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Fig. 3.74. The Willstrop Mersenne-Schmidt telescope with f/1.6 and a 4° diameter field (1984)

mirror solutions, careful baffling is required to prevent light reaching the detector directly from Mi or M3, giving appreciable central obscuration of about 22% of the light. The system is described by McGraw et al. [3.76].

An interesting extension of the Paul or Paul-Baker concept is a proposal by Willstrop [3.77]. He also perforates the primary, but places the third (spherical) mirror sufficiently far behind it that the final image is formed in the plane of Mi (Fig. 3.74). The optical design theory is not changed in principle, but allows a compact telescope with moderate field curvature and excellent correction over a 4° field diameter. Willstrop points out that Mersenne's original proposal in 1636 (Fig. 1.3) showed a perforated primary. He therefore calls his system a Mersenne-Schmidt telescope. The system is admirable since the Mersenne telescope does not suffer from chromatic aberrations. Its one weak feature is the inevitable large central obstruction. This is a more severe restraint than in the Paul or Paul-Baker concepts because of the large separation M2M3 which defines |r3| with the Schmidt geometry centered on the secondary, as with Paul. If, following Paul, both M2 and M3 are spherical to the third order, then r2 = r3 and is relatively long. Since Ra = r2/ri in the afocal Mersenne telescope, the obstruction is high. In the Willstrop proposal, Ra = 0.5 because di = f3 = f2 = fi/2. This axial obstruction is inevitably further increased at the primary perforation by the field supplement. Willstrop analyses the total obstruction in detail for a field of 2° or 4° in diameter. Depending on the stop position and baffle arrangement, he deduces total obstructions (by area) between 41 and 45% for the 4° diameter field. He makes the entirely valid comment that the effective surface area loss at the primary of typical large Schmidt telescopes due to the pupil (plate) shift and field is even larger, about 53%.

Epps and Takeda [3.78] had already proposed a similar system with a perforated primary and third mirror a short distance behind it. We shall see in § 3.6.5.3 that these 3-mirror concepts of Epps and Takeda, Willstrop and Korsch can be extended to give excellent designs with 4 powered mirrors, particularly with 2-axis geometry.

Korsch Telescope

Baker [3.23(d)] also gave design details of a 2-axis (brachy-type), anas-tigmatic predecessor of the concept of Fig. 3.72 (b) suggested by Korsch. A primary concave paraboloid has a 45° flat mirror at its focus which deflects the diverging beam to a concave collimating hyperboloid. The collimated beam is then focused by a concave sphere as in the Paul-Baker telescope. Apart from the brachy-form in which the beam is turned at right angles, this Baker form (Fig. 3.75) is generically the Gregorian equivalent of the PaulBaker telescope above. The hyperbolic form of the concave secondary has the same function as the elliptical convex secondary in the Paul-Baker telescope. The properties of this system, as a fundamental generic type of 2-axis telescope, are further discussed in § 3.6.5.3. As a practical form for modern 3-mirror telescopes, the Korsch design of Fig. 3.72 (b) is much superior.

A modification of the Paul-Baker 3-mirror telescope which is of interest, above all, in smaller sizes for amateurs is the Loveday telescope [3.79]. Details are also given by Rutten and van Venrooij [3.12(i)]. Instead of an independent tertiary, the Loveday design uses the primary with a second reflection, as shown in Fig. 3.76. The final image is at the prime focus since the first two mirrors again form a beam-compressor of the classical Mersenne afocal, anastigmatic pair of confocal paraboloids. The primary can either be used directly as a relatively fast Newton telescope or as a slow Newton telescope with 3 reflections. If the f/no of the primary is /X/Ni, then that of the complete system is //(N1/Ra). Typical values for an amateur telescope given by Rutten and van Venrooij are N1 = 6, Nfin = 24, with Ra = 0.25.

The similarity with the Paul-Baker telescope ceases with the 3-mirror geometry, since the tertiary (i.e. M1) is parabolic in form, not spherical. It therefore suffers from the field aberrations of a parabolic primary with its entrance pupil at the exit pupil of the beam compressor. However, the relative aperture is so weak that the performance is still excellent over the 40 mm field given by Rutten and van Venrooij for a Loveday telescope of

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