As telescopes evolved, it has been discovered that performance of all-reflecting systems - especially those with more strongly curved surfaces - can be improved if mirrors are combined with lens elements. General term for telescope systems using both, reflective and refractive elements in order to form the image is catadioptric. Main working principle of a catadioptric telescope is that aberrations of reflecting and refracting elements cancel each other (kata=against, dioptric=refractive). Lens element(s) used in combination with mirrors can be either placed in the path of incoming light, when it is a full-aperture corrector, or in the converging cone produced by mirror element(s), when it is a sub-aperture corrector. Strictly talking, catadioptric telescope is designed as a synergy of reflective and refractive elements, requiring both for its functioning. That separates it from a mirror telescope using field-corrector, which can function without it.

Systems with sub-aperture correctors are relatively infrequent and, in the commercial telescope arena, often come with second-grade products. This doesn't mean that high-quality catadioptric systems with sub-aperture lens elements can't be built. One example is hyperbolic astrograph, consisting of a hyperbolic primary and either two (Rosin), three (Wynne) or four element (a pair of doublets) sub-aperture lens corrector. Sub-aperture Schmidt corrector can also be used to enhance off-axis performance of a telescope (for instance, to reduce or cancel astigmatism in the Ritchey-Chrétien), but it is seldom used for this purpose in amateur telescopes.

Even a single-element lens corrector in the form of equal or near-equal radii meniscus, can significantly improve performance of a fast paraboloidal mirror (FIG. 98A), or even fast sphere (FIG. 98B). In both instances the corrector is placed in front of the diagonal flat.

Sub-aperture meniscus corrector for a single mirror can be also placed closer to the focal plane, at the bottom of the focuser. Off-axis correction here is nearly as good as with the corrector placed in front of the diagonal flat, while offering advantages of smaller meniscus and lighter, smaller diagonal flat assembly.

A simple doublet corrector of Jones-Bird type can also achieve a good overall correction level in combination with spherical mirror (FIG. 99). Even simpler version, a split meniscus concave toward mirror, with the front surface somewhat stronger (intermediate form toward the above meniscus corrector) , also corrects for spherical aberration and coma, while introducing strong astigmatism and field curvature. It offers better control of secondary spectrum, but has some lateral color.

 Another more recent sub-aperture corrector type for spherical mirror, incorporated in the Cape Newise telescope, consists of a pair of widely separated doublets (one in front of the diagonal, the other at the bottom of the focuser tube). It seems to be capable of very good performance. However, its specifications are not available.

Wynne-type corrector offers excellent correction level with either paraboloidal or hyperboloidal mirrors (it is also used in two-mirror systems), both in regard to monochromatic aberrations and chromatism. However, while one of the three lens elements is a simple near plano-convex or PCX, the other two are strongly curved, thin menisci, very demanding in both, fabrication and positioning/centering. Still, the Wynne corrector is not out of reach for advanced amateur telescope makers and designers. Here is a working example of a Wynne-type corrector for a general paraboloid by C. Cavadore. While this 3-element Wynne-type corrector does good job with smaller, or medium fast mirrors, large, fast amateur mirrors benefit from a more efficient corrector of this type, such as the one illustrated on Fig. 100.

With smaller mirrors, it is easier to achieve high level of correction by combining sub-aperture corrector and hyperboloidal mirror. This is due to simple coma correctors generating under-correction, thus introducing one aberration while correcting for the other. Having the mirror hyperboloidal practically takes spherical aberration out of the equation, allowing corrector design to be optimized for correcting other aberrations. For that reason, hyperbolic astrographs can be designed to a high level of correction with quite simple doublets in place of the correcting element (FIG. 101).

Sub-aperture correctors can also be used with two-mirror telescopes, usually with the goal of improving field quality. As with the above examples of the Newtonian telescopes corrector, they can be either integral part of a system, or an optional add-on. Typical sub-aperture correctors in two-mirror systems are coma-corrector in Dall-Kirkham, or astigmatism/field curvature corrector in aplanatic Cassegrain (Ritchey-Chrétien) telescope. But they also can be used in systems with full-aperture corrector, either to maximize performance, or to allow for easier fabrication of the full-corrector, or both. One such example is aplanatic Houghton-Cassegrain with both, full- and sub-aperture being a plano-convex/concave lens pair (FIG. 138).

In general, aberrations induced by a sub-aperture corrector are determined by its effective diameter, as well as the element shape and power. Ideally, its monochromatic aberrations would nearly balance out with those of the mirror (or mirrors), while the chromatism it induces should be negligible. It is not always possible; in principle, axial monochromatic correction is a priority, followed by acceptable chromatic correction and correction of off-axis aberrations. While sub-aperture correctors can be very complex, a simple single-lens doublet, as illustrated with the above examples, can be very effective. A brief overview of the aberration properties of a thin-lens-element sub-aperture corrector follows.