Tracking turbulence-induced tilt errors with shared and adjacent apertures*†

Tracking apertures which are on axis, off axis, and annular with respect to the pointer optics are considered in terms of their effectiveness in canceling atmospheric turbulence-induced wave-front tilt errors. The off-axis tracker is found to be the least effective, whereras the annular configuration is least sensitive to the wind profile and slewing conditions. The key to minimizing the centroid wander in the focal plane is the proper setting of the low-pass cutoff frequency of the tracking servo. That setting is based on wind velocity, slew rate, and aperture diameters. A too-high setting of the cutoff frequency can actually degrade tracker performance when the tracking aperture is small.