prime focus

Prime focus. When no other mirrors or lenses are introduced into the optical path, the light striking the primary mirror of a reflecting telescope converges at the prime focus, P. The point at which the primary mirror in a reflecting telescope brings incident light to a focus in the absence of a secondary mirror. The prime focus lies within the incident beam but, in very large telescopes, instruments may be positioned at the prime focus without significant loss of performance. Working at the prime focus has the advantage that light loss and image imperfections introduced by secondary mirrors are eliminated, and a relatively low focal ratio and wide field of view can be obtained.