K2m r

There is an infinity of (kt, k2) sets satisfying this condition. A classical form is with a paraboloid primary; hence the conic constants are in this case

which then leads to an hyperboloid secondary for Cassegrain and ellipsoid for Gregory.

If one of the mirrors is a sphere (kt = 0, or k2 = 0), then the total amount of Sphe 3 aberration must be corrected by the other mirror. Thus we obtain from (1.59) two other systems for each form, Cassegrain and Gregory (Table 1.4). Designs with a spherical primary are called Pressman-Camichel telescopes, whilst those with spherical secondary are Dall-Kirkham telescopes. We include in the Table the Ritchey-Chretien (RC) telescopes whose conic constants are given further by Eq. (1.70).

One may define the telephoto effect, T, as the following ratio,

This ratio is usable for comparisons only if the image field is at the vertex of the primary mirror or just behind it towards the light propagation. A high telephoto effect characterizes a compact system. Compared to the Gregory form, T is the highest for the Cassegrain.

Table 1.4 Comparison of the conic constants for stigmatic two-mirror telescopes: Cassegrain, Newton, quasi-flat secondary and Gregory forms

Telescope type

Primary M1

0 0

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