We observed comet C/2025 A6 with the Five-hundred-meter Aperture Spherical radio Telescope (FAST) equipped with the ultra-wideband receiver from 2025 October 23 to November 8, and it was the first detection for this comet with FAST. Through trapezoidal fitting of the OH line profiles, we derived the expansion velocities of the water which showed an increase from 1.5 +/- 0.3 km s(-1) at the heliocentric distance of 0.65 au to 3.0 +/- 0.9 km s(-1) at 0.54 au. Based on these results, we estimated the OH production rates of C/2025 A6 for October 23, October 26, November 4 and November 5 which were (1.0 +/- 0.1) & times; 10(29), (1.2 +/- 0.1) & times; 10(29), (1.4 +/- 0.3) & times; 1029, and (1.5 +/- 0.4) & times; 1029 s-1 respectively. The results show a significant upward trend.

We observed comet C/2025 A6 with FAST telescope equipped with the ultra-wideband receiver from 23^ rd October to 8^ th November 2025 and detected the OH 18-cm lines for the first time. The OH lines underwent a reversal from emission to absorption from 23^ rd October to 5^ th November, which is mainly caused by variations in the heliocentric velocity. Through trapezoidal fitting of the OH line profiles, we derive expansion velocities of the water that rise as the heliocentric distance decreases. Based on these results, we estimated the OH production rates of C/2025 A6 for 23^ rd October, 26^ th October, 4^ th November, and 5^ th November and it presents a significant upward trend.

We present the polarization calibration of the 19-beam receiver at 1420 MHz within the full illumination of the Five-hundred-meter Aperture Spherical radio Telescope from 2018 October to 2023 March. We perform spider observations to characterize the on-axis Mueller matrix of the central beam. The calibrated polarization percentage and polarization angle of a source with strong linear polarization emission are about 0.2% and 05, respectively. Several parameters of the central-beam Mueller matrix show time variability from months to years, suggesting that relatively frequent polarization calibrations are needed. We obtain the Mueller matrix parameters of the 18 off-center beams with a combination of on-the-fly observations and spider observations. The polarization calibration provides consistent fractional Stokes parameters of the 19 beams, although the Mueller matrix parameters of the off-center beams are not as accurate as those of the central beam. The Mueller matrix parameters of the central beam do not show a strong dependence on the reflector surface. However, we notice different off-center Mueller matrix parameters between the eastern and western sides of the reflector surface. We provide average parameters of the 19-beam Mueller matrices that should be applicable to observations from 2020 to 2022, with several caveats. After applying the average parameters, on-axis fractional linear polarization measurements greater than or similar to 10% and on-axis fractional circular polarization measurements greater than or similar to 1.5% can be considered high-confidence detections. For sources with weak polarization, timely polarization calibrations using spider observations are required.

Five hundred-meter aperture spherical radio telescope (FAST) is a multi-disciplinary basic research platform that is used for a wide range of astronomy research. Currently, narrow-band multi-beam feed are primarily employed for observation. The development of ultra-wide broadband feed system is beneficial for covering more scientific objectives, while also giving full play to the advantages of the FAST huge reception area in wider frequency band. This paper focuses on the 0.5-3.3GHz feed design, which has better than 10 dB simulated return loss across the bandwidth with the method of using fitting curves and dielectric loading and adding corrugated structures to improve the performance of quad ridge flared horn (QRFH). A centrally positioned PTFE dielectric rod regulates electromagnetic wave phase velocity, enhancing radiation directivity and main lobe symmetry. The dielectric rod is formed by two concentric layers with the same dielectric constant but different structures. The outer layer is evenly slotted to reduce its dielectric constant. The addition of the above technical means ensures the symmetry of the E-plane and H-plane far-field patterns, making the aperture efficiency of the feed relatively balanced over 6.6:1 bandwidth. This kind of feed is designed with an emphasis on performance, ease of tuning and manufacturability compared to the case of multi-layer dielectric rods.

