Pupils and Principal Rays

It is useful to define an aperture area where all the field beams enter into an optical system; this is called the entrance pupil, or input pupil. In the next medium we obtain an image pupil. At the system output medium we obtain the exit pupil, or output pupil, which is the system conjugate of the entrance pupil. An aperture stop physically defines the beam area in which all beams are allowed to pass, then defining all the pupils of the system. A field stop physically defines the linear size of the field of view. In astronomy, we generally define the entrance pupil - or aperture stop - of a telescope as the first element, the primary mirror.

Given a field angle, the ray passing through the center of the entrance pupil is called the principal ray or chief ray. The principal ray also passes through the center of the successive pupils. Rays which pass at the edge of the pupil are called marginal rays.

If a system is telecentric, then the principal rays in the image space are parallel to the axis and the image focal surface is a plane. This is achieved by setting the entrance pupil at the object focal plane thus providing an exit pupil at infinity.

If a system is homocentric, then the principal rays in the image space are converging towards a common center and the image focal surface is curved and normal to this central direction. This is achieved by monocentric systems whose entrance and exit pupils are both located at the common center. Then in the image space, the principal rays are all normal to the focal surface.

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