In this chapter we will be considering systems with one or two mirrors as well as some lenses. Two-mirror systems are more common and more useful than single-mirror systems, because the image position can usually be outside the optical system, whereas in the case of a single-mirror system the image lies within the incoming beam unless some decentrations are introduced. In this chapter we will only consider centered systems with spherical or aspheric surfaces.
The main advantages of catadioptric systems compared with all-refracting lenses are that they are
1. Very compact,
2. Quite simple for relatively high apertures,
3. Very good color correction because most of the power is in the mirrors, instead of the lenses.
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