Finally, it is instructive to note the third and fifth order surface contributions (SA3 = Sj, SA5) in such cases. The aplanatic system of Table 3.14 gives from calculations with the ACCOS V program the values of Table 3.17. The negative residual sum of SA3 balances the positive residual sum of SA5. The net correction of SA3 produced by the meniscus is 0.100093, whereas its individual surface contributions are some 13 2 times higher. This is a warning that the meniscus is an element with critical radius tolerances. Although the surfaces are spherical, because they are steep it is not an easy element to manufacture.

The system of Table 3.14 has a curved field. In a normal Maksutov-Cassegrain, a flat-field requirement is incompatible with aplanatism. The field can be flattened by an additional field flattener.

For further details of such systems, the reader is referred to Chapters 10 and 11 of Rutten and van Venrooij [3.12] and Chap. 4 of Mackintosh [3.48]. Mixed plate-meniscus systems. The zonal (fifth order) spherical aberration of the Bouwers and Maksutov telescopes soon led to proposals with

Spherical Aberration Plate
Fig. 3.57. Hawkins-Linfoot Schmidt-Bouwers telescope with f/1.2 in the prime focus

aspheric plate-meniscus combinations. Classic work was done by Hawkins and Linfoot [3.50] in 1945. The starting point was a concentric Bouwers telescope7. The chromatic aberration C1 was compensated by a weak achromat in the stop, corresponding to the weak positive lens proposed by Bouwers. In order to achieve a high relative aperture (f/1.2), the zonal aberration was corrected by aspherising the plane external surface of the achromatic corrector (Fig. 3.57), which shows the system both in the original prime focus form and also in the concentric (monocentric) Cassegrain variant. The weak achromat corrector has similar properties to the two-glass Bouwers meniscus of Fig. 3.48, the two glasses having the same refractive index for the central wavelength but different dispersions, the convex part again being of higher dispersion glass. In contrast to the weak positive lens corrector of Fig. 3.50, this corrector has no optical power at the central wavelength: its function is purely limited to chromatic correction (C1) and removal of fifth order spherical aberration. It should therefore be superior to the monocentric Schmidt-Cassegrain of Fig. 3.43, since the asphericity on the plate is far lower,

7 Hawkins and Linfoot point out that D. Gabor [3.51] had already patented a meniscus corrector system of the Maksutov type with annular aperture in 1941 (see also Maxwell [3.52] for details). Konig-Kohler [3.30(b)] and Riekher [3.39(d)] point out that such a system was also patented by Penning [3.53] in 1941. According to Riekher [3.39(d)], Bouwers [3.32] sought a patent in the Netherlands for his concentric meniscus in 1941 and 1942 and in 1945 patented a system using a concentric meniscus with an aspheric correcting plate at its centre of curvature, i.e. a Hawkins-Linfoot system.

giving smaller asymmetries for oblique pencils. Spot-diagrams are shown in Fig. 3.58, which should be compared with Figs. 3.44 and 3.49. Clearly, the optical quality is superb, with spot-diagrams within 0.5 arcsec over the whole field of ±1.0°.

Another possibility is to maintain a purely concentric meniscus with an aspherised singlet positive lens, as shown in Fig. 3.50 without the aspherisa-tion. This monocentric form should be nearly as good as the Hawkins-Linfoot monocentric design.

A further modification is discussed by Rutten and van Venrooij [3.12(f)] under the name "Companar". The corrector system is an aspherised weak positive lens together with a concentric meniscus as above, but is otherwise laid out as a flat-field design with mirrors of roughly equal curvature. The system is designed for a very high aperture ratio in the Cassegrain focus of f/2.5 giving a high obstruction ratio of about 60% of the diameter. At f/3, the aspherisation on the corrector lens is considered unnecessary, so the system is like Fig. 3.50, but in a flat-field version.

The Hawkins-Linfoot (prime focus) design is often referred to as a "SuperSchmidt system". According to comparisons by Konig-Kohler [3.30(c)], such a system still gives a monochromatic extension of a spot-diagram in the tangential section within 0.01 for a system normalized to /' = 100 at a field of ± 30° and for a relative aperture of f/1.5. A classical Schmidt reaches this limit at about ±1.5°. A concentric system is almost as good, monochromat-ically, with an extension of 0.02.

Another Super-Schmidt system of a similar, but even more sophisticated form, was patented by Baker [3.54] [3.55] in 1945. The Hawkins-Linfoot as-pheric achromatic corrector is at the centre of two concentric meniscus shells. This system is discussed by Bahner [3.5], Konig-Kohler [3.30] and Maxwell [3.52]. The Baker Super-Schmidt is shown in Fig. 3.59. The nominal relative aperture is 0.67 and the effective relative aperture 0.82. Further developments are described by Bradford [3.56] and Davis [3.57]. The image quality is remarkable, bearing in mind the extreme speed of the camera: even at the edge of the field of ± 26°, the d80 energy concentration diameter is within 50 ^m for the entire photographic wavelength band. The system length is 3.7/'.

Another form of Super-Schmidt is the Baker-Nunn camera, designed for satellite tracking. This is described in detail by Henize [3.58]. Briefer accounts are given by Bahner [3.5] and Riekher [3.39(e)]. This system (Fig. 3.60) is essentially a modified Schmidt, since it dispenses with the menisci of the meteor camera and uses a symmetrical close triplet group of lenses in the pupil instead of a Schmidt plate. The inner 4 of the 6 lens surfaces are all aspherised. The focal length is 510 mm at f/1. The outer (convex) lenses are of Schott short flint glass KzFS2, the inner (concave) lens of dense crown SK14. KzFS2 is a "special glass" with abnormal dispersion, allowing correction of secondary spectrum (see Fig. 3.31 and Chap. 4). The two glasses have roughly the same refractive index for the central wavelength. If the corrector

Prescription Data



Surfaces Stop

System Aperture Ray aining Gaussian Factor Eff. Focal Len. Total Track Image Space F/# Working F/# Obj. Space N.A. Stop Radius Parax.Ima. Hgt. Parax. Mag. Entr. Pup. Dia. Entr. Pup.

Exit Pupil Dia. Exit Pupil Pos. Maximum Field Primary Wave Lens Units Angular Mag. Fields

Field Type: Angle # X-Value

1 0.000000 2 0.000000 3 0.000000

Value 0.365000 0.405000 0.486000 0.656000 1.014000

Was this article helpful?

0 0
Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

Get My Free Ebook

Post a comment