Study Of Telescope’s Derotation System

During the altazimuth telescope tracking the target, the relative rotation betweenthe mirrors in Coude optical path would give birth to the rotation in field of viewaround the center, known as the image rotation in view field. Meanwhile, thealtazimuth telescope’s field of view is static relative to vertical circle of the Earth andduring the Earth’s rotating vertical circle kept turning relative to the polar axis. So theview of field is rotating, known as the image rotation in object field. In order to obtaina stable image, it’s necessary to compensate these two field rotation, referred to asimage derotation.Commonly used image derotation solutions include optical image derotation,electronic image derotation and physical image derotation. And optical imagederotation is widely used among the large aperture telescopes for its real-time. Thismethod is working by an installing a prism or reflector set before the optical imagingcomponent and control the speed of rotation of prism as half of the speed of imagerotation. Reflector set is generally using three plane reflectors arranged as the "K", soit’s called K mirror.A vertical installed K mirror system is designed for2m diameter altazimuthtelescope. The mathematical relation between the image rotation and the altitude andazimuth rotation movement was analyzed. Then compared the commonly used imagederotation solutions and derived the principle of K mirror’s image in theory.2m altazimuth telescope’s K mirror image derotation system consists of thesupport structure of reflectors, adjusting structure of each mirror and the overallframework, transmission components, motors and encoders, and docking station. Kmirror flexible support structure has been optimized. Optimization method is based onflexibility matrix of the flexible support structure. After the finite element analysisand comparison, the theoretical and simulation results of flexibility do not differ bymore than18%. Besides the supporting structure statics and dynamics simulation results show that the optimal solutions meet the design requirements. The supportingrod’s buckling load is greater than work load, no instability risk.The K-mirror system errors that exist during its working time were analyzedbased on the theory of the rigid mirror imaging’s equivalent rigid motion transformand ray tracing method. The deviation between the rotation axis of the K mirror, Kmirror optical axis and the main optical axis of the telescope system cause the viewof the field on the image plane rotates about the main axis. In theory, the tracks ofthe image point of main axis on the image plane fit the expression of Pascal curve.The the K mirror system’s error was numerical simulated and the trajectory curve ofoptical axis on the image plane is consistent with theoretical predictions. And theinfluence of each error on the radius of the curve was analyzed.A set of K-mirror system aligned with the telescope alignment programs,including three steps: mark azimuth axis’s location; align the optical axis of K mirrorwith the azimuth axis; align the K mirror’s rotating axis with the azimuth axis.