## E

DK,0

the value in Table 3.3, Case 5. Thus, whereas the RC solution produced a modest increase (about 12%) of the astigmatism of the classical Cassegrain, the DK solution virtually compensates it, giving a value of only about one eighth with this geometry. However, the value of this virtue is limited in practice, since the image quality is totally dominated by the coma.

In the afocal case, Eq. (3.69a) or Table 3.6 give with spr1 = 0

Afoc

DK,0

upr 1

The field curvature is unaffected by mirror asphericities and is identical with that of the other solutions of the same geometry.

Figure 3.6 shows the spot-diagrams for the DK solution with the same geometry (NTT) as in Fig. 3.4. The circle again represents 1 arcsec but the maximum semi-field angle is only one tenth, i.e. ±1.5 arcmin instead of ±15 arcmin. These spot-diagrams are for a flat field, since the field curvature effect is negligible over this small field. Since the astigmatism is also negligible, these spot-diagrams effectively represent pure coma.

d) Telescopes with a spherical primary. As with the other solutions, this form also exists in both Cassegrain and Gregory form. The Cassegrain form with a spherical primary is termed the Pressmann-Camichel form by Rutten and van Venrooij [3.12(a)], but this nomenclature has not been so

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