Lenses and mirrors both are used in the infrared, and they have certain advantages and disadvantages. Mirrors are achromatic, testable, available in large sizes, can be readily athermalized, but they almost always get in their own way, because of reflection. In this section several of the more useful forms are reviewed to provide the reader with some reasonable starting points in the designs. The order progresses from single mirrors (in the sense of mirrors with power) through three-mirror systems, to catadioptrics, those with correctors. Then an exploration of variations on the themes.
These include the Newtonian, Herschellian, Pfundian and an eccentric pupil version of the Herschellian. The paraboloid is the right geometric shape for obtaining on-axis imagery that is free from third order aberrations. So it is used for infinite objects.
Herschel simply tilted the paraboloid, as shown in Fig. 11-1, to get the detector out of the way of the incoming beam. Of course, since it is tilted off axis, it suffers from coma and astigmatism. However, it is an extremely simple optical system that has no obscuration and can be set up in the laboratory or in the field.
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