Beam patterns and wideband feed for the five-hundred-meter aperture spherical telescope

The Five-hundred-meter Aperture Spherical Telescope (FAST) is currently being constructed in China. With an illuminated aperture of 300 m in diameter, it will be the most sensitive single-dish radio telescope in the world. Recently we calculate the beam patterns numerically and design a wideband feed for the FAST. A program is developed to construct the FAST models at three different zenith angles of z=0°, 27° and 40°. The FAST beam patterns at 200 MHz, 1.4 GHz and 3 GHz are calculated by utilizing the Shooting and Bouncing Ray method. The results show that the FAST has excellent beam performance as expected, with low sidelobes of better than -20 dB and cross-polarizations of better than -30 dB. With a coaxial horn feed, the FAST at z=0° could reach the maximum aperture efficiency of about 72%, 69% and 67% at 200 MHz, 1.4 GHz and 3 GHz, respectively. We design a wideband quad-ridged horn feed for the FAST which works in the frequency range of 0.6~3.4 GHz with almost constant beamwidths and acceptable variations of phase centers. We conclude that the FAST would be an excellent single-dish radio telescope with a powerful detection capability if it is constructed as expected.