Design and shape control of lightweight mirrors for dynamic performance and athermalization

Abstract

The next generation of space telescopes will need to meet increasingly challenging science goals. For these new systems to meet resolution goals, the collecting area of the primary mirror will need to be increased. However, current space telescope designs are reaching their limits in terms of size and mass. Therefore, new systems will need to include technologies such as lightweight mirrors, segmented or sparse apertures and active optical control. Many of these technologies have no flight heritage, so determining what combinations of technologies will create favorable designs requires detailed modeling and analysis. This thesis examines the design of a lightweight mirror for an advanced space telescope for both dynamic performance and shape control. A parametric model of a rib-stiffened mirror is created in order to quickly analyze many different mirror geometries. This model is used to examine the homogeneous dynamics of the mirror to determine what geometry will maximize the ratio of stiffness to areal density. The mirror model is then used in a full dynamic disturbance-to-performance analysis so that system performance can be examined as a function of changes in the mirror geometry.

(cont.) Next, a quasi-static shape control algorithm is developed to control the mirror using in the presence of thermal disturbances. The traditional method of mirror shape control relies on feedback from 'a wavefront sensor in the optical path. A wavefront sensor reduces the amount of light available for image formation, which causes problems when viewing very dim objects. Therefore, this control algorithm uses feedback from sensors embedded in the primary mirror. Control algorithms using both strain gages and temperature sensors are developed and compared to determine which sensor type results in better performance. The shape control algorithm with temperature sensors is analyzed using the parametric rib-stiffened mirror model to determine what geometries are best for shape control. The dynamic analysis is combined with the thermal control analysis in order to determine what mirror geometries will be favorable for both of these problems.