prime focus

A prime focus reflector telescope is one with no secondary mirror, i.e., the focal plane in use receives its light directly from the primary mirror. The optical design is simple: light reflected off the primary mirror creates an image in the center of the telescope. Using this image does block part of the telescope's light gathering ability. Radio telescopes are very often 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.

Reflector telescopes can be constructed so as to offer prime focus as an option, allowing an instrument to be placed at the focal plane produced by the primary mirror, but alternately placing a mirror in front of that location to deflect the light elsewhere (e.g., for a Newtonian design). In such a case the term prime focus is also used for the instrument-location whose use makes it act as a prime focus telescope. Major telescopes having this capability include Hale Telescope and Subaru Telescope. Some amateur telescopes have the option support a prime focus configuration for photography, i.e., focusing on the photographic film using nothing but the main mirror.