arecibo Single radio elements have limited spatial resolution (the diffraction limit of the telescope). This diffraction limited resolution is proportional to wavelength (as it is also for optical telescopes, but it seems more extreme for radio telescopes due to the huge range of wavelengths over which they are typically used). The diffraction limit for a circular aperture of diameter D is q ~ 1.22 l/D, where q is the angular diameter of the Airy Disk at the half-power point (the full-width-half-maximum, or FWHM) in radians. At a frequency of 5 GHz, even the Arecibo dish has an angular resolution of only about 50 arcseconds. The fully-steerable GBT has a resolution at this frequency of only 150 arcseconds. Because of the limited spatial resolution of single element telescopes, sophisticated techniques have been developed to combine single elements into multiple-element arrays, which work together to form a single...