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Wavefront sensors used in adaptive optics (AO) can misinterpret high spatial frequency wavefront aberrations as low spatial frequency aberrations, known as aliasing. The AO system then corrects for fictitious low-order aberrations, thus adding phase error. Aliasing is a significant contributor to the error budget for XAO. Spatial filtering in the focal plane is a widely accepted practice for Shack-Hartmann Wavefront sensors, and is currently used on SPHERE and GPI. While simulations have shown that the pyramid wavefront sensor (PWS) design is less suscebtible to aliasing than the Shack-Hartmann design, no thorough experimental tests have previously been conducted to verify these simulations. This article describes experiments undertaken using the High Order Testbench (HOT) at the European Southern Observatory headquarters in Munich to explore the effect of a spatial filter on a PWS. The goal of this investigation is to experimentally demonstrate the degree to which a PWS is susceptible to the adverse effects of aliasing. If spatial filtering improves the PWS performance in terms of some relevant metric (e.g. Strehl ratio, contrast, sensitivity), then the optimal aperture size will be identified. In the end, it is shown that the PWS is quite robust against aliasing, but there remain secondary effects caused by spatial filtering that may have useful applications.

Spatial Filtering with a Pyramid Wavefront Sensor on the High Order Testbench