## D

Substitution in Eq. (3.119) gives the form

V^ /Apian VA/ —2RA In the afocal case, since —2 ^ ro and the other quantities remain finite, the astigmatism is zero from (3.120), in agreement with Table 3.6 and the convergence of the classical and aplanatic solutions in this limit case.

The field curvature E S/y is independent of the aspheric form of the mirrors and is therefore identical in the aplanatic case to that of the classical telescope treated above.

The RC aplanatic form of the Cassegrain telescope, is, and will always remain, the ultimate optical design for a 2-mirror telescope, since it combines the compactness and telephoto effect of Cassegrain geometry with optimum aberration correction possibilities (2 conditions) according to the Schwarzschild theorem (§ 3.2.6.1). Its significance increases as |—2| is reduced and the angular field is increased. The development of arrays of electronic detectors, above all CCDs, and multi-object spectroscopy increase the advantages of the RC solution over the classical Cassegrain.

A typical angular field for a modern RC telescope is ± 15 arcmin, as with the ESO 3.5 m NTT. This has a final aperture ratio of f/11 and —2 = —5. Figure 3.4 shows the spot-diagrams 2 for this field for the cases of a classical

2 Spot-diagrams in this book set up to illustrate examples of systems considered to be fundamental in the evolution of the theory of the reflecting telescope are represented in formats given by the ZEMAX-EE optical design program. In the case of all reflecting (catoptric) systems, the "through-focus" matrix representation (see § 3.2.5.3) is used; for systems including refracting elements (catadioptric) the "single-focus field-wavelength" matrix is used. In all cases, the spot-diagram matrix is accompanied by the computer drawing of the system and the input calculation data. The latter has been taken over from computer optimization results without removal of redundant digits in the output data. The same also applies to system data given in tables in the text in this chapter. The removal of redundant digits from the output data is part of the design process of tolerancing and preparation of manufacturing specifications, a process which has not been undertaken in the design of the system examples given in Chapters 3 and 4 of this book.

1 arcsec

IMA : STANDARD

INDEX OF REFRACTION DATA:

Surf Glass 0.500000

0 1.00000000

1 MIRROR 1.00000000

2 MIRROR 1.00000000

3 1.00000000

IMA : STANDARD

INDEX OF REFRACTION DATA:

Surf Glass 0.500000

0 1.00000000

1 MIRROR 1.00000000

2 MIRROR 1.00000000

3 1.00000000

Kj 1 |

## Telescopes Mastery

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