ISBN 9783843916028

978-3-8439-1602-8, Reihe Raumfahrt

Ulrich Lampater
Improvements to the Image Stability of the SOFIA Airborne Telescope

197 Seiten, Dissertation Universität Stuttgart (2014), Softcover, A5

Zusammenfassung / Abstract

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is the largest and most complex airborne platform for infrared astronomy. It operates in the stratosphere at altitudes between 39kft and 45kft (11.8km to 13.7km) and is designed for observations in the wavelength range from 0.3µm to 1600µm. SOFIA features a 2.5m Cassegrain telescope with Nasmyth focus. The main optics are located in a open port cavity in the rear part of a highly modified Boeing 747SP aircraft. The ultimate goal is to reach a nearly diffraction limited image size at a wavelength of 15µm. This requires the image jitter to be limited to 0.5arcsec RMS, with a goal of 0.2arcsec RMS. The airborne platform supports the dumbbell-shaped telescope structure at its center of gravity. Aircraft motion acts as a base excitation force and causes telescope bending, especially during turbulence. The open port cavity causes unsteady pressure fluctuations and acoustic effects that excite the telescope structure in a wide spectral range. In the design process for the observatory end to-end-simulations were performed to estimate the image stability that can be reached with the given design. The simulations relied on design assumptions of the telescope's modal characteristics and the operational load environment. There were inherent uncertainties in those design assumptions, particularly with respect to the cavity acoustics. Furthermore, it became clear during the design process that an image stability of 0.5arcsec RMS cannot be reached without expanding the regime of the pointing control system to include dominant flexible modes of the structure, and it was unclear whether a superposition of acoustic and structural modes would occur with an associated strong increase in image motion. The subject of this thesis is to establish a baseline implementation of the pointing control system algorithms for the first phase of science flights (Early Science), and to determine the modal properties of the telescope structure. The performance of the observatory in Early Science configuration is tested on the ground and in operational conditions in flight. The design concept is validated through the analysis of image motion and image size. The pointing control system is optimized to improve image stability and the potential for reaching the ambitious image stability goal is evaluated.