Fig. 4.37. Basic design (schematic) for an FR without intermediate image for a field of 0.9° diameter at the Cassegrain (RC) focus of the 3.5 m MPIA, f/3 to f/8 telescope [4.15]. mfr = 1/2.67

Fig. 4.38. Spot-diagrams for the FR system of Fig. 4.37 [4.15]. The circle is 0.98 arcsec = 50^m

Lens solutions for modest linear fields in large telescopes may be useful in special cases. A general solution equivalent to the PF with corrector for an FR without intermediate image seems impossible. Wide-field focal reducers (FR) with an intermediate image.

The importance of the quasi-telecentric stop for the performance of an FE or FR without intermediate image has been emphasised above: it is the fundamental factor limiting performance. If the telescope image is allowed to form and a field lens placed near it, the exit pupil of the telescope can, in principle, be re-imaged to an optimum point for an FR with its elements disposed round it like those of a photographic objective round a central stop.

Such a system is really no longer part of the telescope optics in the strict sense: it is effectively a supplementary instrument for direct imaging. In this sense, it is closely associated with spectrograph design: indeed, a normal spectrograph is simply an FR with dispersive means at the transferred pupil. Instrument optics is too large a subject to be treated in this book, but we will give a brief account of some of the work on FR solely considered for direct imaging.

In the study referred to above [4.15] for the MPIA, a lens system with field lens was investigated as shown in Fig. 4.39. The spot-diagrams are shown

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