title: Thick meniscus field correctors.
abstract: Thick meniscus lenses with almost equal curvatures are routinely used for the correction of spherical aberration in optical systems. The use of such quasi-concentric menisci (QCM) for correction of field curvature and astigmatism is less well known. Three limiting forms of QCM can be identified, namely, equal curvatures, true concentric, and zero deviation, and each exhibits radically different field aberrations. A doublet lens has field curvature and astigmatism; the former can be removed by adding a strong negative lens at the image plane, but the latter is uncorrected. Use of a QCM, however, corrects both aberrations, moreover, without causing the chief rays to diverge as the conventional field flattener does. Indeed the chief rays may be made to converge so that the QCM can be used as a field-correcting field lens, and we illustrate its use in a periscopic vehicle sight comprising an objective lens, relay stage, and eyepiece. Even in cases in which a measure of field curvature and astigmatism correction is possible, for example, when a double Gauss-type relay lens is used, the QCM results in smaller high-order aberrations and lower ray heights through the system.
authors.forename: T H
authors.initials: TH
authors.lastname: Jamieson