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at the first meniscus surface, refraction and dispersion of the principal ray take place at this surface. C2 would only be zero if the second surface were plane, 2p, equivalent to a plane-parallel plate for the principal ray. For a concentric meniscus, C2 would correspond solely to the stop-shift term in Eqs. (3.223) for the value of Ci given by the concentric (Bouwers) meniscus. If the second surface is steepened to 2m , the Maksutov form, correcting C1, there is no stop-shift term for C2; but the finite curvature of 2m , combined with the finite thickness, produce a finite C2 residual. With the stop at (a), the incidence angle of the principal ray at 2m produces the same net effect. This gives a numerically almost identical C2 aberration residual in both cases, which is determinant for the image quality with typical fields of ±1.5°, as we see in Fig. 3.35.

Further, more complex variants of the simple meniscus corrector are discussed below. An extremely useful extension is the addition of an achromatic field flattener. Table 3.12 gives the data of the "short" Maksutov above, op-

Table 3.12. Data for the "short" Maksutov system (D = 400 mm and f/3.0) optimized with an additional achromatic field flattener, giving the results of Fig. 3.37

File : C:\ZI Title: MAKSUTOV SHORT^FLAT FIELD

4 arcsec

1 arcsec

SSbJ STAN^D Infinity

STO STANDARD Infinity

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