Diffractive Anisoplanatism and Tracker Bandwidth Limitations
MIT Lincoln Laboratory Lexington United States
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This report develops the concept of diffractive anisoplanatism, a previously omitted effect that places fundamental limitations on tracker performance for directed-energy applications. Diffractive anisoplanatism comprises two effects fist, that diffraction causes the conversion of phase to amplitude on the beacon light used to drive a tracker, effectively hiding some atmospheric phase from tilt measurements second, that diffraction causes a scoring beam propagating to the target to spread into regions outside of the geometric cone sampled by the beacon. These two effects result in a loss of reciprocity between the two beams, such that using the beacon tilt to correct the scoring-beam motion will not drive that motion to zero as conventional wisdom would dictate. This is true even in an idealized situation with no spatial offsets, no latency, no noise or errors, infinite bandwidth, and no time-of-fight delays. Further, this loss of reciprocity worsens as frequency increases, such that our attempts to correct scoring-beam jitter above a scenario-dependent frequency fs will actually make the jitter on target worse.