We present the polarization calibration of the 19-beam receiver at 1420 MHz within the full illumination of the Five-hundred-meter Aperture Spherical Telescope from October 2018 to March 2023. We perform spider observations to characterize the on-axis Mueller matrix of the central beam. The calibrated polarization percentage and polarization angle of a source with strong linear polarization emission are about 0.2% and 0.5^∘. Several parameters of the central-beam Mueller matrix show time variability from months to years, suggesting relatively frequent polarization calibrations are needed. We obtain the Mueller matrix parameters of the 18 off-center beams with the combination of on-the-fly observations and spider observations. The polarization calibration provides consistent fractional Stokes parameters of the 19 beams, although the Mueller matrix parameters of the off-center beams are not as accurate as those of the central beam. The Mueller matrix parameters of the central beam do not show a strong dependence on the reflector surface. However, we notice different off-center Mueller matrix parameters between the eastern and western sides of the reflector surface. We provide average parameters of the 19-beam Mueller matrices which should be applicable to observations from 2020 to 2022 with several caveats. After applying the average parameters, on-axis fractional linear polarization measurements ≳ 10% and on-axis fractional circular polarization measurements ≳ 1.5% can be considered high-confidence detections. For sources with weak polarization, timely polarization calibrations using spider observations are required.

We present the estimation of solar observation with the Five-hundred-meter Aperture Spherical radio Telescope(FAST). For both the quiet Sun and the Sun with radio bursts, when pointing directly to the Sun, the total power received by FAST would be out of the safe operational range of the signal chain, even resulting in damage to the receiver. As a conclusion, the Sun should be kept at least ～2° away from the main beam during observations at～1.25 GHz. The separation for lower frequency should be larger. For simplicity, the angular separation between the FAST beam and the Sun is suggested to be ～5° for observations at 200 MHz or higher bands.

The Five-hundred-meter Aperture Spherical radio Telescope (FAST) has been running for several years. A new ultra-wide bandwidth (UWB) receiver, simultaneously covering 500–3300 MHz, has been mounted in the FAST feed cabin and has passed a series of observational tests. The whole UWB band is separated into four independent bands. Each band has 1,048,576 channels in total, resulting in a spectral resolution of 1 kHz. At 500–3300 MHz, the antenna gain is around 14.3–7.7 K Jy −1 , the aperture efficiency is around 0.56–0.30, the system temperature is around 88–130 K, and the half-power beamwidth is around 7.6′–1.6′. The measured standard deviation of pointing accuracy is better than ∼7.9″ when zenith angle is within 26.4°. The sensitivity and stability of the UWB receiver are confirmed to satisfy expectations through spectral observations, e.g., H i and OH. The FAST UWB receiver has already demonstrated good performance in capturing sensitive observations for various scientific goals.

This paper presents an ultra-wide bandwidth (UWB) low-frequency radio astronomical cryogenic receiver for the Five-hundred-meter Aperture Spherical radio Telescope (FAST). It covers 6.6:1 bandwidth from 0.5 to 3.3 GHz. The receiver consists of a Quad-Ridged Flared Horn (QRFH), a cryogenic microwave unit, an optical transceiver and a warm microwave and frequency mixing unit. A QRFH with a concentric-loaded dielectric spear is developed: the average return losses are larger than 20 dB; the average ports polarization isolation is 43.87 dB; the average dish efficiency is higher than 65%. Many UWB cryogenic low loss components are developed for the fabrication of a cryogenic microwave unit. The average noise temperature lower than 14.2 K and 22.5 K are achieved as referred to the input ports of cryogenic Dewar and the output of horn, respectively. Compared to other similar advanced UWB receivers, such as Parkes 0.7–4.2 GHz (6:1 bandwidth) receiver and FAST 0.27–1.62 GHz (6:1 bandwidth) receiver, wider relative bandwidth of the proposed receiver is achieved and it is a new attempt to expand the bandwidth of UWB low-frequency receiver.

Five-hundred-meter aperture spherical radio telescope (FAST) is a multi-disciplinary basic research platform that is used for a wide range of astronomy research. The L band 19 beam receiver is mainly used for large sky surveys and pulsar searches. The receiver plays an important role in the discovery of new pulsars and the study of fast radio burst (FRB). The design of the multi-beam feed front-end was analyzed to improve the multi-beam feed system layout and the orthogonal mode coupler. This paper describes the improvements and the corresponding test results. These design methods provide design and technical guidelines for the design and upgrading of large aperture antenna feed systems.

为检测微弱的射电信号,要求望远镜接收机噪声性能良好.低噪声放大器（Low Noise Amplifier, LNA）作为接收机前端关键电路,其噪声系数和增益决定了整机的噪声性能.设计了一款1.2–2.2 GHz的低噪声放大器,电路采用两级级联结构,第2级通过引入负反馈,在改善增益平坦度和拓宽带宽的同时减小噪声,级间经过后级输入阻抗优化后仅需一个隔直电容.并引入有损输出匹配网络,实现高增益、低噪声、良好回波损耗和较为平坦的宽带LNA设计.测试结果表明,在1.2–2.2 GHz频段增益30–33 dB,噪声温度平均值为47 K,输出1 d B压缩点大于11.3 dBm.测试性能良好,可用于该频段接收机系统中.

