First demonstration of super-resolution with a single-dish radio telescope

In this paper we report the first successful demonstration of angular super-resolution (SR) obtained with a filled-aperture radio telescopeobserving in the K-band of the electro-magnetic (EM) spectrum. In conventional optical imaging systems the spatial resolution is basically limited by the wavelength of the EM radiation and the finite numerical aperture of the optical system. In this context,the concept of SR refers to various methods for improving the angular resolution of an optical imaging system (with a single entrance pupil) beyond the classical diffraction limit. The increased resolving power of single-dish telescopes through SR would make full use of their intrinsic fast scanning capabilities, as compared to interferometers. Despite the potential scientific applications, few efforts have been devoted to the development of SR imaging techniques for Astronomy. In recent years, some complex and ambitious techniques have been proposed to achieve SR with astronomical telescopes, but none of thesetechniques has gone beyond the stage of basic principles or can be used as a real SR imaging method. Variable-transmittance pupils represent one viable approach to achieving SR in radio astronomy. In 1952 Toraldo di Francia suggested that the classical limit of optical resolution could be improved interposing a filter consisting of finite-width concentric annuli of different amplitude and phase transmittance at the entrance (or exit) pupil of an optical system, now also known as Toraldo Pupils (TPs). In this paper we show thatby exploiting the active surface of the Sardinia Radio Telescope (SRT) to emulate a TP at the entrance pupil of the telescope it was possibleto achieve a main beam narrower than the nominal far-field pattern expected by diffraction. We have also used the SR beam to map an astronomical source and we show that we are able to recover some of the compact structurenot visible using the nominal diffraction-limited telescope beam.