  The future large adaptive telescopes will trigger new constraints for the
calibration of Adaptive Optics (AO) systems equipped with pre-focal Deformable
Mirrors (DM). The image of the DM actuators grid as seen by the Wave-Front
Sensor (WFS) may evolve during the operations due to the flexures of the
opto-mechanical components present in the optical path. The latter will result
in degraded AO performance that will impact the scientific operation. To
overcome this challenge, it will be necessary to regularly monitor and
compensate for these DM/WFS mis-registrations either by physically re-aligning
some optical components or by updating the control matrix of the system. In
this paper, we present a new strategy to track mis-registrations using a
pseudo-synthetic model of the AO system. The method is based on an invasive
approach where signals are acquired on-sky, before or during the scientific
operations, and fed to the model to extract the mis-registration parameters. We
introduce a method to compute the most sensitive modes to these
mis-registrations that allows to reduce the number of degrees of freedom
required by the algorithm and minimize the impact on the scientific
performance. We demonstrate that, using only a few of these well selected
signals, the method provides a very good accuracy on the parameters estimation,
well under the targeted accuracy, and has a negligible impact on the scientific
path. In addition, the method appears to be very robust to varying operating
conditions of noise and atmospheric turbulence and performs equally for both
Pyramid and Shack-Hartmann WFS.

Instrumentation and Methods for Astrophysics