Reflecting Telescopes Help
The troubles that plague refracting telescopes, particularly lens sag, can be largely overcome by using mirrors instead of lenses as objectives. A first-surface mirror , with the silvering on the outside so that the light never passes through glass, can be ground so that it brings light to a focus that does not vary with wavelength. Mirrors can be supported from behind, so it is possible to make them in diameters several times larger than lenses without encountering the sag problem.
Isaac Newton designed a reflecting telescope that was free of chromatic aberration. His design is still used in many reflecting telescopes today. The newtonian reflector employs a concave objective mirror mounted at one end of a long tube. The other end of the tube is open to admit incoming light. A small, flat mirror is mounted at a 45° angle near the open end of the tube to reflect the focused light through an opening in the side of the tube containing the eyepiece (Fig. 19-11a).
Fig. 19-11 . The newtonian reflector ( a ) has an eyepiece set into the side of the tube. In the cassegrain reflector ( b ), the eyepiece is in the center of the objective mirror.
The flat mirror obstructs some of the incoming light, slightly reducing the effective surface area of the objective mirror. As a typical example, suppose that a newtonian reflector has an objective mirror 20 cm in diameter. The total surface area of this mirror is approximately 314 centimeters squared (cm 2 ). If the eyepiece mirror is a 3-cm square, its total area is 9 cm 2 , which is about 3 percent of the total surface area of the objective.
Newtonian reflectors have limitations. Some people find it unnatural to “look sideways” at objects. If the telescope has a long tube, it is necessary to use a ladder to view objects at high elevations. These annoyances can be overcome by using a different way to get the light to the eyepiece.
Figure 19-11b shows the design of the cassegrain reflector . The eyepiece mirror is mounted closer to the objective than it is in the newtonian design. It is not angled, but it is convex. The convexity of this mirror increases the effective focal length of the objective mirror. Light reflects from the convex mirror and passes through a small hole in the center of the objective containing the eyepiece.
The cassegrain reflector can be made with a physically short tube and an objective mirror with more curvature than that of a newtonian telescope having the same diameter. As a result, the cassegrain telescope is less massive and less bulky. Cassegrain reflectors with heavy-duty mountings are physically stable, and they can be used at low magnification to obtain a wide view of a large portion of the sky.
Practice problems of these concepts can be found at: Optics Practice Test
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