A new transformer matching method for ultra-wideband (UWB) low noise amplifier is presented, which effectively extends the bandwidth and has little effect on the circuit noise. Low-frequency gain compression are realized by combining Frlan-style transformer and RC feedback loop topology in the interstage. The output matching network is composed of the Rabjohn-style transformer and LC network after Norton transformation. The power gain is enhanced by using two-stage cascode topology. The proposed UWB LNA is based on a 0.25um GaAs PHEMT process, and the post-layout simulation results show the maximum power gain of 21 dB, minimum NF of 1.6 dB in the frequency range of 4-15 GHz. The input reflection coefficient is less than -7dB. The output reflection coefficient is less than 10dB. The average output 1-dB compression point is 14dBm. The chip size is 0.82 mm(2).

While looking up to the sky, humans always wondering who we are, where we come from and whether we are alone. In the vast universe, are there other civilizations? For thousands of years, man has merely observed the universe through the visible spectrum, while the radiation from celestial bodies covers the entire electromagnetic spectrum.

The Five-hundred-meter Aperture Spherical radio Telescope (FAST) located in Guizhou, China, is a very sensitive single dish telescope. Due to the large size of the telescope, optical fiber is used for the transmission of the 3-km astronomical signal from the telescope to the signal processing center. The optical fibers are suspended in the air above the telescope reflector, very easy to slide when the telescope feed cabin moves, resulting in phase drifts for the transmission signal. This phase drift has a negative impact on the observation mode of very long baseline interferometry, and can be compensated by the frequency transfer system in the FAST. In this manuscript, we propose a new phase drift compensation scheme, which is denoted as data-aided channel equalization scheme. The proposed scheme is based on a hypothesis of linear phase relationship between different wavelengths in the same optical fiber, and uses the channel response information of the data-aided channel to conduct signal recovery for the astronomical signal channel. Not only the phase drift, but also the frequency-dependent distortion of the broadband transmission link can be compensated. The proposed scheme has simple transmission structure, and the function part is well modularized, so that the Astronomer users can easily turn it on or off. In the proof-of-concept experiments, the estimation deviation can be significantly reduced by estimated channel responses averaging over training sequence repetitions, showing very high accuracy of the astronomical signal channel estimation.

This paper presents an ultra-low noise L-band radio astronomical cryogenic receiver for the Fivehundred-meter Aperture Spherical radio Telescope(FAST) telescope. The development of key low noise microwave parts of coupling low noise amplifier(Coupling-LNA) and conical quad-ridge orthogonal mode transducers(OMT) and reasonable system integration achieve outstanding performance of receiver. It covers the frequency range of 1.2 GHz to 1.8 GHz. Novel cryogenic Coupling-LNAs with low noise, large return loss, high dynamic range and the function of coupling calibration signals are developed for the proposed receiver. Amplification and coupling function circuits are integrated as a single Coupling-LNA with full noise temperature of 4 K at the physical temperature of 15 K. Its return loss is more than 18 d B, and output1 d B compression power is +5 d Bm. A cryogenic dewar is fabricated to provide 55 K and 15 K cryogenic environment for OMT and Coupling-LNAs, respectively. The receiver’s system noise temperature is below9 K referred to feed aperture plane. Benefiting from optimal design and precise mechanical treatment, good scattering performance of OMT and equalized radiation patterns of horn are achieved with an antenna efficiency above 75%.

We report the discovery of a highly dispersed fast radio burst, FRB~181123, from an analysis of $\sim$1500~hr of drift-scan survey data taken using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The pulse has three distinct emission components, which vary with frequency across our 1.0--1.5~GHz observing band. We measure the peak flux density to be $>0.065$~Jy and the corresponding fluence $>0.2$~Jy~ms. Based on the observed dispersion measure of 1812~cm$^{-3}$~pc, we infer a redshift of $\sim 1.9$. From this, we estimate the peak luminosity and isotropic energy to be $\lesssim 2\times10^{43}$~erg~s$^{-1}$ and $\lesssim 2\times10^{40}$~erg, respectively. With only one FRB from the survey detected so far, our constraints on the event rate are limited. We derive a 95\% confidence lower limit for the event rate of 900 FRBs per day for FRBs with fluences $>0.025$~Jy~ms. We performed follow-up observations of the source with FAST for four hours and have not found a repeated burst. We discuss the implications of this discovery for our understanding of the physical mechanisms of FRBs.

