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Optics for binocular telescopes

Low power telescopes such as those used in pairs in binoculars do not have a reticle at the intermediate, real, image plane. Therefore there is no need to correct aberrations of the intermediate images as it is in pointing devices. Generally, in order to keep the telescope simple and short the Petzval sum is not corrected. In most cases the solution is to introduce astigmatism in the eyepiece. If it is permissible to use the accommodation of the eye as a degree of freedom then it is possible to remove the astigmatism over the entire field of view and yet to obtain ever better imagery. The final virtual anastigmatic images presented to the eye by the telescope are formed on a curved surface convex towards the observer's eye. The eye will be focused at a close distance for axial images and relaxed for the extreme off-axis images. Then the telescope is to be designed for different conjugate positions at different field angles. Furthermore the telescope is to be designed for different exit pupil locations at different field angles in order to minimize vignetting when the exit pupil is not sufficiently large. Designing for different conjugate and pupil position at different field angles is almost always possible with most optical design codes using multiple configuration mode.