LSST Telescope Guider Loop Requirements Analysis, and predictedPerformance

摘要

The LSST Telescope has critical requirements on tracking error to meet image quality specifications, and will require closing a guiding loop, with the telescope servo control, to meet its mission. The guider subsystem consists of eight guiding sensors located inside the science focal plane at the edge of the 3.5deg field of view. All eight sensors will be read simultaneously at a high rate, and a centroid average will be fed to the telescope and rotator servo controls, for tracking error correction. A detailed model was developed to estimate the sensors centroid noise and the resulting telescope tracking error for a given frame rate and telescope servo control system.rnThe centroid noise depends on the photo-electron flux, seeing conditions, and guide sensor specifications. The model for the photo-electron flux takes into consideration the guide star availability at different galactic latitudes, the atmospheric extinction, the optical losses at different filter bands, the detector quantum efficiency, the integration time and the number of stars sampled. A 7-layer atmospheric model was also developed to estimate the atmospheric decorrelation between the different guide sensors due to the 3.5deg field of view, to predict both correlated and decorrelated atmospheric tip/tilt components, and to determine the trade-offs of the guider servo loop.