Family of Flat Field Meniscus Corrector Catadioptric Lenses Of The Maksutov Type

Abstract

The full aperture meniscus corrector catadioptric lens is a compact, fast flat-field configuration which should find application in wide field, long focal length systems which place a premium on high acutance image quality; i.e., aerial photography. Additionally, since the lens power is mainly in the primary mirror, the meniscus corrector catadioptric lens is capable of covering a much wider spectral bandwidth than can be achieved with a refractor of comparable aperture and focal length. The basic configuration consists of a primary mirror, a weak meniscus corrector lens and a single positive lens close to the image plane, to flatten the field. The under-corrected spherical aberration of the primary mirror is essentially balanced out by the over-corrected spherical aberration contributed by the weak meniscus lens. When design requirements dictate a smaller f-number, the single corrector lens generally evolves into a multiple lens configuration used in either the single pass or double pass mode. Axial color correction is achieved by either the achromatic single glass meniscus lens, or the two glass meniscus doublet. Characteristically, the higher order spherical aberration is corrected by the meniscus lens thickness as the f-number is decreased. However, for large scale systems the corrector thickness may become prohibitive. For the large scale designs the utilization of an aspheric figure on one of the corrector surfaces will produce a significantly thinner lens element. Similarly, the benefits of aspheric surfaces can be realized in the field flattening lenses. For a wide field of view, covering a square/circular field format, the obstruction is quite large; in fact, complete occultation is common in these instances. Consequently, the wide field versions will have to be used in the slit field mode with the resultant bar obstruction. However, this poses no serious problems in aerial photographic applications since the utilization of these designs would be either in the push broom or the scanning mode which would require auxiliary mirrors.