chapter 13, Catadioptric Systems

Excerpt

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.

13.1 General considerations

Because the image in a two-mirror system is formed close to the primary mirror, there almost always needs to be a hole in the primary mirror. This restricts the maximum image diameter to essentially the size of the hole, which is considerably less than the diameter of the primary mirror. For example, if the size of the hole is 30% of the diameter of the primary mirror, the linear obstruction ratio of 0.30, gives a reduction in effective aperture of 0.09( = 0.3²), which is quite common. These systems are therefore always narrow-field systems. A typical application is a mirror-telephoto system with an aperture of f/8, and focal lengths on the order of 500 mm to 1000 mm. A 1000-mm system like this for a 35-mm camera has a diameter of 1000/8 mm = 125 mm. The image diameter is about 40 mm, so the linear obstruction ratio will be about 40/125 = 0.3.