Diffractive anisoplanatism and tracker bandwidth limitations

This work develops the concept of "diffractive anisoplanatism," a phenomenon that limits tracker performance for directed-energy applications by introducing differences between point-source-beacon tilt measurements and scoring-beam centroid motion. Our theoretical analysis of this phenomenon, checked against wave-optics simulations, highlights two relevant effects: diffractive conversion of phase to amplitude in the beacon light, and diffractive spreading of the scoring beam into regions outside of the geometric cone sampled by the beacon. In this work, we derive expressions for the variance and power spectral density of the differential jitter between beacon tilt and scoring-beam centroid motion. Additionally, we find a scenario-dependent frequency f_S above which corrections of atmospheric tilt will increase, rather than decrease, scoring-beam centroid jitter on target. Diffractive anisoplanatism provides a fundamental-physics limit on tracker performance that should be considered alongside other practical limitations.