Adaptive Optics for the CHARA Array

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On the summit of Mt. Wilson overlooking Los Angeles sits a Y-shaped array of six 1-m aperture telescopes operating as an optical/near-infrared interferometric array with baselines from 33 to 331 meters, or angular resolutions of better than 1 milliarcsecond. This far exceeds the resolution of the largest ground-based and orbiting telescopes, and is even an order of magnitude better than will be possible with the extremely large telescopes (ELTs) planned for the next decade. Since construction of the Mt. Wilson array was completed in 2005 by Georgia State University's Center for High Angular Resolution Astronomy (CHARA), it has yielded the first direct detection of gravity darkening on a single star, the first angular diameter for a halo population star, the first image of a single, main-sequence star, and the first direct image of an interacting binary, among other advances. Almost every new measurement reveals that our understanding of stellar astronomy was in some aspect incomplete or simplistic.In the first CHARA array upgrade since construction, Georgia State's Dr. T. ten Brummelaar and collaborators have carefully considered the most cost-effective approaches to enhance the sensitivity of the instrument. They intend to redesign the tip-tilt/wavefront sensor to use an improved detector and eliminate 19 reflections, plus enhance the wavefront control through the addition of a simple low-order adaptive optics (AO) system. Together, these modifications should yield sensitivity increases of ~50x for many targets, with greater improvements in the near-IR. New important areas of study include highly reddened targets such as protoplanetary disks around young stars, active galactic nuclei, and close binary stellar systems, including those containing neutron stars and stellar black holes.Access to the Mt. Wilson interferometer is afforded to consortium members but also to the general astronomical community through an open allocation process managed by the National Optical Astronomy Observatories (NOAO). Through the array, CHARA is training the next generation of scientific and technical experts in the field of interferometry, and scientific results emerging from the facility are incorporated in undergraduate courses as well as public outreach activities. Funding for upgrading the Mt. Wilson optical/near-IR interferometer is being provided by NSF's Division of Astronomical Sciences through its Advanced Technologies and Instrumentation program.