Model-based low-order wavefront sensing and control with a fully physical digital twin on MagAO-X

Residual low-order aberrations and post-adaptive-optics tip-tilt jitter are major drivers of stellar leakage in small-inner-working-angle coronagraphs and limit high-contrast imaging performance. The aim of this work is to develop a fully physical and differentiable digital twin for the focal plane low-order wavefront sensor on MagAO-X. This digital twin can be used to provide accurate non-linear and fast linearised wavefront sensing and control to deal with low-order aberrations. We built a physics-based optical model of the focal plane low-order wavefront sensor system and calibrated it using laboratory and on-sky measurements. The calibrated model was used for non-linear phase retrieval, deriving a model-based linear reconstructor and generating the corresponding reference image for real-time control. The calibrated digital twin accurately reproduces the sensor response and recovers the dominant low-order aberrations. In the laboratory, non-linear reconstruction achieves residual floors of about 6 to 7 nm over the pupil, compared with about 11 nm for the linear reconstructor. The same digital-twin framework transfers across different coronagraph and wavelength configurations. On-sky, the model can be recalibrated with limited changes, achieves reconstruction performance close to the residual atmospheric floor, and provides a linear reconstructor that enables stable closed-loop low-order control at kilohertz speeds. A fully physical digital twin can unify accurate low-order wavefront reconstruction, fast linearised control, and transfer both across observing configurations and from lab to on-sky operation within a single interpretable framework.