Design of Highly Stable Optical Support Structure

Abstract

As spaceborne optical systems increase in diameter to achieve improved resolution, the stability requirements imposed on structures approach values which were unthinkable only several years ago. To achieve the capabilities of these apertures optical path errors must not exceed a specific fraction of the wavelength of light, and this fraction, typically x/20 rms in the focal plane, is independent of system size. Thus, from a percentage error basis, large optical support structures represent a far more formidable development task than do smaller systems. The Large Space Telescope (LST) sponsored by MSFC/NASA is a case in point. The Optical Telescope Assembly (OTA) is shown (Fig. 1) installed in the LST spacecraft. The vertex-to-vertex spacing of the primary and secondary mirror is 193 inches. To achieve satisfactory optical performance, this spacing must be maintained constant to a precision of -±11.1 for observation periods up to 10 hours. During this time it may be necessary to alter the spacecraft attitude with respect to the sun, which would change the temperature levels and gradients within the structures. It is believed that by exploiting the use of graphite-epoxy in a novel manner, the stringent alignment require-ments can be satisfied with a nominally passive structure.