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For the normalized case of Table 3.2 and the values m2 = —4 and Ra we have

TC,0

the second term being the field coma of the classical Cassegrain. The total field coma is therefore 12.62 times worse than for the classical Cassegrain, compared with factors of 8.27 and 28.56 for the DK and SP solutions respectively. The field coma is therefore only some 50% worse than that of the DK solution.

It is possible to combine the condition for freedom from lateral decentering coma for the form of the secondary given by (3.386) with the condition for a spherical primary. The corresponding primary is spherical if

the ± sign being required because Ra = L/f' and f is positive for the Cassegrain, negative for the Gregory case. With |m2| = ro for the afocal form, Ra = ± 1/2. For finite values of m2, Ra grows slowly: for |m2| = 4, Ra = ± 0.533; for |m2| = 2, Ra = ± 0.666. So this combined solution of zero lateral decentering coma with a spherical primary is only possible with high obstruction ratios Ra. In the Cassegrain case this reduces the field coma according to (3.388).

An interesting variant [3.114] is a "Schiefspiegler" using off-axis segments, shown in Fig. 3.98, which maintains the advantages while overcoming the obstruction problem. The insensitivity to lateral decenter means that the secondary is insensitive to shifts perpendicular to the axis so far as coma in the axial image 12 is concerned. Of course, the strong field coma of this centered solution remains, but the whole field remains constant so far as coma is concerned. Tilt of the off-axis segment of the secondary about its centre is also relatively insensitive, in the sense that the point in the field corresponding to coma freedom wanders from the centre field point to a point in the field where the field coma is compensated by the rotation coma. This

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