## Info

Classical Gregory (CG)

+2.320

Aplanatic Gregory (AG)

+2.447

DK Gregory (DKG)

+1.450

SP Gregory (SPG)

+7.821

the usual sign changes for these parameters. The values correspond to the relatively large decenter of |S//'| = 10-4 (+1 mm for f = ±10 m) and for a final relative aperture N = 10. Only in the SP case is there a major difference between the Cassegrain and Gregory cases, provoked by the large second term of (3.366) and the sign change of Ra.

An important limit case is the afocal 2-mirror telescope, corrected for spherical aberration. The general formula (3.363) gives at once, for this limit case with |m2| ^ rc

(Su'Jcomat = — or--72 (1 — bs2) (206 265) arcsec , (3.367)

where N and / refer to the primary. We see that, in this limit case, the secondary only influences the decentering coma by its form bs2, not by its geometry in the system. For the classical Mersenne form of the afocal telescope, with bs2 = —1, this gives the simple result

(SuP)Comat = — 1r-r2 f/ (206 265) arcsec , (3.368)

totally determined by the scale and speed of the primary. If we take the same primary as assumed in Table 3.20, with N = 2.5 and / = —2500 mm for S =1 mm, then the angular lateral decentering coma in both classical Cassegrain and Gregory cases is +2.475 arcsec, slightly larger than for |m2| = 4 in Table 3.20. The difference is small because |m2| = 4 is quite a high value. As |m2| ^ 1, the limit case of a plane M2, then the decentering coma vanishes according to (3.358) because M2 has no optical power.

The afocal DK form gives, with bs2 = 0 in (3.367), for both Cassegrain and Gregory forms

(5u'p)comat „ _ ^_ =---2~r (206 265) arcsec , (3.369)

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