Dual-Beam Delay Calibration For VERA

We present the technique of instrumental delay calibration for the dual-beam system of VLBI Exploration of Radio Astrometry (VERA), namely, the horn-on-dish method, in which artificial noise sources are mounted on the antenna feedome base and a wide-band radio noise is injected into the dual-beam receivers after reflection by a subreflector. We introduce the basic concept of calibration with the horn-on-dish method, and also present results of the experiments to evaluate its calibration accuracy. Detailed comparisons between model path calculations and measured paths from the noise sources show that the horn-on-dish method can calibrate the dual-beam delay difference in the antenna structure and receiver within an similar to 0.1 mm level. We estimated that the systematic error in the calibration does not exceed 0.127 mm, which was evaluated at an elevation angle of 15 degrees. This error corresponds to an astrometric error of similar to 11 mu as with VERA's maximum baseline. An experimental confirmation of the systematic error in the horn-on-dish method has also been obtained by using a pair of 10m and VERA's 20 m antennas at. the Mizusawa station, demonstrating that the systematic difference between the dual-beam delay difference measured with noise sources and that for celestial objects was 0.118 mm, being consistent with the above estimate.