  The concept of super-resolution refers to various methods for improving the
angular resolution of an optical imaging system beyond the classical
diffraction limit. Although several techniques to narrow the central lobe of
the illumination Point Spread Function have been developed in optical
microscopy, most of these methods cannot be implemented on astronomical
telescopes. A possible exception is represented by the variable transmittance
filters, also known as "Toraldo Pupils" (TPs) since they were introduced for
the first time by G. Toraldo di Francia in 1952. In the microwave range, the
first successful laboratory test of TPs was performed in 2003. These first
results suggested that TPs could represent a viable approach to achieve
super-resolution in Radio Astronomy. We have therefore started a project
devoted to a more exhaustive analysis of TPs and how they could be implemented
on a radio telescope. In the present work we report on the results of extensive
microwave measurements, using TPs with different geometrical shapes, which
confirm the correctness of the first experiments in 2003. We have also extended
the original investigation to carry out full-wave electromagnetic numerical
simulations and also to perform planar scanning of the near-field and transform
the results into the far-field.
