Improvement of the surface accuracy of the nobeyama 45m telescope using radio holographic metrology

The Nobeyama 45m telescope[1, Fig.1] was designed for operation at millimeter wavelengths. The surface of the telescope consists of 600 panels, the mean accuracy of which is about 60 \mu m rms. At initial setting of panels, laser ranging theodolite was used for surveying the surface. However, the surface accuracy of the main reflector has been limited by the measuring error in the method. To improve the accuracy in surveying the surface, an instrument based on radio holographic technique[2, 3] has been developed. Signals at 19.45GHz from the Japanese communications satellites CS-2a and 2b were used for the measurements. The radio holography using geostationary satellites has following advantages for achieving high accuracy. First, as the elevation angle of the telescope is almost constant, it is easier to reduce the influences of pointing errors and the gravitational deformations during the measurements. Second, signal to noise ratio is much higher than using astronomical radio sources.