Va

where y is the semi-aperture and va the Abbe number n' — 1

Fig. 3.48. Two-glass concentric (monocentric) Bouwers-Cassegrain telescope

measuring the dispersion between wavelengths F and C. Then the change in chromatism due to the Maksutov radius change is

VA VA V2 V2

In the Bouwers proposal, the same change must be generated by a change of va, giving from (3.308)

VA VA VA V2 va

Setting (dC1)s = (dC1)M gives with dr2 = Az dvA = —vA~d— (3.312)

n r2

Since thin-lens theory is used, (3.312) is only an approximation. If va is taken as 60 for typical crown glasses and d/r2 is taken as ~ -1/10 for a typical Bouwers meniscus with n' ~ 1.5, we have dvA ~ 2.7 , a very small change of dispersion which limits the glass choice.

Equations (3.223) show that the correction of C1 will not normally correct the spherochromatism; but it reduces it compared with the simple meniscus.

Figure 3.49 shows spot-diagrams for a Bouwers achromatic, monocentric meniscus-Cassegrain, to be compared with the Linfoot monocentric Schmidt-Cassegrain of Figs. 3.43 and 3.44 using a singlet, non-achromatic plate. The mirror geometry was analysed in connection with the Linfoot monocentric system above. The glasses chosen (BK10 and BK3) reflect the limited choice and have dvA = 1.9, less than that required from (3.312) above. The front stop is abandoned, for the same reasons given for the Maksutov. The finite thickness of the meniscus, together with the crude approximation for dvA, lead to lateral chromatism C2. For this reason, a singlet (BK7) field flattener has been introduced, allowing excellent correction with a flat field. The geometry is lightly modified to 400 mm, f/3.11 - f/6.0. The optical quality is virtually constant over the ±1° field, which could be extended apart from the obstruction limitation of the concentricity.

Rutten and van Venrooij [3.12(c)] also give data for another achromatic variant of the concentric system, a classical Bouwers telescope with an additional weak lens in the concentric stop (Fig. 3.50).

The additional lens must be weakly positive. It has the advantage over the two-glass solution of Fig. 3.48 that a single glass type is used in both meniscus and additional lens. It has the same basic chromatic characteristics as the Maksutov but gives better performance because the concentric symmetry is preserved apart from small asymmetries of oblique pencils at the stop lens. Its performance is comparable at f/3 with a classical Schmidt with a singlet plate, as is shown by the spot-diagrams for such a prime focus system with aperture 400 mm in Fig. 3.51. This may be compared with the spot-diagrams

1 arcsec

Value 0.365000 0.405000

Weight 1.000000 1.000000 1.000000 1.000000 1.000000

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