Radio telescopes function in much the same way as optical telescopes. A parabolic reflector collects energy from a large area, determined by the reflector's radius. This energey is brought to focus on a detector. Comon focuses for radio telescopes included cassigrain, prime and offset. Unlike optical astronomy, the longer wavelengths of radio frequencies are not suited to direct imaging detectors. Usually a point detector is employed. With a single reflector, this requires many transit passes, or scanning to build up an image.