## F 4 [ml m2 1 m2m2 1mf 2 bs2

for the general case of a 2-mirror telescope.10 Eq. (3.403) can also be written, introducing f ' = f '/m2, as d(Si)2 = ( Vi y i m22 - 1

in which dd 1 is expressed as a proportion of the primary focal length f. In the limit case of the afocal telescope with m2 = ro, this reduces to the simple form d(S! )2

Afoc

Vib s2

dd fi

which depends only on the aspheric form of the secondary.

10 In a private communication (1997), Dr. Bahner indicated a form of equation equivalent to Eq. (3.403) which he considered appreciably simpler and therefore preferable. The difference arises simply because he introduced the parameters f ' and L into the term in square brackets. Because of the innumerable relationships possible for the paraxial parameters indicated by Eqs. (2.54) to (2.90), almost all the formulae in this book can be expressed in many forms. Which form one chooses is a matter of individual preference. The form of Eq. (3.403) has been chosen here because all the terms in the square bracket contain only the fundamental dimensionless numerical parameters m2 and bs2. It also reduces easily to the simple forms of Eqs. (3.405) to (3.408) for the most important special cases.

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