We present a pilot HI survey of 17 Planck Galactic Cold Clumps (PGCCs) with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). HI Narrow Self-Absorption (HINSA) is an effective method to detect cold HI being mixed with molecular hydrogen H$_2$ and improves our understanding of the atomic to molecular transition in the interstellar medium. HINSA was found in 58\% PGCCs that we observed. The column density of HINSA was found to have an intermediate correlation with that of $^{13}$CO, following $\rm log( N(HINSA)) = (0.52\pm 0.26) log(N_{^{13}CO}) + (10 \pm 4.1) $. HI abundance relative to total hydrogen [HI]/[H] has an average value of $4.4\times 10^{-3}$, which is about 2.8 times of the average value of previous HINSA surveys toward molecular clouds. For clouds with total column density N$\rm_H >5 \times 10^{20}$ cm$^{-2}$, an inverse correlation between HINSA abundance and total hydrogen column density is found, confirming the depletion of cold HI gas during molecular gas formation in more massive clouds. Nonthermal line width of $^{13}$CO is about 0-0.5 km s$^{-1}$ larger than that of HINSA. One possible explanation of narrower nonthermal width of HINSA is that HINSA region is smaller than that of $^{13}$CO. Based on an analytic model of H$_2$ formation and H$_2$ dissociation by cosmic ray, we found the cloud ages to be within 10$^{6.7}$-10$^{7.0}$ yr for five sources.

The Five-hundred-meter Aperture Spherical radio Telescope (FAST) has passed national acceptance and finished one pilot cycle of ‘Shared-Risk’ observations. It will start formal operation soon. In this context, this paper describes testing results of key fundamental parameters for FAST, aiming to provide basic support for observation and data reduction of FAST for scientific researchers. The 19-beam receiver covering 1.05–1.45 GHz was utilized for most of these observations. The fluctuation in electronic gain of the system is better than 1% over 3.5 hours, enabling enough stability for observations. Pointing accuracy, aperture efficiency and system temperature are three key parameters for FAST. The measured standard deviation of pointing accuracy is 7.9″, which satisfies the initial design of FAST. When zenith angle is less than 26.4°, the aperture efficiency and system temperature around 1.4 GHz are ∼0.63 and less than 24 K for central beam, respectively. The sensitivity and stability of the 19-beam backend are confirmed to satisfy expectation by spectral H i observations toward NGC 672 and polarization observations toward 3C 286. The performance allows FAST to take sensitive observations for various scientific goals, from studies of pulsars to galaxy evolution.

This paper describes the design, construction, and performance of the wideband orthomode transducers (OMTs) for the L - (1.2–1.8 GHz), the S - (2–3 GHz) and the P - (0.56–1.12 GHz) band receiver systems of the Five-hundred-meter Aperture Spherical radio Telescope (FAST). These OMTs operate at the cryogenic temperature of 70K to reduce their thermal noise contribution to the receiver chains. The development on the FAST L - and S -band quad-ridged waveguide (QRWG) OMTs is carried out based on the theoretical mode analysis. In view of the miniaturization of FAST cryogenic receiver system at P -band, a novel wideband compact bowtie dipole OMT is designed with an octave bandwidth as well as a length of only quarter wavelength. The proposed L -, S - and P -band OMTs are designed and optimized by using Ansys High Frequency Structure Simulator (HFSS), and then manufactured, tested at room temperature. Measurement of FAST cryogenic receiver system noise is also performed with the L -, S - and P -band OMTs installed. The measured results fully comply with the design specifications.

This paper reports on the time and frequency standard system for the Five-hundred meter Aperture Spherical radio Telescope (FAST),including the system design,stability measurements and pulsar timing observations.The stability and drift rate of the frequency standard are calculated using 1-year monitoring data.The UTC-NIM Disciplined Oscillator (NIMDO) system improves the system time accuracy and stability to the level of 5 ns.Pulsar timing observations were carried out for several months.The weighted RMS of timing residuals reaches the level of less than 3.0 μs.

This paper proposes a system based on Delta-Sigma modulation to solve the influence of the deflection reflecting surface and the large-scale traction movable feed system on the astronomical signal received by the feed in FAST. Compared with the traditional solution, this solution focuses more on optimizing from the transmitting end. Through oversampling technology and noise -shaping technology, the quantization noise generated by the transmitting end from analog signal to digital signal is reduced. Experimental results show that the amplitude jitter, phase difference, and frequency spectrum of the system have been significantly optimized.