tism, as emerges from the analysis of Wynne [3.60]. This system rivals the Super-Schmidt systems discussed above.

Wynne [3.59] also analysed a system in which the front (thick) meniscus of Fig. 3.62 is split into two, giving a three element corrector with spherical surfaces. The basic purpose was to maintain a large total thickness of the menisci, which is favourable for the spherical aberration and field correction, but reduces the mass and thickness of the single front element. The nearly concentric airspace produced has some design advantages but these are offset by the increase in complexity and number of surfaces of the system.

Richter-Slevogt or Houghton-type lens correctors at the pupil: We have seen that the recognition by Schmidt in 1931 of the advantages of a corrector element in a pupil shifted in front of the primary mirror of a telescope unleashed a remarkable harvest of optical design solutions using aspheric plates and concentric or quasi-concentric menisci. In this period, a third generic variant was also discovered: the replacement of the Schmidt aspheric plate by a lens system with spherical surfaces. According to Kohler [3.30(d)] [3.46] and Riekher [3.39(f)], the first such system was probably published by Sonnefeld in 1936 [3.62]. This was an extremely fast system using a doublet corrector and a Mangin mirror (see § 3.6.4.4) and is not closely linked to the basic Schmidt geometry. However, in 1941 Richter and Slevogt [3.63] applied for a patent for a Schmidt-type solution replacing the aspheric plate by an afocal doublet with spherical surfaces, an important design form. In 1944, the same design principle was patented by Houghton [3.64]. In the English literature, such systems are usually called Houghton-type systems, but equal credit should be given to Richter and Slevogt.

For a full account of the many interesting possibilities of this generic type, the reader is referred to Rutten and van Venrooij [3.12(g)].

Houghton's patent gave a lens corrector for a spherical primary. Above all for smaller telescopes this was a very significant advance. His aim was to show

that a two- or three-lens corrector with spherical surfaces could achieve good correction instead of the technically difficult Schmidt plate. In principle, then, any design considered above with an aspheric plate combined with one or two mirrors can be considered as a candidate for a Houghton-type system where the plate is replaced by a lens corrector. If aspheric surfaces are introduced, such as in the Baker-Nunn system of Fig. 3.60, the system becomes a hybrid of Houghton and aspheric plate or plates.

A basic Houghton type described in [3.12(g)] is the Buchroeder design with the geometry of the basic Schmidt, i.e. the corrector at the centre of curvature of a spherical mirror (Fig. 3.63). This type of corrector is particularly attractive for amateurs since it not only has spherical surfaces but also only one glass type. The corrector is afocal, thereby eliminating longitudinal chromatic aberration C1. Furthermore, four surfaces all have the same radius, either positive or negative, the other two being plane. This greatly simplifies the manufacture. Of course, as a single glass corrector generating spherical aberration, it is bound to suffer from spherochromatism proportional to the dispersion of the glass. Also, since the system only has spherical surfaces, it will - like the Maksutov - be limited in relative aperture by fifth order spherical aberration. Rutten and van Venrooij give spot-diagrams for their standard aperture of 200 mm at f/3. Over the curved field, the spot-diagrams remain well within 25 ^m for the whole photographic spectral range (C - h) out to a field of ± 30 mm (nearly ± 3°). Although this is not as good as a classical Schmidt of this size, the quality is excellent compared with photographic grain for small sizes of telescope.

Another very interesting Houghton-type analysed by Rutten and van Ven-rooij is ascribed to Lurie [3.158, 3.159]. In this form, a doublet corrector is placed at a pupil somewhat inside the focus, even shorter than the Wright-

Vaisala camera (Fig. 3.40) which it resembles in a Newton form (Fig. 3.64). However, the Lurie design has an extra degree of freedom with its doublet corrector compared with a single plate in the Wright-Vaisala camera. This enabled Lurie to produce an aplanatic design with a spherical mirror. The field is limited by astigmatism, but this is less than that of the Wright-Vaaisaalaa camera. Such a system, designed for f/4, is virtually diffraction limited over the visual spectral range (C - F) and a field of 1.5° diameter. The improved axial performance compared with the f/3 Buchroeder version arises essentially from the layout at f/4 for the aperture of 200 mm, which reduces the fifth order spherical aberration and spherochromatism. In the Newton form the optical length of the f/4 Lurie system is </', i.e. about 650 mm, compared with an optical length > 2/', i.e. about 1250 mm, for the f/3 Buchroeder system. The price of this compactness is the astigmatism limitation in the field for the Lurie system, although this to some extent flattens the field according to the effective field curvature (2Sjjj + Sjv) - see Table 3.3. In view of its compactness and use of only spherical surfaces, Rutten and van Venrooij are certainly right in drawing the attention of amateurs to the merits of this form. They also show an equally compact Maksutov-Newton telescope with the same geometry. Unlike the Lurie-Houghton design, the Maksutov-Newton they show is not aplanatic and therefore seems less interesting.

More sophisticated designs use two different glasses, enabling correction of spherochromatism.

Figure 3.65 shows spot-diagrams for a Lurie-type design, somewhat modified from that shown in Fig. 3.64 to allow a more direct comparison with the Wright-Vaisala system of Fig. 3.40. The Lurie system now has the afocal corrector just in front of the focus, as with the aspheric plate in Fig. 3.40. This increases the length somewhat, but also relaxes the design. In both systems

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