Operating data from wind induced vibration of a large radio telescope

Next generation radio telescope designs face two serious technical challenges in pointing accuracy. The first is that improved resolution requires more precise pointing, and the second is that increased size makes that pointing accuracy even harder to achieve. New telescopes, such as the 50m LMT/GTM, will require sub-arcsecond pointing in significant wind, whereas current large radio telescopes point only to a couple of arcseconds without wind. A commonly proposed solution to the pointing problem is laser metrology. In this approach, structural deformations are measured, enabling correction of the resulting pointing errors. These measurements are typically slow, allowing only quasi-static effects to be removed. However, the low natural frequencies of large structures allow a significant response to the frequency content of the wind. This effect is difficult to calculate accurately because of both the limited knowledge of the actual wind power spectrum and the complex interaction of the wind with the structure. To investigate the dynamic behavior of a large radio telescope in the wind, we conducted pointing measurements with the Nobeyama Radio Observatory (NRO) 45m telescope. We measured the pointing error in elevation and cross-elevation as a function of time and wind speed, and examined the frequency content of the results; These measurements were the first ever to include not only on-sky pointing, but also simultaneous operating data from accelerometers on the structure. This operating data enables significant extensions to the pure astronomical data. These include more positive identification of structural modes, localization of the major structural contributors to the pointing error, and the nature of the wind loading. As a result, these measurements will be very helpful in understanding the wind-induced vibrations of structures, which is essential to the design of future large telescopes.