Most modern reflecting telescopes have relative apertures of about f/3 and f/8 for the primary and first secondary foci in accordance with the suggestions of Bowen. The angular field which can be used at the first secondary focus is limited by the size of available plates for large instruments but can approach ±1° for smaller systems. The factors influencing the choice of the field corrector system in the first secondary focus are discussed. It is an important point whether the Ritchey-Chrétien form of the mirrors is strictly maintained—giving an optimum field without the corrector—or whether the aspheric constants are allowed to vary as free parameters. The differences are small but significant. The performance of a number of secondary focus correctors consisting of one, two, and three elements is discussed, spot diagrams being given in each case. Systems with fixed Ritchey-Chrétien mirror constants are inferior to those with free mirror constants. Test methods for the manufacture of the mirrors of telescopes of this type are compared. A doublet type corrector is suitable for compensation testing of primary mirrors or for secondaries tested from the back, but the testing of the latter from the front is more difficult. Several possible techniques are discussed.
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