance due to wind with sufficient accuracy to inform relevant design decisions, including control bandwidths. The model is designed to be sufficiently computationally efficient to allow rapid exploration of the impact of design parameters or uncertain/variable input parameters, and includes (i) a parametric wind model, (ii) a detailed structural dynamic model derived from a finite element model, (iii) a linear optical response model, and (iv) a control model. Model predictions with the TMT structural design are presented, including the parametric variation of performance with external wind speed, desired wind speed across the primary mirror, and optical guide loop bandwidth. For the median mountaintop wind speed of 5.5 m/s, the combination of dome shielding, minimized cross-sectional area, and control results in acceptable image degradation.

AMTD: Update of Engineering Specifications Derived from Science Requirements for Future UVOIR Space Telescopes

The Advance Mirror Technology Development (AMTD) project is in Phase 2 of a multiyear effort, initiated in FY12, to mature by at least a half TRL step six critical technologies required to enable 4 meter or larger UVOIR space telescope primary mirror assemblies for both general astrophysics and ultra-high contrast observations of exoplanets. AMTD uses a science-driven systems engineering approach. We mature technologies required to enable the highest priority science AND provide a high-performance low-cost low-risk system. To give the science community options, we are pursuing multiple technology paths. A key task is deriving engineering specifications for advanced normal-incidence monolithic and segmented mirror systems needed to enable both general astrophysics and ultra-high contrast observations of exoplanets missions as a function of potential launch vehicles and their mass and volume constraints. A key finding of this effort is that the science requires an 8 meter or larger aperture telescope.

Structural-thermal-optical performance (STOP) sensitivity analysis for the James Webb Space Telescope

The James Webb Space Telescope (JWST) is a key component of NASAs Origins Program to understand the origins and future of the universe. JWST will be used to study the birth and formation of galaxies and planets. The mission requires a large (25m2 aperture) but extremely stable (150 nm RMS wave front error) optical platform, where performance is a tightly coupled function of numerous physical processes. Distortion due to thermal loading is a significant error source. The process by which predicted heat loads are mapped to optical error is termed Structural-Thermal-Optical Performance (STOP) modeling. Thermal-optical performance is a function of heat loads, thermal properties (conductivities, radiative coupling coefficients), structural properties (moduli, geometry, thermal expansion coefficients, ply layup angles), and optical sensitivities. Sensitivities, the gradients of performance with respect to design parameters, give a direct way to identify the parameters that have the largest influence on performance. Additionally, gradients can identify the largest sources of uncertainty, and thus contribute to improving the robustness of the design, either via redesign or by placing requirements on parameter variability. The paper presents a general framework for developing the analytical sensitivities of the STOP prediction using the Chain Rule. The paper focuses on solving for the sensitivities of the steady-state, conduction-only, problem, using discipline modeling tools (thermal, structural, and optical) to compute the terms in the STOP gradients. The process is demonstrated on the SDR2 Rev. 1 cycle of the JWST modeling effort.