Adaptive DFT-Based Interferometer Fringe Tracking
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Adaptive DFT-Based Interferometer Fringe Tracking Edward Wilson Intellization, 454 Barkentine Lane, Redwood Shores, CA 94065, USA Email: [email protected]
Ettore Pedretti Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA Astronomy Department, University of Michigan, 914 Dennison Building, Ann Arbor, MI 48109, USA Email: [email protected]
Jesse Bregman NASA Ames Research Center, Mail Stop 269-1, Moffett Field, CA 94035, USA Email: [email protected]
Robert W. Mah NASA Ames Research Center, Mail Stop 269-1, Moffett Field, CA 94035, USA Email: [email protected]
Wesley A. Traub Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA Email: [email protected] Received 1 June 2004; Revised 29 October 2004 An automatic interferometer fringe tracking system has been developed, implemented, and tested at the Infrared Optical Telescope Array (IOTA) Observatory at Mount Hopkins, Arizona. The system can minimize the optical path differences (OPDs) for all three baselines of the Michelson stellar interferometer at IOTA. Based on sliding window discrete Fourier-transform (DFT) calculations that were optimized for computational efficiency and robustness to atmospheric disturbances, the algorithm has also been tested extensively on offline data. Implemented in ANSI C on the 266 MHz PowerPC processor running the VxWorks real-time operating system, the algorithm runs in approximately 2.0 milliseconds per scan (including all three interferograms), using the science camera and piezo scanners to measure and correct the OPDs. The adaptive DFT-based tracking algorithm should be applicable to other systems where there is a need to detect or track a signal with an approximately constant-frequency carrier pulse. One example of such an application might be to the field of thin-film measurement by ellipsometry, using a broadband light source and a Fourier-transform spectrometer to detect the resulting fringe patterns. Keywords and phrases: fringe tracking, DFT, interferometry, IOTA, real time.
1.
INTRODUCTION
The infrared-optical telescope array (IOTA), shown in Figure 1, is a 3-aperture-long baseline Michelson stellar interferometer located on Mount Hopkins near Tucson, Arizona. Three 45 cm collectors can be located along a 15 m by 35 m L-shaped array, supplying visible and near-IR light to pupilplane beam combiners. The operational details and scientific accomplishments of IOTA have been well documented in [5, 6] and at http://cfa-www.harvard.edu/cfa/ oir/IOTA. This paper reports on the development of an algorithm designed and used to simultaneously minimize the optical
path differences (OPDs) for the three baselines (A-B, A-C, and B-C) provided by IOTA’s three apertures. 1.1.
Fringe tracking goals
Details of the relevant interferometric derivations are covered thoroughly in other references such as [1, 2]. From a signal processing perspective, it is important to know that the governing physics of stellar pupil-plane interferometry result in an id
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