Comparing realtime optical gain measurement and methods on MagAO-X

Abstract

A lingering technical challenge for pyramid wavefront sensors (PyWFS) is their change in response between calibration and correction residuals, a quantity known as optical gain (OG). Given the prevalent use of PyWFSs in current and planned high contrast adaptive optics (AO), understanding and reliably measuring OG for realtime control unlocks advanced correction and post processing techniques. The OG quantity as an unknown inhibits a system's ability to stably correct non common path errors, reconstructing wavefronts, and PSF reconstruction. This work compares kinds of optical gain measurement techniques on MagAO-X, a visible light extreme AO instrument on the 6.5m Magellan Clay telescope. We present a set of on-sky measurements of OG across three techniques: 1) An on-sky calibration that acquires OG per spatial mode, 2) realtime measurements of the instantaneous Strehl Ratio (SR) on the pyramid tip, and 3) realtime measurement of known, high-frequency probe signal on the WFS itself. We compare these on-sky results with performance diagnostics to asses how faithfully OG is returned. We conclude with future steps for active control of OG on MagAO-X.