impossible with one glass from (4.79) for a compact system). Although the above individual powers seem high, it must be remembered they are normalized to /' = 1 and are of a feasible order of magnitude for relatively small lenses near the Cassegrain image correcting field of the order of 1° or less in diameter.

Equation (4.82) demonstrates the important advantage of 2-glass doublet solutions in thin correctors for strict RC telescopes. We shall give examples confirming this. For a "thin" system, both chromatic conditions C1 and C2 can be fulfilled, an important advantage in view of the residual negative power required for the correction of astigmatism and field curvature. But we shall see the effects of secondary spectrum in C2 become significant.

As in the case of the classical Cassegrain, a powerful means of relaxing the condition (4.79) for a thin corrector of one glass type is to vary the as-pheric constants on the mirrors to produce a quasi-RC telescope. As in the quasi-classical Cassegrain case above, we will assume a solution of the 2-mirror system giving (S/)Tei = 0, but introducing finite changes of coma and astigmatism of AS/-/ and which will be functions of £ from (4.64). By analogy with (4.75), we have:

ES/V = (S/V )Teï + (S/V ) The first three conditions give

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