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.
The 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.