Operating data and pointing measurements of a large radio telescope

The most challenging problem facing the next generation of large, high-frequency radio telescopes is to maintain the pointing accuracy of the final structure. The required pointing tolerances are very precise (typically <0.0003 degrees, or 1 arc-second), because the size of the radio beam on the sky decreases at higher observation frequencies and also decreases with increased telescope diameter. A major difficulty is that such large structures are affected significantly by the wind. A critical unknown in any future plans for achieving required telescope performance is identifying the source and frequency content of the pointing errors. The importantstructural contributors to such errors are not obvious. The large dish-shaped primary reflector presents the largest physical area to the wind. However, the pointing accuracy is generally more sensitive to the lateral position of the secondary mirror, which is suspended far above the dish on a tripod or tetrapod structure. As a result, it is not clear which structure is more important in wind-induced errors. To address this, we examined the operating data taken at the Nobeyama 45m radio telescope, located in Japan. More than 40 channels of accelerometer data were taken simultaneously with an on-sky pointing measurement. In this paper, we identify the most significant structural contributors to wind-induced quasi-static and dynamic pointing errors, and determine the relevant frequency range.