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Register a new userBeam Patterns of the Five-hundred-metre Aperture Spherical Telescope: Optimisation
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The Five-hundred-metre Aperture Spherical Telescope (FAST) uses adaptive spherical panels to achieve a huge collecting area for radio waves. In this paper, we try to explore the optimal parameters for the curvature radius of spherical panels and the focal distance by comparison of the calculated beam patterns. We show that to get the best beam shape and maximum gain, the optimal curvature radius of panels is around 300 m, and a small shift in the focal distance of a few cm is needed. The aperture efficiency can be improved by ~10% at 3 GHz by this small shift. We also try to optimise the panel positioning for the best beam, and find that panel shifts of a few mm can improve the beam pattern by a similar extent. Our results indicate that accurate control of the feed and panel positions to the mm level is very crucial for the stability of FASTs observational performance.
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The Search for Extraterrestrial Intelligence (SETI) attempts to address the possibility of the presence of technological civilizations beyond the Earth. Benefiting from high sensitivity, large sky coverage, an innovative feed cabin for Chinas Five-hundred-meter Aperture Spherical radio Telescope (FAST), we performed the SETI first observations with FASTs newly commisioned 19-beam receiver; we report preliminary results in this paper. Using the data stream produced by the SERENDIP VI realtime multibeam SETI spectrometer installed at FAST, as well as its off-line data processing pipelines, we identify and remove four kinds of radio frequency interference(RFI): zone, broadband, multi-beam, and drifting, utilizing the Nebula SETI software pipeline combined with machine learning algorithms. After RFI mitigation, the Nebula pipeline identifies and ranks interesting narrow band candidate ET signals, scoring candidates by the number of times candidate signals have been seen at roughly the same sky position and same frequency, signal strength, proximity to a nearby star or object of interest, along with several other scoring criteria. We show four example candidates groups that demonstrate these RFI mitigation and candidate selection. This preliminary testing on FAST data helps to validate our SETI instrumentation techniques as well as our data processing pipeline.
The discovery of ubiquitous habitable extrasolar planets, combined with revolutionary advances in instrumentation and observational capabilities, has ushered in a renaissance in the search for extra-terrestrial intelligence (SETI). Large scale SETI activities are now underway at numerous international facilities. The Five-hundred-meter Aperture Spherical radio Telescope (FAST) is the largest single-aperture radio telescope in the world, well positioned to conduct sensitive searches for radio emission indicative of exo-intelligence. SETI is one of the five key science goals specified in the original FAST project plan. A collaboration with the Breakthrough Listen Initiative has been initiated in 2016 with a joint statement signed both by Dr. Jun Yan, the then director of the National Astronomical Observatories, Chinese Academy of Sciences (NAOC), and Dr. Peter Worden, the Chairman of the Breakthrough Prize Foundation. In this paper, we highlight some of the unique features of FAST that will allow for novel SETI observations. We identify and describe three different signal types indicative of a technological source, namely, narrow-band, wide-band artificially dispersed, and modulated signals. We here propose observations with FAST to achieve sensitivities never before explored.
We present a pilot study of extragalactic HI 21-cm absorption lines using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). We observed 5 continuum sources with HI absorption features firstly identified in the 40% data release of the Arecibo Legacy Fast Arecibo L-Band Feed Array (ALFA) Survey (ALFALFA), including two systems later detected by the Westerbork Synthesis Radio Telescope (WSRT). Most of our observations were carried out during the FAST commissioning phase, and we have tested different observing modes, as well as data reduction methods, to produce the best spectra. Our observations successfully confirmed the existence of HI absorption lines in all these systems, including two sources that were marginally detected by ALFALFA. We fitted the HI profiles with single or double of Gaussian functions, and calculated the HI column densities of each source. The HI absorption profiles obtained by FAST show much higher spectral resolution and higher S/N ratio than the existing data in the literature, thus demonstrating the power of FAST in revealing detailed structures of HI absorption lines. Our pilot observations and tests have enabled us to develop a strategy to search for HI absorption sources using the data from the FAST extragalactic HI survey, which is one of the key projects undertaken at FAST. We expect that over 1,500 extragalactic HI absorbing systems could be detected with survey data, based on sensitivity level we achieved in pilot observations.
We report new HI observations of four z$sim$0.05 star-forming galaxies undertaken during the commissioning phase of the Five-hundred-meter Aperture Spherical Radio Telescope (FAST). FAST is the largest single-dish telescope with a 500 meter aperture and a 19-Beam receiver. Exploiting the unprecedented sensitivity provided by FAST, we aim to study the atomic gas, via the HI 21cm emission line, in low-$z$ star-forming galaxies taken from the Valparaiso ALMA/APEX Line Emission Survey (VALES) project. Together with previous ALMA CO($J=1-0$) observations, the HI data provides crucial information to measure the gas mass and dynamics. As a pilot HI survey, we targeted four local star-forming galaxies at $zsim0.05$. In particular, one of them has already been detected in HI by the Arecibo Legacy Fast ALFA survey (ALFALFA), allowing a careful comparison. We use an ON-OFF observing approach that allowed us to reach an rms of 0.7mJy/beam at a 1.7km/s velocity resolution within only 20 minutes ON-target integration time. We demonstrate the great capabilities of the FAST 19-beam receiver for pushing the detectability of the HI emission line of extra-galactic sources. The HI emission line detected by FAST shows good consistency with the previous ALFALFA results. Our observations are put in context with previous multi-wavelength data to reveal the physical properties of these low-$z$ galaxies. We find that the CO($J=1-0$) and HI emission line profiles are similar. The dynamical mass estimated from the HI data is an order of magnitude higher than the baryon mass and the dynamical mass derived from the CO observations, implying that the mass probed by dynamics of HI is dominated by the dark matter halo. In one case, a target shows an excess of CO($J=1-0$) in the line centre, which can be explained by an enhanced CO($J=1-0$) emission induced by a nuclear starburst showing high velocity dispersion.
The sub-mm sky is a unique window for probing the architecture of the Universe and structures within it. From the discovery of dusty sub-mm galaxies, to the ringed nature of protostellar disks, our understanding of the formation, destruction, and evolution of objects in the Universe requires a comprehensive view of the sub-mm sky. The current generation single-dish sub-mm facilities have shown of the potential for discovery, while interferometers have presented a high resolution view into the finer details. However, our understanding of large-scale structure and our full use of these interferometers is now hampered by the limited sensitivity of our sub-mm view of the universe at larger scales. Thus, now is the time to start planning the next generation of sub-mm single dish facilities, to build on these revolutions in our understanding of the sub-mm sky. Here we present the case for the Atacama Large Aperture Submillimeter Telescope (AtLAST), a concept for a 50m class single dish telescope. We envision AtLAST as a facility operating as an international partnership with a suite of instruments to deliver the transformative science described in many Astro2020 science white papers. A 50m telescope with a high throughput and 1$^circ$ FoV with a full complement of advanced instrumentation, including highly multiplexed high-resolution spectrometers, continuum cameras and Integral Field Units, AtLAST will have mapping speeds thousands of times greater than any current or planned facility. It will reach confusion limits below $L_*$ in the distant universe and resolve low-mass protostellar cores at the distance of the Galactic Center, providing synergies with upcoming facilities across the spectrum. Located on the Atacama plateau, to observe frequencies un-obtainable by other observatories, AtLAST will enable a fundamentally new understanding of the sub-mm universe at unprecedented depths.
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