Flip-flop modulation method used with a pyramid wavefront sensor to correct piston segmentation on ELTs

Abstract

The next generation of ground-based optical telescopes, such as the European Southern Observatory's Extremely Large Telescope (the ELT), will have large support structures (spiders) for the secondary mirror. These spiders have the effect of segmenting the pupil. Without careful control of the wavefront, segment piston (petal modes) errors can develop. We present a flip-flop modulated/unmodulated method for the pyramid wavefront sensor (PWFS) enabling the PWFS to sense petal piston modes. This flip-flop modulation method uses a single PWFS operating in two states: a modulated state and an unmodulated state. An independent controller is used in each state; the modulated state controls the atmospheric turbulence and the unmodulated state only controls petal piston modes. In simulation, we show the flip-flop method working with the wavefront sensor in both K-and R-bands, providing an improvement of 9.9% and 13%, respectively, over a standard modulated PWFS.

Non-modulated pyramid wavefront sensor

“…12. We note that the slope intensity decreases with increasing partial correction (i.e., the seeing value); this means that when the seeing is changing, the flat wavefront slope reference should be changed as well (Engler et al 2022).…”

“…We considered a well established approach, tested on-sky on a 8m-class telescope: the one presented in Esposito et al (2015) and Esposito et al (2020) for the optical gain compensation, along with the one presented in Agapito et al (2021) for online gain optimization. We note that other methods to deal with optical gain do exist, which may be considered (see Korkiakoski et al 2008;Deo et al 2018Deo et al , 2019Chambouleyron et al 2020;Deo et al 2021;Chambouleyron et al 2021), but in this case, we are not interested in finding a general solution for this issue. Our objective is to understand whether a nonmodulated PWFS calibrated for a certain seeing value can operate with a different seeing value and if one method developed for a diffraction-limited IM can work also for the SIMPC presented in this paper.…”

“…However, the non-modulated PWFS, as demonstrated in Vérinaud (2004), is able (in principle) and excluding nonlinearity, to correct almost perfectly the lower spatial frequencies and to offer an increased sensitivity that is particularly attractive for extreme AO (XAO) systems and for differential piston sensing. In fact, for example, Cerpa-Urra et al ( 2022) proposed a second stage AO system with a non-modulated PWFS, Pinna et al (2014) proposed a PWFS-based system with a small modulation radius to balance atmospheric and differential segment piston modes correction, while Engler et al (2022) proposed a mixed approach where PWFS modulation alternates between modulated and non-modulated. In the latter case, the measurement done during non-modulated frames are used for the correction of differential segment piston modes.…”

Real-time Experimental Demonstrations of a Photonic Lantern Wave-front Sensor

“…To test the loop, we injected a fixed amount of a single Zernike mode, scanning in both mode amplitude and mode index. At each point in the scan, after injecting the wavefront error (WFE) and letting the loop settle, we sampled the closed-loop correction 20 times over the course of 2 s. This procedure is idealized in the sense that it neglects the effect of AO residuals, which will degrade the performance of the sensor in real-world operation; nevertheless, we believe that a comprehensive treatment of WFS performance in the presence of realistic AO residuals (e.g., as was done in Engler et al 2022 for the pyramid) outside the scope of this work. We plan on pursuing such characterizations with future on-sky tests.…”

Making the unmodulated Pyramid wavefront sensor smart

“…However, this modulation reduces its sensitivity, especially to low-order modes, decreasing its performance on faint stars and limiting its loop speed. Additionally, it strongly reduces its ability to sense petal-piston modes, which are crucial for upcoming giant segmented telescopes (Bertrou-Cantou et al 2022;Hedglen et al 2022;Engler et al 2022). Even when modulated, the PWFS still exhibits a nonlinear response to large aberrations.…”