CROSS WIDTH: 100
REFERENCE : CHIEF RRY
Fig. 3.54. Spot-diagrams for the "short" version of the Maksutov-Cassegrain of Fig. 3.53 and Table 3.14 for an optimum curved field
The "long" system not only increases the length by almost 50%; it also slightly increases the diameters of the elements unless vignetting is accepted. The slight improvement in astigmatism does not justify the major increase in length, because the colour error C2 is determinant.
An attractive solution for manufacture is to combine the secondary with the back surface of the meniscus by evaporating aluminium on to its central area. The system is shown in Fig. 3.55, the data in Table 3.16 and the spot-diagrams in Fig. 3.56.
The radius of the secondary is no longer a free parameter except within the limits over which the meniscus can be optically "bent". The system given has m2 = — 4.34 and has a substantial amount of "outward" coma as a result, a total spread of about 2.0 arcsec at the corner of a miniature camera format (semi-field = 0.21°), with field curvature. The aperture and radius of the primary are the same as those of the previous system.
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