Measurement of Atmospheric Opacity in K-band of Nanshan 25 m Radio Telescope

The 25-meter radio telescope at Nanshan Observatory of Xinjiang Astronomical Observatory of the Chinese Academy of Sciences has been equipped with four cryogenic receivers in the centimeter band, namely, L, S/X, C and K band receivers. The newly installed Q-band cryogenic receiver (30-50 GHz) is the first millimeter-wave receiver of the 25-meter radio telescope, which is currently in the stage of installation and debugging. In order to better carry out millimeter wave observation at Nanshan Observatory, millimeter wave amplitude calibration is very important, and atmospheric opacity is the key to accurate millimeter wave amplitude calibration. To this end, we use the K-band cryogenic receiver (22-24.2 GHz) in the highest frequency band of the four centimeter wave cryogenic receivers of the existing 25-meter radio telescope to control the antenna to take skydip for elevation scanning in fixed azimuth, and obtain the corresponding power values at 18 elevation angles respectively. The power values are selected as the intermediate frequency signals (100-600 MHz) of the K-band receivers. The atmospheric opacity of K-band is obtained by using Levenberg-Marquette non-linear fitting of the signal power value at 200 MHz frequency point (corresponding radio frequency signal of K-band cryogenic receiver is 22.1 GHz). By comparing the measured data, the maximum error of the three groups of test results is 4.5%, which verifies the feasibility of the fitting method. We intend to apply this method to Q-band cryogenic receiver atmospheric opacity measurement after the installation and debugging of Q-band receiver in the future, so that it can improve the accuracy of millimeter-wave amplitude calibration.