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.
Reflecting telescopes can be made to offer prime focus as an option, the prime focus being a location within the telescope at which instruments can be placed. Major telescopes with this capability include Hale Telescope and Subaru Telescope. Amateur telescopes can be made such that the prime focus can be used for photography, i.e., focusing on the photographic film using nothing but the main mirror.
William Herschel Telescope (WHT)