Adaptive DFT-Based Interferometer Fringe Tracking

Abstract

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

Robust determination of optical path difference: fringe tracking at the Infrared Optical Telescope Array interferometer

“…Other algorithms for fringe packet tracking which have been tested at IOTA are the subjects of separate publications. [7][8][9][10][11] 2. The Instrument IOTA is a long-baseline optical interferometer located at the Smithsonian Institution's Whipple Observatory on Mount Hopkins, AZ, comprising three 45-cm diameter telescopes which can be positioned at 17 stations on an L-shaped track, where the arms are 15 m toward the south-east and 35 m toward the north-east.…”