Adaptive Optics Pathfinders & Phasing Testbeds for a New Era of Extremely Large Telescopes

Abstract

The large apertures of the upcoming generation of Extremely Large Telescopes (ELTs) will enable unprecedented angular resolutions that scale as ∝ λ/D and higher sensitivities that scale as D^4 for point sources. However, all will have pupil segmentation caused by mechanical struts holding up the secondary mirror [European Extremely Large Telescope (E-ELT) and Thirty Meter Telescope (TMT)] or intrinsically, by design, as in the Giant Magellan Telescope (GMT). These gaps will be separated by more than a typical atmospheric coherence length (Fried Parameter) and introduce petal modes into the system characterized by differential piston. Commonly used wavefront sensors, such as a pyramid wavefront sensor (PyWFS), struggle with phase wrapping caused by >λ/2 differential piston wavefront error (WFE). We have developed the holographic dispersed fringe sensor (HDFS), a single pupil-plane optic that employs holography to interfere the dispersed segments onto different spatial locations in the focal plane to sense and correct differential piston between the segments. This allows for a very high linear dynamic piston sensing range of approximately ±14 μm with 50 nm RMS WFE resolution. The GMT has selected a two-channel phasing system comprised of a holographic dispersed fringe sensor (HDFS) to drive the differential piston to within ±λ/2 of the "white light fringe", or zero differential piston, then a PyWFS to complete final fine phasing. We have begun the initial attempts at phasing a segmented pupil utilizing the HDFS on the High Contrast Adaptive optics phasing Testbed (HCAT) and the Extreme Magellan Adaptive Optics instrument (MagAO-X) at the University of Arizona. Additionally, we have demonstrated use of the HDFS as a piston sensor on-sky for the first time. We were able to phase each segment to within ±λ/11.3 residual piston WFE (at λ = 800 nm) of a reference segment and achieved ~50 nm RMS residual piston WFE across the aperture in poor seeing conditions. MagAO-X is a current extreme adaptive optics (ExAO) run by the Extreme Wavefront Control Lab (XWCL) that operates at the 6.5 meter Magellan Clay Telescope at the Las Campanas Observatory in Chile. One of the many things that makes MagAO-X a novel ExAO system is that it is shipped back and forth from its lab at the University of Arizona's Steward Observatory and the Las Campanas Observatory in Chile. This allows its team of scientists and engineers to make hardware and software upgrades and keep the instrument functioning at a cutting-edge level. We present the optomechanical designs for a recent round of upgrades including a new high-speed low order wavefront sensing camera and a 1,000-actuator MEMS non-common path corrector DM (1k NCPC DM). MagAO-X is a pathfinder for ExAO technology on the ELTs, specifically the GMT's ExAO instrument, GMagAO-X. We present the PDR level optomechanical design of GMagAO-X.