## Cjt

the field coma, a "long" version of this variant makes no sense at all. The advantage of such a system for amateurs is the economy of one convex spherical surface. Since only the central area of the convex meniscus face is used for the secondary, a high quality of its figure is largely assured. The combined element also has advantages in maintaining centering. Nevertheless, the apla-natic solution of Fig. 3.53 is optically much superior and normally worth the extra effort of producing a separate secondary if photography (direct imagery) is intended.

Some comment is necessary on the thickness of the meniscus corrector in Maksutov telescopes. The situation is not the same as that of strictly concentric (Bouwers) designs. In the latter, the contribution of the meniscus thickness to the compensating spherical aberration is a direct function of the thickness: the first surface produces a massive overcorrection, the second a massive undercorrection. The undercorrection is smaller because the second surface has a radius longer by Ar with d = ri - r2 = Ar , since the spherical aberration contribution of a surface depends for a parallel beam roughly on the third power of (1/r) - see the first equation of (3.20) in which A and A ( are proportional to (1/r) for a parallel beam. The difference between the two contributions must compensate the spherical aberration of the mirror system. The shallower the meniscus, the thicker it must be.

In Maksutov designs, concentric symmetry has been abandoned to achieve the chromatic condition C1 = 0, giving from (3.283)

as the paraxial condition. For the glass BK7, this gives d = (1.766)Ar Maksutov [3.38] [3.12(e)] recommended d = (1.70) Ar , which compensates for the focus shift due to the spherochromatism. The data of Tables 3.14 and 3.16 above follow this formula quite closely, the factors being 1.723 and 1.716 respectively.

Since concentricity has been abandoned, a degree of freedom is available in Maksutov designs which is not available in the Bouwers telescope: the freedom to "bend" the meniscus optically irrespective of its position while maintaining the chromatic condition (3.313). The "bending" changes the "shape factor" involved as (1/r1) in Eq. (3.270) [3.3] [3.6] for the "thin-lens" approximation, giving a variation in the spherical aberration produced. To a third order, then, the thickness of a Maksutov meniscus becomes arbitrary. Maksutov [3.38] recommended a thickness ratio of D/10 which is often followed. Wright [3.49] and Rutten and van Venrooij [3.12(e)] show that the fifth order spherical

aberration (zonal error) is reduced by increasing the thickness d, but the transverse chromatic aberration C2 is made worse. So increase of d improves the axial quality at the cost of field correction. The improvement in zonal error comes essentially from the increase of ray height y at the second surface compared with the first.

The system of Table 3.14 has a fairly conventional thickness d = D/7.67. In Table 3.16, a thickness d = D/4.44 was taken, giving a massive support for the secondary on the convex surface. The d80 energy concentration of the axial spot-diagram of Fig. 3.56 is 0.33 arcsec; that of Fig. 3.54 with the thinner meniscus is 0.35 arcsec. This difference is too small to justify the additional cost and weight of a 90 mm meniscus in the one case compared with the 52 mm meniscus in the other case. However, the 52 mm meniscus has a somewhat lighter task, as the larger secondary mirror compensates somewhat more of the aberration generated by the primary.

Surface |
SA3 (= Si) |

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