Real-time atmospheric turbulence profile estimation using modal covariance measurements from multiple guide stars

An accurate and timely model of the atmospheric turbulence profile is an important input into the construction of tomographic reconstructors for laser tomography adaptive optics (LTAO) and multi-conjugate adaptive optics (MCAO) using multiple guide stars. We report on a technique for estimating the turbulence profile using the correlations between the modal reconstructions of open-loop wavefront sensor (WFS) measurements from natural or laser guide stars. Laser guide stars can provide an estimate of the turbulence profile along the line of sight to any suitable science target. Open-loop WFS measurements, acquired at the MMT telescope, have been analyzed to recover an estimate of the C_n² profile. This open-loop WFS data can be used to yield turbulence estimates in near real-time, which can be used to update the tomographic reconstructor prior to closed-loop operation. This method can also be applied in closed-loop, using telemetry data already captured by multi-guide star adaptive optics (AO) systems, by computing estimates of the wavefront modal covariances from the closed-loop WFS residual error signals and the deformable mirror (DM) actuator positions. This will be of particular value when implemented with accurate position feedback from the AO system's DMs, rather than the input actuator commands, as is possible with an adaptive secondary mirror. We plan the first tests of the technique with the MMT's adaptive secondary and five Rayleigh laser guide stars.