Recently, the Kunming 40-meter Radio Telescope (hereafter Kunming 40m Radio Telescope) at Yunnan Observatories, Chinese Academy of Sciences, participated in the international Very Long Baseline Interferometry (VLBI) joint observations organized by the European VLBI Network (EVN). This observation successfully got the fringes by real-time data processing. This is the first fringe of 4 Gbps ultra-wideband observation by the 40m Radio Telescope, making it the second VLBI station in China with 4 Gbps ultra-wideband observation capability, following the Shanghai Tianma 65m Radio Telescope. Compared to 2 Gbps, the 4 Gbps observation theoretically increases the signal-to-noise ratio by approximately 1.4 times.
VLBI is a technique that combines radio telescopes located tens of thousands of kilometers apart to simultaneously observe one same target source, effectively creating a huge single telescope. Compared to single-dish observations, VLBI offers significantly higher spatial resolution, allowing for the detection of structures in radio sources at sub-milliarcsecond or even higher. The first image of the black hole's shadow was obtained using VLBI.
The joint VLBI observation for this session was conducted in the X-band (center frequency at 8.4 GHz), using 32 BBC channels, each with a bandwidth of 32 MHz and 2-bit sampling, the total data rate is 4096 Mbps. Kunming station used VLBI terminal DBBC2 for this observation, while the data recording terminal used Mark6. Participating stations in this observation included nine EVN European stations and three Chinese stations (Shanghai Tianma 65m, Xinjiang Urumqi 26m, and Yunnan Kunming 40m). Figure 1 shows the cross-correlation fringes between the German Effelsberg 100m (Ef) and the Kunming 40m (Km) telescopes for this observation. Figure 2 shows the cross-correlation processing results for 2-second data from the 4th scan of this observation, indicating that the correlation amplitude of the Ef-Km baseline is ranking third, only lower than Yebes (Ef-Ys) and Tianma (Ef-T6). Kunming had high data quality.
The Kunming 40m Radio Telescope was established in 2006 and has approached its life expectancy. To ensure the safe operation of the telescope, all VLBI observations were suspended at the beginning of 2022. In order for the 40-meter Radio Telescope to continue its mission’s involvement in the Chang'e lunar exploration project and other observations, from June to November 2023, Yunnan Observatories collaborated with Shanghai Observatory and other relevant institutions to conduct a major renovation and maintenance of the Kunming 40-m telescope. Key components of the telescope were upgraded, enabling it to resume normal status. By the end of 2023, the telescope passed the acceptance inspection conducted by an expert panel and was officially put back into service.
As an associate members of EVN, the telescope’s first observation of 2024 also marked its first EVN VLBI observation after the renovation and upgrade. The success of this observation indicates that the systems of the Kunming 40m Radio Telescope are functioning normally after the renovation, with improved performance. Additionally, it demonstrates the capability to participate in ultra-wideband astronomical VLBI observations, reaching the international forefront level.
Figure 1, the cross-correlation fringes between the Effelsberg 100m (Ef) and the Kunming 40m (Km) telescopes for this observation. Image by CHEN.
Figure 2, the cross-correlation processing results for 2-second data from the 4th scan of this observation, Green means the channel has fringes. The red frame shows the fringes of Kunming 40m’s channel. Image by CHEN.
