The Giant Magellan Telescope Organization is designing and building a ground-based 25-meter extremely large telescope. This project represents a significant increase in complexity and performance requirements over 8-10 meter class telescope control systems. This paper presents how recent software and hardware technologies and the lessons learned from the previous generation of large telescopes can help to address some of these challenges. We illustrate our model-centric approach to capture all the functionalities and workflows of the observatory subsystems, and discuss its benefits for implementing and documenting the software and control systems. The same modeling approach is also used
to capture and facilitate the development process.
The Terrestrial Planet Finder Interferometer (TPF-I) mission requires a set of formation-flying collector telescopes that direct the incoming light to a beam combiner where the beams are combined and detected to identify habitable planets. A baseline TPF collector design, using a primary mirror of 4.2 meters in diameter, is used here to conduct a dynamic study. The objective is to investigate the effects of dynamic response of the spacecraft on the system optical performance at the presence of disturbances that arise from the reaction wheel assembly and thruster loading, respectively. Frequency responses where the frequency is associated with the flywheel speed are presented in the paper. The results focus on the surface oscillation of the primary mirror and the point at which the secondary mirror is located. Transient response simulations under the baseline four thruster-assembly configuration were conducted using various duty cycles and thrust levels determined by the TPF formation rotation requirements. This paper will also describe an investigation conducted using new IMOS (Integrated Modeling of Optical Systems), which is an open, multi-disciplinary, and Matlab-based dynamic/optical system simulation code. A pre-processor that is able to generate the sub-structure modal models required by ISYSD (Integrated System Dynamics) was developed in new IMOS. ISYSD is used to develop a high-fidelity system dynamic model by integrating the sub-structure modal models. Finally, the paper will summarize current and future work in order to meet the TPF dynamic requirements.
Mechanism interface mechanics play an important role in the static and dynamic dimensional stability of deployable optical instruments. Friction mechanics in deployment mechanisms has been found to be a source of kinematic indeterminacy allowing elastic energy to be stored throughout the structure. At submicron scales, microslip mechanics allow this behavior to persist well below the classical Coulomb friction limit. This paper presents the design of a cryogenic tribometer for measuring this behavior in candidate mechanism interfaces in both room temperature and cryogenic environments. Room temperature results are presented and compared to a proposed generalized microslip model form. This model form is intended to allow the parametric characterization of microslip behavior caused by smooth nonconforming contact as well as roughness-induced microslip. Spherical ball-on-flat interface geometries were used with two unlubricated material combinations: 440C stainless steel ball on a 440C stainless steel flat and a silicon nitride ball on a 440C stainless steel flat. Consistent parameters were identified for the generalized microslip model from steady cyclic shear responses for both of these interface cases. While these parameters exhibited a measurable sensitivity to normal preload levels, the model form appears to provide the necessary level of robustness. Non-ideal transient shear phenomena including rate dependence were also observed but should play only a secondary role in future modeling efforts.
The Terrestrial Planet Finder mission requires extreme dynamic stability at cryogenic temperatures in order to carry out its objectives of searching for and observing extraterrestrial planets. As a result, the ability to meet its ambitious science goals will be significantly enhanced by increasing its vibrational damping at cryogenic temperatures. Given the low inherent structural damping at cryogenic temperatures, significant reduction in vibration amplitude could be gained with only modest increases in damping on the structure. To examine the use of vibrational damping options to improve the dynamic stability of cryogenic structures, Jet Propulsion Laboratory has conducted a series of experiments to measure the damping levels of various materials at cryogenic temperatures and to search for the materials with higher cryogenic damping. This paper summarizes our experimental observations on the material damping of silicon foam and silicon carbide foam materials at cryogenic temperatures. These foam materials have been independently developed by Schafer Corporation and have properties that enable their applications in space environments with a range of temperature from 25K to 500K. These materials have been used for mirrors, and uses for foam based structures such as optical mounts and benches are currently in development. As observed from the measured damping, these two foam materials have higher damping than aluminum at cryogenic temperatures, and the damping level is relatively insensitive to temperature change from room to cryogenic temperatures. As a result, these materials may be potential candidates to achieve increased levels of cryogenic damping for the Terrestrial Planet Finder mission.
This paper describes a unique experimental facility designed to measure damping of materials at cryogenic temperatures for the Terrestrial Planet Finder (TPF) mission at the Jet Propulsion Laboratory. The test facility removes other sources of damping in the measurement by avoiding frictional interfaces, decoupling the test specimen from the support system, and by using a non-contacting measurement device. Damping data reported herein are obtained for materials (Aluminum, Aluminum/Terbium/Dysprosium, Titanium, Composites) vibrating in free-free bending modes with low strain levels (< 10-6 ppm). The fundamental frequencies of material samples are ranged from 14 to 202 Hz. To provide the most beneficial data relevant to TPF-like precision optical space missions, the damping data are collected from room temperatures (around 293 K) to cryogenic temperatures (below 40 K) at unevenly-spaced intervals. More data points are collected over any region of interest. The test data shows a significant decrease in viscous damping at cryogenic temperatures. The cryogenic damping can be as low as 10-4 %, but the amount of the damping decrease is a function of frequency and material. However, Titanium 15-3-3-3 shows a remarkable increase in damping at cryogenic temperatures. It demonstrates over one order of magnitude increase in damping in comparison to Aluminum 6061-T6. Given its other properties (e.g., good stiffness and low conductivity) this may prove itself to be a good candidate for the application on TPF. At room temperatures, the test data are correlated well with the damping predicted by the Zener theory. However, large discrepancies at cryogenic temperatures between the Zener theory and the test data are observed.