Proceedings PaperImproved method for pointing characterization of large radio telescopes
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In order to upgrade existing large radio telescopes or develop new ones, it is necessary to employ sophisticated active controls to meet the higher requirements on surface precision and pointing accuracy. However, in order for these high- performance controllers to maintain stability, they require an accurate characterization of the telescope structure. A finite element model (FEM) is sufficient to prove controller concepts, but does not have the level of accuracy required for final controller implementation. This results in a need for experimental characterization of the structure. A significant problem is that the structural behavior of the telescope is typically measured at the encoders, while the critical performance is the actual pointing on the sky. Conventional pointing measurements are excellent for obtaining the actual pointing direction, but are insufficient for structural characterization. Conversely, conventional physical measurements are excellent for determining structural behavior, but are not suitable for high accuracy calculation of the final pointing. We describe a new method for taking pointing measurements to quantify the static and dynamic tracking errors in the telescope. This is accomplished by combining pointing measurements at a high sample rate with simultaneous data taken from sensors on the structure. In the simplest form, the method allows improvement of the telescope controller and some indication of the relative importance of static and dynamic effects. More complete implementations of the approach can provide information about the major contributors of pointing error, improvements to the FEM, and extraction of the force distribution history on the structure. Such data will be essential if future telescope upgrades and designs are to take advantage of complex control and metrology.