Forecasting Wavefront Corrections in an Adaptive Optics System

TL;DR

This paper demonstrates that linear auto-regressive models can significantly forecast wavefront corrections in adaptive optics, reducing errors and improving image quality, with minimal computational cost, while more complex models offer no additional benefit.

Contribution

It introduces a simple, effective linear forecasting method for wavefront correction in adaptive optics and evaluates machine learning models, finding linear models suffice for current conditions.

Findings

Linear auto-regressive models reduce wavefront error by up to 5 times for Tip-Tilt.

Forecasting filters can be updated quickly with minimal telemetry data.

More complex machine learning models do not outperform linear models in this context.

Abstract

We use telemetry data from the Gemini North ALTAIR adaptive optics system to investigate how well the commands for wavefront correction (both Tip/Tilt and high-order turbulence) can be forecasted in order to reduce lag error (due to wavefront sensor averaging and computational delays) and improve delivered image quality. We show that a high level of reduction ($\sim$ 5 for Tip-Tilt and $\sim$ 2 for high-order modes) in RMS wavefront error can be achieved by using a "forecasting filter" based on a linear auto-regressive model with only a few coefficients ($\sim$ 30 for Tip-Tilt and $\sim$ 5 for high-order modes) to complement the existing integral servo-controller. Updating this filter to adapt to evolving observing conditions is computationally inexpensive and requires less than 10 seconds worth of telemetry data. We also use several machine learning models (Long-Short Term Memory and dilated convolutional models) to evaluate whether further improvements could be achieved with a more sophisticated non-linear model. Our attempts showed no perceptible improvements over linear auto-regressive predictions, even for large lags where residuals from the linear models are high, suggesting that non-linear wavefront distortions for ALTAIR at the Gemini North telescope may not be forecasted with the current setup