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Proceedings Paper

The Large Binocular Telescope azimuth and elevation encoder system
Author(s): David S. Ashby; Tom Sargent; Dan Cox; Jerry Rosato; Joar G. Brynnel
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Paper Abstract

A typical high-resolution encoder interpolator relies on careful mechanical alignment of the encoder read-heads and tight electrical tolerances of the signal processing electronics to ensure linearity. As the interpolation factor increases, maintaining these tight mechanical and electrical tolerances becomes impractical. The Large Binocular Telescope (LBT) is designed to utilize strip-type encoders on the main axes. Because of the very large scale of the telescope, the accumulative length of the azimuth and elevation encoder strips exceeds 80 meters, making optical tape prohibitively expensive. Consequently, the designers of the LBT incorporated the far less expensive Farrand Controls Inductosyn® linear strip encoder to encode the positions of the main axes and the instrument rotators. Since the cycle pitch of these encoders is very large compared to that of optical strip encoders, the interpolation factor must also be large in order to achieve the 0.005 arcsecond encoder resolution as specified. The authors present a description of the innovative DSP-based hardware / software solution that adaptively characterizes and removes common systematic cycle-to-cycle encoder interpolation errors. These errors can be caused by mechanical misalignment, encoder manufacturing flaws, variations in electrical gain, signal offset or cross-coupling of the encoder signals. Simulation data are presented to illustrate the performance of the interpolation algorithm, and telemetry data are presented to demonstrate the actual performance of the LBT main-axis encoder system.

Paper Details

Date Published: 16 July 2008
PDF: 9 pages
Proc. SPIE 7019, Advanced Software and Control for Astronomy II, 701920 (16 July 2008); doi: 10.1117/12.790151
Show Author Affiliations
David S. Ashby, Large Binocular Telescope Observatory, Univ. of Arizona (United States)
Tom Sargent, Steward Observatory, Univ. of Arizona (United States)
Dan Cox, Steward Observatory, Univ. of Arizona (United States)
Jerry Rosato, Steward Observatory, Univ. of Arizona (United States)
Joar G. Brynnel, Large Binocular Telescope Observatory, Univ. of Arizona (United States)

Published in SPIE Proceedings Vol. 7019:
Advanced Software and Control for Astronomy II
Alan Bridger; Nicole M. Radziwill, Editor(s)

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