Fig. 3.6. Spot-diagrams for a DK Cassegrain telescope with the geometry of the ESO 3.5 m NTT (f/11; ra2 = Compare with Fig. 3.4 where the field is 10 times larger
generally applied as that of the Dall-Kirkham (DK) above. We shall designate it as the SP (spherical primary) solution. The manufacturing advantage of a spherical primary is obvious. However, the ease of testing concave primaries compared with convex Cassegrain secondaries makes this solution, in general, less attractive for amateurs than the Dall-Kirkham, particularly as the field coma is far worse, as we prove below. Spherical primaries have more interest for the largest professional sizes, particularly in segmented construction (see RTO II, Chap. 3). The SP form has great generic interest in connection with more sophisticated telescope solutions using more mirrors, as we shall see in § 126.96.36.199. But 2-mirror SP solutions have also been proposed, for the giant (25 m) USSR telescope in the 1970's, or more recently for the French TEMOS project of more modest size, for which the enormous field coma was to have been corrected by refracting field correctors near the image, a very difficult undertaking as we shall see in Chap. 4, or by additional mirror correctors avoiding chromatic aberration.
The 2-mirror SP telescope is defined by (bs1)gP = 0 in the equation (3.59)
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