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8 June 1995 Encoders for the altitude and azimuth axes of the VLT
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Abstract
The Very Large Telescope (VLT) utilizes high accuracy absolute encoders for the altitude and azimuth axes. The size of the telescope makes it impossible to use encoders of the glass disc type, like have been used on the ESO NTT telescope. The solution was found in using the laser Doppler displacement measurement (LDDM) technique. Although this technique is primarily developed for linear displacements, it can also be used for rotational motions. The arguments that have led to the specifications of the VLT encoders are described. The principle of operation of the LDDM, together with the application for rotational motion, is explained. The application of the LDDM for large scale, high accuracy rotational motions is unique. Therefore, a prototype of the encoder was made. The results of the prototype tests, which confirmed the theoretical analysis, are presented. The mounting of the LDDM on the available diameters of the VLT required a detailed analysis of the mechanical tolerances and thermal effects. These aspects are evaluated. A description of the system integration and calibration is presented.
© (1995) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Martin Ravensbergen, Roberto Merino, and Charles P. Wang "Encoders for the altitude and azimuth axes of the VLT", Proc. SPIE 2479, Telescope Control Systems, (8 June 1995); doi: 10.1117/12.211444; https://doi.org/10.1117/12.211444
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