S,Afoc,DK 32 N2 f exactly half that of the classical Mersenne form, 1.238 arcsec. Finally, for the afocal SP form, |m2| ^ <x in (3.366) gives

(Sup)comA s,AfocSP = —16 1 0 — 2jrj f(206265) arcsec ,

again dominated by the second term. The values for the Cassegrain and Gregory cases with the primary defined above are —3.025 arcsec and +7.975 arcsec respectively. The large second term arises from the limit case of Eq. (3.137)

All the above formulae are valid for the third order lateral decentering coma of a 2-mirror telescope with any stop position if the spherical aberration is corrected; but only for a stop at the primary if the spherical aberration is not corrected - see Eq. (3.354).

The formula (3.363) was derived above from the Schiefspiegler approach because the comparison is physically instructive and revealing. However, it can also be derived directly from the recursion formulae in the sense shown in Fig. 3.97. So far as the principal ray parameters are concerned, the secondary mirror is treated as a primary with a telecentric beam incident on it from infinity. Eqs. (3.336) are therefore applied to the principal ray. For the primary, upri = 0 and its coma (Sii)1,$ = 0. For the secondary with the telecentric incident beam ypr2 = +S , the lateral decenter, and

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