Abstract
Aims. With its high sensitivity, the pyramid wavefront sensor (PyWFS) is becoming an advantageous sensor for astronomical adaptive optics (AO) systems. However, this sensor exhibits significant non-linear behaviours leading to challenging AO control issues. Methods. In order to mitigate these effects, we propose to use in addition to the classical pyramid sensor a focal plane image combined with a convolutive description of the sensor to fast track the PyWFS non-linearities, the so-called optical gains (OG). Results. We show that this additional focal plane imaging path only requires a small fraction of the total flux while representing a robust solution to estimating the PyWFS OG. Finally, we demonstrate the gain that our method brings with specific examples of bootstrapping and handling non-common path aberrations.
Highlights
The pyramid wavefront sensor (PyWFS), which was proposed for the first time in 1996 by Ragazzoni (1996), is an optical device used to perform wavefront sensing
In the first section we introduce the concept of the linear parameter-varying system (LPVS) to describe the PyWFS, which opens the possibility of estimating the optical gains (OG) frame by frame instead of considering a time-averaged quantity
One way to deal with this behaviour while keeping a matrix computation formalism is to consider the PyWFS as a LPVS
Summary
The pyramid wavefront sensor (PyWFS), which was proposed for the first time in 1996 by Ragazzoni (1996), is an optical device used to perform wavefront sensing. Inspired by the Foucault knife test, the PyWFS is a pupil plane wavefront sensor performing optical Fourier filtering with a glass pyramid with four sides that is located at the focal plane The purpose of this glass pyramid is to split the electromagnetic (EM) field into four beams producing four different filtered images of the entrance pupil. Previous studies (Korkiakoski et al 2008, Deo et al 2019a) have demonstrated that one of the most striking effect of this undesirable behaviour is a time-averaged frequency-dependent loss of sensitivity when the PyWFS is working in presence of non-zero phase This detrimental effect can be mitigated by providing an estimation of the so-called optical gains (OG), which are a set of scalar values encoding the loss of sensitivity with respect to each component of the modal basis. We illustrate this OG tracking strategy in the context of closed-loop bootstrapping and handling non-common path aberrations (NCPAs)
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