NEWTONIAN REFLECTOR

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. 1a).
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

Fig. 1. 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.

diameter. The total surface area of this mirror is approximately 314 centimeters squared (cm²). If the eyepiece mirror is a 3-cm square, its total area is 9 cm², 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.