Telescopes, microscopes, and similar compound systems often require achromats for objectives, since the longitudinal chromatic aberration from a singlet objective is so large that eyepieces cannot easily correct it. Such achromats can limit large-aperture systems because the curvatures required of their components are much stronger than the curvatures of singlets with the same net optical power.
The hybrid diffractive-refractive telescope formed by combining a diffractive eyepiece with an unachromatized refractive objective is shown to eliminate the need for such bulky objectives, since a
diffractive eyepiece is capable of correcting the large longitudinal chromatic aberrations of singlet refractive objectives. By splitting the holographic eyepiece into two elements, paraxial lateral color may
also be corrected. First-order design considerations in these hybrid telescopes are presented, and a practical hybrid telescope layout is developed in which 1) primary chromatic aberration is eliminated, 2)
paraxial lateral color is corrected, and 3) a useful eye relief is obtained.