Sunrise III: The Wavefront Correction System
Thomas Berkefeld, Alexander Bell, Reiner Volkmer, Frank Heidecke, Tobias Preis, Thomas Sonner, Eiji Nakai, Andreas Korpi-Lagg, Achim Gandorfer, Sami K. Solanki, Jose Carlos del Toro Iniesta, Yukio Katsukawa, Pietro Bernasconi, Alex Feller, Tino L. Riethmüller, Alberto Álvarez-Herrero, Masahito Kubo, Valentín Martínez Pillet, H. N. Smitha, David Orozco Suárez, Bianca Grauf, Michael Carpenter
TL;DR
Sunrise III demonstrates a redesigned Correlating Wavefront Sensor-based wavefront correction system tailored for a 1 m balloon-borne solar telescope operating at 640 nm. The core approach combines a fast correlation-tracker–assisted tip-tilt correction with a six-element Shack-Hartmann WFS for slow focus and coma sensing, integrated into a real-time control loop that also adjusts the telescope M2 with automated alignment. Key contributions include a higher-bandwidth tip-tilt correction (up to ~$130$–$160$ Hz), an autonomous CW-AO/CW-COM software suite, and a robust, conduction-cooled electronics architecture suitable for extreme flight conditions; performance during the 2024 flight surpasses previous balloon missions, achieving milliarcsecond-level stabilization over multi-hour observations. The improved stability enables high-quality, long-duration solar time series and demonstrates the feasibility of larger balloon-borne telescopes for both solar and night-time astronomy.
Abstract
This paper describes the wave-front correction and image stabilisation system (CWS) developed for the Sunrise III balloon-borne telescope, and provides information about its performance as measured during the integration into the telescope and during the 2024 science flight. The fast image stabilisation is done by a correlation tracker (CT) and a fast tip-tilt mirror, low order aberrations such as defocus and coma are measured by a six-element Shack-Hartmann wavefront sensor (WFS) and corrected by an active telescope secondary mirror for automated focus and manual coma correction. The CWS is specified to deliver a stabilised image with a precision of 0.005 arcsec (rms). The autofocus adjustment is specified to maintain a focus stability of 0.01 waves in the focal plane of the CWS.