Dual-Beam Delay Calibration for VERA

Abstract

Abstract 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 $\sim$0.1mm level. We estimated that the systematic error in the calibration does not exceed 0.127mm, which was evaluated at an elevation angle of 15$^\circ$. This error corresponds to an astrometric error of $\sim$11$\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 20m 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 0.118mm, being consistent with the above estimate.