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An aperture stop is the physical opening that limits the angular cone of rays from an on-axis object point. Its images through preceding and following optics are the entrance and exit optical pupils; it differs from a field stop, which limits the extent of the imaged field.

An optical pupil is an image of the aperture stop. The entrance pupil is its image viewed through optics before the stop; the exit optical pupil is its image through the optics after it. optical pupil reimaging maps the telescope aperture onto devices such as a deformable mirror.