Design and Analysis of Three Axis Gimbal Mount for testing a Large Size Light Weight Mirror of a Space Borne Telescopic Optics

Optical imaging telescopes for space applications are typically designed and built to perform at their diffractionlimited performance, where-in their near theoretical limit of performance to be achieved. To obtain such performance, the mirrors front surfaces of the telescopes are typically fabricated and characterized to a surface accuracy λ/60 RMS and λ/10 PTV (where λ = 632.8 nm). Further, typical pointing and geometrical positioning accuracies of different optical elements in the telescope (such as primary & secondary mirror) are required to be controlled to less than 5 arc seconds and 10 microns respectively. The stability of the above performance for given operational and survival conditions is also very critical. The loads that influence the stability of performance of the system may be manufacturing process of each mirror & their mounts, assembly and interface loads, static pre-loads, dynamic launch loads, gravity release effects, on-orbit thermal and other environmental loads. During fabrication and testing of mirror performance on ground, the gravity induced self weight deformations becomes very critical. If the mirror are very large in size and are light weighted, the self weight deflections due to gravity becomes very significant and gravity release support systems thus become very essential. When the mirror is tested with mirror axis in horizontal orientation, the gravity induced self weight deformations will be quite different from that of Whiffle Tree mount deflections with mirror axis in vertical orientation due to different stiffness characteristics of the Light weight mirror in two different orientations and support conditions. To test such a large light weight mirror with mirror axis in horizontal orientation, special mounting techniques such as 2-axis / 3-axis Gimbal mounts or strap mounts are used to provide minimal gravity induced deflections. Thus, the Scope was to design, and analyze a semi-kinematic three axis Gimbal mount for testing a typical 1200 mm diameter light weight annular mirror with mirror optical axis in horizontal direction and the gravity vector is perpendicular to it. To carryout testing and characterization of such large size mirror in the above configuration, the proposed semi-kinematic mount, when realized shall provide minimal gravity induced deflections.