Abstract. The Subarcsecond Telescope And BaLloon Experiment, STABLE, is the fine stage of a guidance system for a high-altitude ballooning platform designed to demonstrate subarcsecond pointing stability, over one minute using relatively dim guide stars in the visible spectrum. The STABLE system uses an attitude rate sensor and the motion of the guide star on a detector to control a Fast Steering Mirror in order to stabilize the image. The characteristics of the thermal-optical-mechanical elements in the system directly affect the quality of the point spread function of the guide star on the detector, and so, a series of thermal, structural, and optical models were built to simulate system performance and ultimately inform the final pointing stability predictions. This paper describes the modeling techniques employed in each of these subsystems. The results from those models are discussed in detail, highlighting the development of the worst-case cold and hot cases, the optical metrics generated from the finite element model, and the expected STABLE residual wavefront error and decenter. Finally, the paper concludes with the predicted sensitivities in the STABLE system, which show that thermal deadbanding, structural preloading and self-deflection under different loading conditions, and the speed of individual optical elements were particularly important to the resulting STABLE optical performance.Keywords: optical telescope, high-altitude balloon, thermal /structural/optical analysis Address all correspondence to: Michael Borden, NASA Jet Propulsion Laboratory, M/S 321-525N. 4800 Oak Grove Dr., Pasadena, CA, USA, 91109; Tel: +1 651-303-2923, Email: mike.b.borden@gmail.com IntroductionAs astronomers and planetary scientists face shrinking budgets and growing competition for flight opportunities, they are increasingly looking to alternative low-cost platforms that can support science-grade data collection. High-altitude balloons (HABs) are one such platform showing increasing promise for this application. In fact, the planetary science decadal survey for 2013-2022 explicitly called out these platforms for their scientific merit, by suggesting that: "significant planetary work can be done from balloon-based missions flying higher than 45,000 feet… these facilities offer a combination of cost, flexibility, risk tolerance, and support for innovative solutions ideal for the pursuit of certain scientific opportunities."1 These platforms can reach altitudes that are above much of the earth's atmosphere, offering the large coherence lengths (the propagation distance over which a wave retains its coherence) that provide nearspace-like image quality even across spectral bands that are absorbed by the atmosphere and are therefore inaccessible to ground-based observatories. Flights are available at a fraction of the cost of a launch vehicle, and ballooning centers can often recover the payload system so it can be refurbished and reused. These advantages make HABs an attractive solution for certain types of

Microstructure evolution and pattern formation during directional solidification of dilute Sn-Pb system were analysed. Planar, cellular and dendritic morphologies were obtained by adequate thermal gradient and interface velocity control. The samples were obtained from pure elements in concentrations of 0?05, 1?0 and 4?0 wt-%Pb. Growth was carried out in a Bridgman device in the velocity range of 0?38-30 mm s 21 at thermal gradients from 1?3 to 2?6uC mm 21 . The samples were examined using metallographic techniques to determine their morphology and dimensional characteristics. The system was analysed to describe the different stages of evolution of the morphology and transitions. It was observed that both primary cellular and dendritic spacing decreased with increasing velocity. However, in the cellular-dendritic transition zone, an important increase in spacing was detected and basically associated with convective effects during irregular cellular growth.

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