Shared apertures in tracking turbulence-induced wavefront tilt errors

In many optical pointing and tracking systems it is not practical to beam split the incoming wavefront in order to derive a tracking signal. The tracker optics are typically placed adjacent to a larger pointing mirror or telescope. We have found that this configuration is not optimum in tracking turbulence-induced wavefront errors. Instead, the tracker should either be in the region of the pointer central obscuration or be in a thin annulus surrounding the pointer. Wherever the tracker is located, its signal should be low-pass filtered, perhaps by the servo itself. Low frequencies correspond to large-scale turbulence, and such turbulence appears the same to both the pointer and the tracker.