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Fig. 3.65. Spot-diagrams for a modified Lurie-Houghton design with aperture 400 mm at f/3.5 and geometry like the Wright-Vaisala system of Fig. 3.40

the field is limited by astigmatism. However, even at f/3.5, the Lurie system (with all spherical surfaces) has only about half the astigmatism of the Wright-Vaisala system at f/4.0. At a field of ±1° this Lurie gives an image spread on the optimally curved field of about 2 arcsec, compared with about 4 arcsec for the Wright-Vaisala (Fig. 3.41) on its optimum (flat) field.

It is possible to improve the astigmatism of the Lurie even further by bending the two corrector lenses to give a double-meniscus corrector, with its elements either concave or convex to each other. However, this worsens the correction of spherical aberration and coma.

Cassegrain solutions equivalent to the prime focus solutions of Figs. 3.63 and3.64 also give excellent performance [3.12(g)].

In general, as stated by Rutten and van Venrooij, such Houghton-type systems are of great interest for amateur size telescopes. For professional instruments, the maximum size will be limited, as in the Maksutov case, by the limitations of high quality optical glass of appreciable thickness. All these solutions can find application in spectrograph optics.

3.6.4.4 Mangin types and refracting combinations. Although the use of a meniscus, as a separate element more or less concentric with the pupil for correcting the aberration of a concave spherical primary, was only invented in the 1940s [3.32] [3.38] [3.51] [3.53], its invention as a double-pass element attached to the primary is much older. It was first proposed by Mangin [3.65] in 1876 for systems projecting light beams (searchlights). Since such projection systems are simply small-field telescopes in reverse, many telescope forms involving the Mangin principle have been proposed.

The original form (Fig. 3.66) of the Mangin system for projection (searchlights) is analysed in detail in the older literature, e.g. Czapski-Eppenstein [3.11(c)]. Both the front and back (mirror) surfaces are spherical, the doublepass meniscus correcting the third order spherical aberration. The back surface reflection had the great advantage that the chemically silvered reflecting surface available at that time could be protected from behind by paint and

was far more robust than a front-silvered paraboloid. The system was designed to have the front surface concentric with the focus. Various improvements subsequently aspherised either or both the surfaces.

For telescopes, the basic Mangin form is very limited in application, since - with the pupil in the normal position at the primary - the system has the same order of non-aplanatism as the single paraboloid (Newton) telescope. Furthermore, the axial imagery suffers not only from spherochromatism but also from longitudinal chromatic aberration C1. This arises because there are only two free parameters, the radius difference and the thickness of the meniscus, whereas a Maksutov meniscus has the further free parameters of its position and its optical "bending". In the Mangin form, the radius difference for a given thickness must be fixed to correct Sj, the only condition correctable. With spherical surfaces, the system also suffers from fifth order spherical aberration which is reduced, as with the Maksutov, by increasing the thickness of the meniscus.

Details of more modern developments of telescopes using the Mangin principle are given by Maxwell [3.52] and Rutten and van Venrooij [3.12(h)]. The first obvious improvement is to achromatise the meniscus in a way analogous to the two-glass achromatic Bouwers meniscus of Fig. 3.48. Maxwell [3.52] shows such an achromatic version due to Rosin and Amon [3.66]. It uses the Bouwers principle of two glasses with the same refractive index for the central wavelength (1.6134), but different dispersion (va = 57.3 and 43.9), the convex part being of higher dispersion glass as in the Bouwers system. Unless the stop is shifted from the Mangin mirror, even the achromatic form can only be used for limited fields at high relative apertures. If the stop is shifted to some compromise point of the centres of curvature of the two surfaces, a major improvement in field performance at the cost of doubling the length of the system is possible. Maxwell [3.52] shows a number of more sophisticated designs using Mangin elements. An interesting one by Silvertooth [3.67] uses both a Mangin primary and secondary, the secondary being on the front face of a full-aperture meniscus corrector. This system also has a 4-element corrector in the beam converging on the focus.

A practical design for an achromatic Mangin prime focus telescope is given by Rutten and van Venrooij [3.12(h)]. Here the two glasses do not have the same index at the central wavelength, so that an additional small refraction takes place at the glass interface. For their standard aperture of 200 mm, they select a relative aperture of f/5. Since the glasses have "normal" dispersions, the secondary spectrum limits the axial performance, according to their calculations, although the power of the Mangin element is weak compared with a normal refracting objective and therefore gives much less secondary spectrum. This defect could be much alleviated by using "special" glasses with better secondary spectrum characteristics, but such glasses are much more expensive. The field is limited by coma which is about half that of a Newton telescope. It can be improved by the glass choice and, they claim, removed by an airspace in the Mangin meniscus (compare secondary spectrum compensation in the Schupmann Medial telescope below). Rutten and van Venrooij state that a "normal glass" version would need to have its relative aperture reduced to f/10 to give fully acceptable secondary spectrum characteristics, too long to have much interest. This confirms the viewpoint of Maxwell [3.52] that the basic achromatic Mangin form is of limited interest. This is a pity, since the back-reflection remains a favourable feature for protecting an aluminium coating or, even more, silver with its superior reflectivity in the visible. Also, the basic Mangin consists of a single block and avoids centering problems between separated elements. The Mangin possibility should be borne in mind for other, more complex designs such as those cited by Maxwell [3.52] or the Sonnefeld system [3.62] referred to in § 3.6.4.3 above.

Lens telescopes with catadioptric compensation-Schupmann Brachymed-ial and Medial: Although we are entering here somewhat into the domain of refracting telescopes, these designs should be considered as an interesting consequence of the basic Mangin technique of combining lenses with mirrors. An excellent account of the historical development is given by Riekher [3.39(g)]. He describes the Dialyte telescope of Plossl (1850) in which a singlet objective is compensated for its aberrations by a smaller, nearly afocal doublet of two (normal) glasses, giving an achromatic combination (Fig. 3.67).

Fig. 3.67. Dialyte telescope due to Plossl (1850)

The longitudinal aberration C1 of a set of separated thin lenses is given, following Eq. (3.308) by

In the Dialyte corrector, we assume the ray height y to be the same for both elements, giving

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