AAS 203rd Meeting, January 2004
Session 38 Observation and Instrumentation : Optical
Poster, Tuesday, January 6, 2004, 9:20am-6:30pm, Grand Hall

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[38.10] The CHARA Array's Visible Band Fringe Tracker

C. E. Ogden, T. ten Brummelaar, D. H. Berger (CHARA / Georgia State University)

The CHARA Array is a Y-shaped cluster of 1-meter telescopes used to make interferometric measurements of stars. The optical paths from each telescope to the beam combiner must be matched in order to detect interference fringes. Atmospheric turbulence introduces a varying piston error in each telescope's optical path, which must be removed to stabilize the fringe position.

The CHARA array separates the fringe tracker from the science combiner to keep the instrument flexible. The science combiner operates in the K' filter, leaving the visible band, from 0.6 \mu m to 1 \mu m for the fringe tracker. After beam combination, the visible light passes into the spectrograph, which is composed of a prism and a fast readout CCD. When the optical path difference (OPD) of two telescopes is near zero, the spectrum is modulated by sinusoidal fringes, called "channel fringes." The spatial frequency of the channel fringes is proportional to the OPD, and can be used as the error signal for a fringe tracking servo.

A brief overview of the fringe tracking servo is presented, and current technical challenges are discussed. First channel fringe data are included, as well as a discussion of what they imply about atmospheric turbulence on Mount Wilson.

Construction of the CHARA Array was made possible by grants from the National Science Foundation, the W. M. Keck Foundation, and the David and Lucile Packard Foundation, and by the generous support of Georgia State University. This research was also funded in part by the Michelson Fellowship Program sponsored by the NASA Jet Propulsion Laboratory.

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Bulletin of the American Astronomical Society, 35#5
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