Optical calibration and performance of the adaptive secondary mirror at the Magellan telescope

www.nature.com/scientificreports OPEN Received: 1 May 2018 Accepted: 28 June 2018 Published: xx xx xxxx Optical calibration and performance of the adaptive secondary mirror at the Magellan telescope Runa Briguglio 1, Fernando Quirós-Pacheco2, Jared R. Males3, Marco Xompero1, Armando Riccardi 1, Laird M. Close3, Katie M. Morzinski3, Simone Esposito1, Enrico Pinna Alfio Puglisi1, Lauren Schatz4 & Kelsey Miller4 1 , In this paper we describe the procedure for the optical calibration of large size deformable mirrors, acting as wavefront correctors for adaptive optics systems. Adaptive optics compensate the disturbance due to the atmospheric turbulence to restore the telescope resolution. We will showcase in particular the activities performed for the Adaptive Secondary Mirror (ASM) of the Magellan Adaptive Optics system (MagAO), which is an instrument for the 6.5 m Magellan Clay Telescope, located at Las Campanas Observatory, in Chile. The MagAO ASM calibration is part of the MagAO-2K project, a major MagAO upgrade that started in 2016 with the goal of boosting adaptive optics (AO) correction at visible wavelengths to image exoplanets. For the first time, the optical quality of MagAO mirror is reported. We describe the procedures developed to achieve high SNR interferometric measurements of the ASM modes under the presence of dome convection noise and telescope vibrations. These measurements were required to produce an improved control matrix with up to 500 modes to close the AO loop on sky with enhanced performances. An updated slaving algorithm was developed to improve the control of actuators vignetted by the central obscuration. The calibrations yielded also a new ASM flattening command, updating the one in use since the MagAO commissioning in 2013. With the new flattening command, a 22 nm RMS surface error was achieved. Finally, we present on-sky results showing the MagAO performance achieved with the new calibrations. Large ground-based telescopes are often equipped with Adaptive Optics (AO) systems1 to compensate for the wavefront distortions introduced by the atmospheric turbulence and restore, at least partially, the telescope diffraction-limited resolution. In particular, AO is an indispensable capability in those fields in astrophysics where high resolution is required, like exoplanets and protostellar disks imaging. In an AO system, a wavefront sensor (WFS) measures at high cadence the optical turbulence and sends commands to a deformable mirror (DM) acting as the wavefront corrector. The WFS and the DM are placed in a closed-loop control configuration. While all the early AO systems and many of the current ones feature an optical relay to re-image the telescope entrance pupil on a small post-focal DM, some current telescopes have been transformed into fully adaptive ones, by replacing the conventional (rigid) secondary mirror with an adaptive one (adaptive secondary mirror, ASM). With respect to a post-focal DM, an ASM offers a number of key advantages, namely: it delivers a corrected wavefront to any focal station of the telescope, with a minimal number of optical surfaces; thermal emissivity is substantially reduced and the optical throughput is increased; a lower impact of the high spatial frequency, manufacturing errors and actuators print-through. The technology of large ASM was developed in the 2000 s, and was first demonstrated at the MMT on Mt. Hopkins, Arizona, and then implemented at the Large Binocular Telescope (LBT)2, in Mt. Graham, Arizona, the Magellan Clay Telescope3, in Las Campanas Observatory (LCO), Chile, and 1 INAF Osservatorio Astrofisico di Arcetri, L. E. Fermi 5, 50125, Firenze, Italy. 2GMTO, 465 N. Halstead St., Suite 250, Pasadena, CA, 91107, USA. 3Steward Observatory, Department of Astronomy, University of Arizona, Tucson, AZ, 85721, USA. 4College of Optical Science, University of Arizona, Tucson, AZ, 85721, USA. Fernando Quirós-Pacheco, Jared R. Males, Marco Xompero, Armando Riccardi, Laird M. Close and Katie M. Morzinski contributed equally to this work. Correspondence and requests for materials should be addressed to R.B. (email: ) Scientific REPOrtS | (2018) 8:10835 | DOI:10.1038/s41598-018-29171-6 1 www.nature.com/scientificreports/ more recently, at the ESO VLT-UT44 in Paranal Observatory, Chile. The ASM technology has been also selected for the next generation of extremely large telescopes (ELT): the Giant Magellan Telescope (GMT) will feature a 7 segments ASM5, while the wavefront corrector of the European ELT (E-ELT) will be a flat, deformable quaternary mirror6. Given the large format of the ASM technology, the same concept has been also proposed as an adaptive primary for the space telescope in the LATT project7. Therefore, large deformable mirrors have become a key component of major AO systems for ground-based telescopes and will likely play a role in future space telescope projects too. All these systems share the same working principle. The optical surface is a thin glass shell (TS) 1 to 2 mm thick, with magnets bonded on its back. The actuators are voice coil motors, each facing a corresponding magnet and exerting a contactless force on it. The actuators are mounted inside a plate (called reference body, RB) behind the TS. For the system already in use at an observatory, including MagAO ASM, the RB is a thick Zerodur glass meniscus; for future systems such as the GMT and E-ELT deformable mirrors, Silicon Carbide has been considered. The RB provides a reference for position capacitive sensors co-located with the actuators. Thanks to such internal metrology, the actuator forces are controlled in a local, position closed loop by the ASM electronics. In nominal working conditions the TS floats in front of the RB at a given distance: such gap is 60 μm for the MagAO ASM and will be larger for future systems. A larger gap allows a greater movement range for the TS, valuable to implement field stabilization and chopping mode. When the AO loop is closed, the WFS sends commands to the ASM and the actuators modify the TS surface to compensate for the wavefront aberrations. The Magellan Adaptive Optics system (MagAO)3 was designed as a modification of the LBT AO systems, and specifically tailored for visible light AO imaging as demonstrated in L. Close et al.8 Similarly to the LBT AO systems, the MagAO features a pyramid WFS and a concave ASM, with a 1.6 mm thick TS. Given the particular design of the Magellan telescope, the MagAO ASM is 85 cm in diameter (≈ 5 cm less than the LBT), and is actively controlled by 585 actuators (87 actuators less than the LBT). Furthermore, due to the 0.29 central obscuration of Magellan, the first three inner rings of actuators are vignetted. This particular feature required the development of specific slaving algorithms to control these un-illuminated actuators (see Section 4.4). MagAO was integrated and tested at the Arcetri Test Tower (ATT) in Italy in 2011–2012. Laboratory tests inc (...truncated)