The contrast performance of current extreme adaptive optics (XAO) systems can be improved by adding a second AO correction stage featuring its own wavefront sensor (WFS), deformable mirror (DM), and real-time controller. We develop a dynamical model for such a cascade adaptive optics (CAO) system with two stages each controlled by a standard integrator and study its control properties. We study how such a configuration can improve an existing system without modifying the first stage. We analyze the CAO architecture in general and show how part of the disturbance is transferred from low to high temporal frequencies with a nefarious effect of the second stage integrator overshoot and suggest possible ways to mitigate this. We also carry out numerical simulations of the particular case of a first stage AO using a Shack–Hartmann WFS and a second stage AO with a smaller DM running at a higher framerate to reduce temporal error. In this case, we demonstrate that the second stage improves imaging contrast by one order of magnitude and shortens the decorrelation time of atmospheric turbulence speckles by even a greater factor. The results show that CAO presents a promising and relatively simple way to upgrade some existing XAO systems and achieve improved imaging contrasts fostering a large number of science case including the direct imaging of exoplanets.
We introduce the RISTRETTO instrument for ESO VLT, an evolution from the original idea of connecting the SPHERE high-contrast facility to the ESPRESSO spectrograph (Lovis et al 2017). RISTRETTO is an independent, AO-fed spectrograph proposed as a visitor instrument, with the goal of detecting nearby exoplanets in reflected light for the first time. RISTRETTO aims at characterizing the atmospheres of Proxima b and several other exoplanets using the technique of high-contrast, high-resolution spectroscopy. The instrument is composed of two parts: a front-end to be installed on VLT UT4 providing a two-stage adaptive optics system using the AOF facility with coronagraphic capability and a 7-fiber IFU, and a diffraction-limited R=135,000 spectrograph in the 620-840 nm range. We present the requirements and the preliminary design of the instrument.
Higher contrast on images using Adaptive Optics (AO) systems is a topic on demand to be develop for high contrast imaging, especially on objects with small angular separations mixed with low entrance flux. Is for this, that a new system architecture is presented on this proceeding, where an already existing low order system (the 1st stage) is retrofitting a high order, fast system (the 2nd stage). The benefits of such a design is to minimize interventions in the hardware and software of the existing system, while increasing the number of degrees of freedom and speed of the combined system. The 1st stage is completely independent, and does not see the corrections done by the 2nd stage. We present here the analytical formulation and simulations results of such a system. First the system is described and the methodology is presented, where an analytical model was used to roughly determine system parameters for the 1st stage and 2nd stage, and then, a numerical simulation is performed using Octopus for verification.
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