A Prime Focus Reflector Telescope is one with no secondary mirror, i.e., the Focal Plane used is directly off the Primary Mirror. The optical design is simple: light reflected off the primary mirror creates an image in the center of the telescope. This position does block part of the telescope's light gathering ability. Radio telescopes are generally built this way. For optical telescopes, the equipment to record the image, e.g., a CCD camera, is kept compact to reduce its blockage of incoming light.
William Herschel Telescope (WHT)