Focal-plane-assisted pyramid wavefront sensor: Enabling frame-by-frame optical gain tracking

Aims. With its high sensitivity, the pyramid wavefront sensor (PyWFS) is becoming an advantageous for astronomical adaptive optics (AO) systems. However, this exhibits significant non-linear behaviours leading to challenging AO control issues. Methods. In order mitigate these effects, we propose use in addition classical a focal plane image combined with convolutive description of fast track PyWFS non-linearities, so-called optical gains (OG). Results. We show that additional imaging path only requires small fraction total flux while representing robust solution estimating OG. Finally, demonstrate gain our method brings specific examples bootstrapping and handling non-common aberrations.