Observations of Extragalactic Radio Sources with the 40m Radio Telescope of the Yunnan Observatory

To open new opportunities on radio astronomical research with the recently constructed YNAO 40m radio telescope, we carried out test observations of extragalactic radio sources. Three compact radio sources (3C273, 3C295, and 0716+714) were observed with the ON/OFF mode. In such a mode the telescope was pointed first toward the source and then 2 degrees off the source. In each cycle there are 2 minutes on-source observation and 2 minutes off-source observation. The observational wavelength bands were alternated between the S and X bands but with short time intervals. We present here the S- and X-band flux-density curves of the three sources. There are unexpected drifts on various time scales in these curves. The sensitivity limits of the observations are about 1 Jy. Through the observation, we have identified problems of the telescope stability and environmental radio-frequency interference. By pointing the telescope to the Zenith without tracking, we found ≤1% per hour drifts in the flux-density curve, which are much less than the drifts observed in an ON/OFF mode for a source. In the S band, there is a strong interference signal at the frequency 2142MHz. This interference signal has several harmonics, so that it polluted our data severely. The frequency interference in the X band is weaker than in the S band. We finally propose measures to improve the telescope capacity. These include developing of a counter-interference system, using a radiometer for measuring the K factor to eliminate gain drifts of devices, and performing drift-scan observation without telescope tracking to avoid other unstable factors.