Meniscus Systems

a variety of optical catadioptric systems in which a spherical (or, less frequently, an elliptical) mirror, or a system of mirrors and lenses, is preceded by one or more achromatic menisci. Meniscus systems were invented in 1941 simultaneously and independently by D. D. Maksutov and D. Gáabor. Meniscus systems use meniscus lenses with surfaces that have nearly the same radii of curvature. Lenses of this type are compensators; that is, they have little effect on the general direction of the rays but appreciably change the distortions (aberrations) of optical images of the optical systems of which they are a part.

A meniscus is nearly achromatic (that is, it is free from chromatic aberration) with respect to a parallel pencil of rays if the

Figure 1. Optical diagrams of the simplest meniscus systems; these systems are free from chromatic aberration and can correct spherical aberration and astigmatism. (M) Achromatic meniscus, (Z) concave spherical mirror, (F) focus of the system.

quantity (R₁ — R₂)/d is close to 0.6, where R₁ and R₂. are the radii of curvature of the meniscus surfaces and d is the thickness of the meniscus (see Figure 1 ,a and b). At the same time, R¹ and R₂ may be selected such that the negative spherical aberration of the mirror (mirrors) is compensated by the positive spherical aberration of the meniscus. The coma in a meniscus system depends on the distance between the meniscus and the mirror, and at a certain position of the meniscus the coma is equal to zero. The astigmatism of the simplest meniscus systems is low. The curvature of the image field in a meniscus system is significant. For this reason, photography in these systems is performed on appropriately curved photographic film. However, the use of an additional corrective Piazzi-Smyth lens, which corrects the curvature of the field as well as the distortion, makes it possible to perform photography using meniscus systems on flat plates and films. Single-meniscus systems with high aperture ratios exhibit low chromatic aberration, which is known as chromatism of magnification. This effect is eliminated by using a pair of oppositely oriented menisci (Figure 2,a, b, and c). The same goal is attained in other cases by a light retouching of one of the surfaces of the

Figure 2. Double achromatic menisci in which the dispersion of the first lens is compensated by that of the second

Practical applications in astronomy include meniscus systems in telescopes (which are also called Maksutov telescopes), schematically represented in Figure 3. The first scheme (Figure 3,a) provides adequate field of view (up to 5°) and aperture ratio (up to 1:1.2). The second scheme (Figure 3, b), which is the meniscus analog of the Cassegrain reflector system, provides a moderately large field of view (of the order of 1°) at a long focal length and, therefore, at a lower aperture ratio (1:10 to 1:15). The third scheme (Figure 3,c), which is the meniscus analog of a Gregorian reflector system (telescope), resembles the second scheme in its features.

Figure 3. Optical diagrams of meniscus telescopes, in which an additional correcting lens is used to correct the curvature of the image field and the distortion: (a) system with an opening in the meniscus designed for the introduction of a photographic plate, (b) and (c) meniscus analogs of the Cassegrain and Gregorian reflectors with a second mirror cemented to the meniscus and with an opening in the primary mirror for allowing the image to be projected beyond the mounting of the mirror. (M) Meniscus, (Z_P) principal spherical (elliptical) mirror, (F) focus of the system, (Z_s) secondary mirror, (L) correcting lens, (f) field to be photographed or observed.

Cassegrain-type meniscus systems are used in photographic objectives of long focal length. The shortness of these objectives for a relatively large diameter of the entrance pupil gives them an important advantage over telephoto lenses.

Meniscus systems are more compact than other optical systems with comparable parameters, which simplifies the control of meniscus telescopes by clockworks. Their principal surfaces are of a simple form (spherical), which simplifies manufacture and permits precise optical control of the meniscus systems. Correction of all the principal aberrations leads to high-quality images not only in the center of the field of view but also at the edges of large fields of view.