a two-mirror reflector telescope system in which the two concave parabolic mirrors, called the primary and secondary mirrors, have a joint focus. A parallel beam of rays is incident upon the large primary mirror and converges at the focus; the beam is then intercepted by the secondary mirror (Figure 1, a), which is mounted beyond the focal point. The emerging beam, which is parallel once again but now has a narrower cross section, then passes through a perforated central aperture in the primary mirror. A variant of the Mersenne reflector system has a convex parabolic secondary mirror (Figure 1, b) mounted in front of the focus of the primary mirror. The Mersenne reflector system is used to supply light to slitless stellar spectrographs and was proposed by M. Mersenne in 1636.
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