TABLE OF CONTENTS The design of a telescopic system begins with a first-order layout of the powers and spacings of the objective, erectors, field lenses, prisms, and eyepiece, as required to produce the desired magnification, field of view, aperture (pupil), eye relief, and image orientation. Then the individual components are designed so that the telescope, as an entire system, is corrected. Often the eyepiece is designed first; the design is carried out as if the eyepiece were imaging an infinitely distant object through an aperture stop located at the system exit pupil. That is, the rays are traced in the reverse direction from the direction in which the light travels in the actual instrument. Usually a principal ray is traced from the objective (or the aperture stop) through the eyepiece to locate the exit pupil, then an oblique bundle can be traced in the reversed direction (from the eye) to evaluate the off-axis imagery. Almost all optical design is done in this manner, by tracing rays from long conjugate to short, largely for convenience, because the focus variations (due to aberrations and small power changes) are smaller and more readily managed at the short conjugate.
The erectors, if there are any, are usually designed next; their design is frequently included in the eyepiece design by considering the erector and eyepiece as a single unit. (Alternatively, the erector may be considered as a part of the objective; the choice is usually determined by the location of the reticle.) Usually...
