Abstract
The two mirrors of a Ritchey–Chrétien telescope have shapes chosen to give aplanatic imagery, generally at a focal ratio around ƒ/8. If the secondary is replaced by one giving a materially different focal ratio, the spherical aberration of the combined system can be corrected by an appropriate mirror form, but the coma cannot also be corrected, and it may be large. It is shown that both coma and astigmatism can be corrected by the addition of an afocal thin doublet lens between the secondary mirror and the focus, together with a modification to the secondary. The analysis leads to a discriminant that determines the range of two-mirror systems for which such correction is possible. Two numerical examples are given, for conversion of the 2.5-m du Pont ƒ/7.5 telescope to work at ƒ/15 and ƒ/30.
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