With at least two optical surfaces we may obtain afocal systems by setting the resulting power to zero. For instance this is achieved by a thick lens in air if, from (1.28a), its thickness is d = n(R1 —R2)/(n — 1). Other basic systems are the four Mersenne two-mirror designs with confocal paraboloids in L'Harmonie Universelle, 1636 (, see also Sect. 2.3) and the two-lens designs by Galileo of the Cassegrain form and by Kepler of the Gregory form (Fig. 1.17). Without knowing anastigmatism (see Sect. 1.9), Mersenne invented such afocal systems, and with its Cassegrain form of an afocal system, he also invented the basis of the first
mirror telephoto effect which provides a long focal length by a compact design (see Wilson ).
Afocal systems can be characterized by a compression ratio or an expansion ratio which allows defining the relative change in aperture diameter of the conjugate beams (cf. Sect. 2.4).
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