Reflectors: Newtonian reflectors

The Newtonian system provides access to the image formed by the primary mirror, without altering the effective focal length of the system. The essential optical parts are illustrated in figure 1. A flat front-surfaced mirror is placed at 45◦ on the optic axis of the primary mirror so that the image is formed just outside the cylindrical beam which the primary mirror collects. The rim of the secondary mirror describes an ellipse and the mirror is referred to as the Newtonian flat or elliptical flat. Adjustment of the position and tilt of the Newtonian flat is fairly easy to control and, from this point of view, the system is very convenient. For some purposes the position of the focus, which is towards the open end of the telescope, is inconvenient for direct observation and the use of subsidiary equipment. For these reasons, large telescopes are not frequently used with a Newtonian focus.

Figure 1. The Newtonian system.